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MG TD TF 1500 - What am I missing?

I recently picked up a lumpy idle and stuttering acceleration at low rpm. Engine is a stock TF XPEG. Valve clearances were checked, new plugs (BP6ES) fitted and the timing checked. All in spec.

Removed the carb air cleaners, because I suspected that after recently replacing a popped core plug under the exhaust manifold (new gaskets fitted - no leaks), the shaft balance between the carburettors may have been altered when I removed and replaced them as a single unit. The car was also running bit rich on a couple of plugs. The carburettors are H4s and are an early MGA adaptation and have worked well to date.

Adjusted the balance between the discontented carbs and still a lumpy idle. So I checked the float levels and reset them at around 7.16" (11.2mm). Floats and needle valves are all good and move smoothly with no hanging up. The needles are well centred and the pistons move freely. I then checked the jet to bridge height on each carb. They were a bit out so I reset them to around 0.07" below. Fuel could be seen in the jet nozzle but not flowing out in to the carb. Oil pressure is good at 50-60 and compression is 155 -165 psi all four cylinders. No over heating and oil is clean.

Although it now idles and runs much better there are sooty black deposits on the plugs. While this could be prolonged idling (temp never goes over 85 deg) it does still suggest that the carbs are running too rich?

What am I missing?
M Hyde

I wonder, have you checked that the jets are returning properly when the choke is pushed in? The black plugs point to this. They don't need to stick very far down to make a massive difference to mixture.

Good luck, David
D A Provan

All good thanks David on that count. I guess I should disconnect the connection rod between the bases of the jets before balancing the carbs?
M Hyde

Thinking about getting a Colour Tune also. The TF 1500 with NKG plugs has an almost 15mm 'plug reach - does 14mm Colour Tune work with that?
M Hyde

Hi Merv
i usually set the jets to .065", with the step on the needle dead level with the bottom face of the piston- and normally it needs very minor adjustment at that.

What happens as you lift the pistons with it idling-? The idle should very slightly increase with the first 1/8" of lift then drop off with more lifting----If you can lift the pistons more than 1/8" and still have increased idle or it doesn't drop off,then it is too rich and the jets need lifting.

willy
William Revit

Many thanks Willy. I have noticed an anomaly in that if I set the jet to bridge height at (say) 0.07", that is 7 flats down on the RH carb and almost 9 down on the LF carb (connecting lever disconnected between the choke levers and jets pushed up by hand)?

Also several old sources suggest 12 flats down as a start for some reason that I can't fathom. Seems far too rich.
M Hyde

Strange that you have two different jet heights, maybe one of the nuts holding the jet guides in place has come loose, might pay to check them but if they're ok it could be just a mis-match of parts-----main thing is equal depth from the top of the jet to the bridge., if that's equal then all should be ok and adjustable/balanceable

If it's rich at idle then yes the plugs might darken up but then when you correct the mixture the plugs won't magically clean themselves ,you need to go for a blast out the road to do that

lifting the pistons and listening for revs change is your best guide for setting idle mixture

willy
William Revit

Yes I do get a rev increase with piston lifting and then it drops back to idle rpm. I have levelled the two jet heights, but they are too deep still IMO. I also put in new plugs and will run the car on the road tomorrow. I am borrowing Tony Slattery's A/F ratio gauge this week to fine tune the process and hopefully get rid of the over rich situation. Thanks Willy.
M Hyde

Yep
The rev increase should be hardly noticeable, if you can lift the piston more than 1/8" and still have increased speed then it's too rich-

Lean----Lifting the piston will make the revs drop off

Normal---Lifting the piston slowly, there should be a very slight/just noticeable increase in idle speed up to approx 1/8" lift then with further lifting the speed drops back off

Rich-----If you can lift the piston more than 1/8" and the revs still pick up instead of dropping off as the piston is lifted the mixture is too rich

willy
William Revit

I have a TD. About the middle of last year, I noticed the onset of a rough idle and low-rev acceleration splutter as you are experiencing. Also, start up from cold was no longer a smooth procedure as it used to be.

An extensive (half)rebuild and calibration of the carb system, new plugs and timing adjustments, resulted in almost zero improvement of the idle, although I was able to achieve near optimum mixture adjustments using a ColorTune (an excellent tool) when revs were set to about 1200rpm.

Two weeks ago, I had the distributor cap off and was checking general cleanliness, etc. The points gap was, as expected, set to 0.015". However I noticed that only the tip of the feeler gauge would fit between the open points faces and I was unable to slide the gauge right through the gap. Only after removing the points set was I able to see excessive burning of the points faces, which included a globule of metal (1mm thick) melted into one face. I figured the dwell setting would be all over the place!

I then fitted and adjusted a replacement breaker set. Magically on start-up, the engine (cold) came to life instantly, with the choke applied for only about three seconds.
Even after only about 1 minute of warm up, I was able to achieve a smooth idle down to about 400 rpm and even lower after 10 mins of running. The exhaust sound was quite smooth and splutter free and just about perfect when idle was set to about 700rpm.

I couldn't believe my luck after all the effort put in with the carbs initially. So, worth checking the points, if you haven't already done so.

David
David Padgett

Tried to add to my post but it won't let me so here is version 2
Yep
The rev increase should be hardly noticeable, if you can lift the piston more than 1/8" and still have increased speed then it's too rich-

Lean----Lifting the piston will make the revs drop off

Normal---Lifting the piston slowly, there should be a very slight/just noticeable increase in idle speed up to approx 1/8" lift then with further lifting the speed drops back off

Rich-----If you can lift the piston more than 1/8" and the revs still pick up instead of dropping off as the piston is lifted the mixture is too rich

Kind of Mr. Slattery to lend his meter, do you know what scale it has as in A/F, CO, or Lambda I have a conversion chart if thjat helps but I'd imagine he'll run you through the process--yell out though if you need some conversion help-----You need to be looking for at least 13:1 for a good stable idle and up to 12.5:1 if you're running fuel with ethanol in it

willy

and yeah there's some dodgey ignition points about at the moment that appear to have plated contacts instead of solid material---as soon as the surface burns off they run crap, but those that I've struck tend to make the idle erratic as though they are running really lean and surge on light throttle around town, you'd swear you had a blocked idle jet if it were an ordinarry carby
William Revit

David , you described my symptoms exactly.

Remembering the old maxim about most fuel problems being electrical, I first started with the distributor, the rotor and then the timing, leads and plugs. I have a Petronics system with no points. Timing was spot on at idle and at max advance. I did clean the rotor and add a little oil to the shaft. The distributor was only recently rebuilt in Melbourne.

My TF 1500 has H4 SU's that, by their numbers, came from an early MGA I believe. Midel in Sydney set them up a couple of years ago. Not that that means too much.

Good advice thanks Willy. When I lift the pistons a tad, the revs do increase and drop off, but in the third category you described. It is still too rich. I thought of getting a Colourtune as Mike Leadbeater has had good results with that, but Tony kindly suggested trying his A/F set up from RayTech in Adelaide. I don't yet have a clear idea which model he has, but he said he would run me through it.

Thanks again Merv
M Hyde

I should add Willy, that I have no idea why my carbs seem slightly different in their mixture range. For example starting from the highly recommended jet to deck height of around 0.07", there are 7 flats up to fully up (lean) on the RH carb and 9 flats to fully up on the LH carb. If I use the 0.07" setting it is hard to start. I then went to two full turns (12 flats) down from fully up on both carbs and it will start and run after a difficult warmup. That of course significantly increases the deck height and hence the rich running. I need to wind them up clearly, but with the initial Right to Left difference that becomes a bit of a guessing game and can could plugs quickly at idle. Hopefully the A/F meter will help with that.
M Hyde

The main thing is to have both jets the same distance down from the bridge--regardless of differing numbers of turns and have both needles exactly level with the bottom face of the piston------then when you change mixtures do both carbs exactly the same number of flats
The A/F ratio gauge in the exhaust will only read the average of both carbs so it's important to adjust both equally from your base setting-
William Revit

Good point Willy. I took a quick drive this morning, about 15 kms at around 3000 rpm, minimal to no idling. Straight home and plugs out. Still some sooty deposits on the plug bases but 1 and 2 show tan colour appearing on the electrodes even after a short drive. Plugs 3 and 4 are the overall sooty ones.

I feel that this suggests that the LH (rear) carb is too rich, even though the bridge heights are the same as the RH one?

I had noted earlier that if I set the same 0.07" depth at the bridge on both jets, it took 7 flats to fully up on the RH carb and 9 flats to fully up on the LH carb. This suggests some variation in the jet positioning perhaps?

I appreciate your help with this!






M Hyde

Sorry I asked the same question again about flats variation between the carbs, I do understand you advice above. Right now the bridge heights are both at 0.075" to get it running (plug sooting issue above withstanding). So I should re-set at 0.065' on both, and see how that goes?
M Hyde

Got me thinking--yep try .065"---and where are you measuring that from, it's .065" from the level flat surface of the bridge, not from the top of the jet tube guides and while you're in there check that the needle shoulders are dead level with the bottom faces of the pistons

bit weird this 7-9 flats to fully up, -when they're both fully up how high are the tops of the jets
The normal setup is set the top of the tubes flush with the top of the bridge then down 2 turns--that usually puts you around that .060"-.065" spot---but i've had some that won't reach the level spot and that means just measuring down to the equal .065"

It could be that the upper cork seal on the jet tube has failed and fuel is getting sucked up the outside of the tube but that's very very rare
William Revit

Willy,
I am measuring with a vernier from the top of the jets to the flat surface of the bridge. As for the needle shoulders, they are of equal length and seem to be correctly installed. The pistons are flat on the bridge. When the jets are wound right up they are 'almost' level with the jet tube guides. The jet tube guides themselves are down very slightly from the bridge.
M Hyde

yep ok---forget all about the 7-9 flats-it's just how it is---not all of them will screw right up till the jet pokes out the top-it's not important.
-Because the jets won't reach up high enough to be level with the bridge for an initial height, the only option you have is measuring the .065" from the bridge and go from there-
William Revit

Thanks again Willy. Will do. I have a glitch just now as my Lucas starter solenoid has packed up. A PO back in 1970 had installed an early MGB type push-button solenoid on the firewall under the bonnet. While useful for many reasons, it is probably unrepairable and I need to replace it or fit an original cable type, before proceeding. Merv
M Hyde

I can add that I believe that I have the carbs balance correct and also the mixture levels and jet to bridge heights right now

However, I think the source of the problem is that I am getting float levels that are just too high in the bowls. I have to lower the saddle/forks a lot (!) to get to the point where there is not a dribble out of the mixture jet at 0.07". I have replaced the floats (Carbs are AUC6020 (Z)/6031 and 6033. Bowls H3495 and H3496. So early MGA. Floats are the 1 7/8" brass ones (new). The needle and seats are new viton ones.

So I have either too much pressure from the pump (never a problem before) or there is some mismatch in the parts in the pump or the float bowl set up?
M Hyde

Ok -just to check
Are you setting the float fork to 7/16" with the arm 'just' sitting on the needle--not compressed
Is there a gasket(fibre/copper washer)between the needle/seat and the float chamber lid---is the seat tightened in the lid ---if there's a leak here the float level will rise
Unlikely to be excess fuel pressure if all as was but just to check, is the pump still mounted under the rear of the car or has it been moved
It does sound like a leaking needle and seat issue, maybe picked up some rubbish when you had the carbs off
How old is the flex hose ,it might be deteriorating inside and sending little bits of rubbish through
Just another thought are the float bowls still flexi mounted----if not then maybe there's only a single fibre/copper washer between the float and carb housings which would lift the float levels,,if so it might need packing down a bit there with a couple more washers-
You mentioned you have new floats---are they the same depth/weight as the oldies and same shape on the top (flat/curved)-maybe retry the originals
The fuel level should be below the top of the jet tube by a few mm and equal in both carbs
William Revit

Merv, just trying to think out of the box--
Are the vent tubes clear--
Also, and i've been caught with this--the sealing washer between the vent tube banjo fitting and the top of the float chamber cap---does it have the cutouts in it's inside diameter, i've had a kit with full circle washers for there that were a snug fit on the banjo bolt and restricted the venting out the tube----this causes the fuel to be pushed up the jet tube.
on the washers you could see they had been cut/pressed or whatever they do making them but the 3 little pieces were still intact in the washer------------
William Revit

Great thoughts thanks Willy.

I checked my carbs against the early MGA records and these are AUC 6020Z (6031 and 6033) carbs with bowls H 3495 and H3496 - all standard and matching it seems. There is no flex hose as they have the fixed bolt on tube from the bowl to the carb base.

I also checked with Midel on the floats as well and they are correct. To be safe I fitted the new floats (matched sizes, flat tops and weights) and needle and seat (viton) assemblies that I had. I did find that one of the earlier needles was sticking in the seat on occasion. Also, the vent tubes were not venting properly. The PO had fitted longer plastic tubes to brass the ones to exit under the car and these were crimped off. So your last suggestion is spot on there! I manually filled the floats to 3/4 full and ran the fuel pump with the jets at 0.07". No squirting, pump stops (the pump is 3.5 psi on my gauge), floats engage and the fuel reaches just below the jet orifice. I might be almost there with a little fine tuning!
M Hyde

Ahh very good-
On the flex hose bit I was meaning the supply hose between the car's body and the carburettors--
-Your carbs being H4's wouldn't have the jet tube hoses under the bowls but there should be a flexible 'rubber' mounting where the bowl bolts up to the bottom of the body--If this has been removed and replaced by fibre washers there needs to be extra washers between the float bowl and the carb body to match the thickness of the discarded 'rubber' thing to get the fuel level correct in the jet tube---In saying this if your fuel level is now settling below the level of the tops of the jet tubes you're not far off, it should be ok but worth investigating to see if your bowls are flexi or solid mounted--a 4 cylinder car should always have the flexible fittings-
willy

William Revit

Very interesting point Willy. Here is an image of the attachment of the 'arm' to the bowl on the base of the carbs. They are solid mounted. The photo shows that there is only a single fibre washer. Are you thinking that there should be a different spacer there or more washers to lower in the level of fuel? That could explain why I need to have a lot of bend in the float forks to get a proper fuel level?


M Hyde

It'll depend on how long your banjo bolts are, you should be able to get an extra washer or two in there though--
I must apologize though, I'd forgotten the early MGA carbs like yours were solid mounted, it wasn't until 1600's that they went floppy-----I 'think' the banjo bolt is longer for the floppy mounts
Here's a pic of the floppy setup

https://www.ebay.com.au/itm/391481511095

Solid mount should be ok i guess---even though i like the floppy setup as it takes all the engine vibration out of the float bowl---there must be plenty of T types running about solid mounted without me overthinking it
William Revit

I think you may have hit on a key aspect Willy. The early MGA's - seem to show as having those rubber grommets, but mine don't have them. I have ordered some of the rubber grommets anyway. They could be Alternative 1 or Alternative 2 in the attached image. I suspect mine are the ones with the grommets. In the meantime I will pull off the bae bolt and see how long its is.


Very best Merv


M Hyde

I have checked on the MGA 'sites and the 2 alternatives for the attachment of the bowl to the carb body shown in the schematics, both have the rubber grommets, just at different positions with a different banjo bolt. Either way they would lower the bowl I believe.
M Hyde

Most of the MGA guys I have spoken to suggest a solid mount. Perhaps a spacer might be better.
M Hyde

Merv
looking at your pic. the banjo bolt there is for solid mounting only---the bolt for the floppy mount has a step on it to stop it tightening up past a set point.
the only choice you'll have with that bolt is extra washers if the thread is long enough
have a read through this----it explains the changes
https://mgaguru.com/mgtech/carbs/cb111a.htm
William Revit

Thanks Willy. I did read that today and fitted some extra fibre washers between the banjo bolt and the carb body. It seems to help with the level, fuel pump stopped ticking and the fuel level at the jet seems good. A metal spacer may do a good job as well
M Hyde

A metal spacer would need a fibre washer either side of it as well to seal-
Just out of interest, on the mounting flange of the carburettors, where they bolt up to the manifold, on the side edge of the flange up near the top mounting bolt there should be a code stamped into the flange, a letter and a couple of numbers, if you could have a look for that it'd be interesting---someone previously might have removed it while recoing them but i'd be interested to know what you have stamped there-

willy
William Revit

That's the way I fitted the metal spacer/s (two brass washers) Willy. No leaks. On the mounting flange (there seems be) "6N3"is stamped on the front one. I can't read the other. There is the phenolic spacer after that to the manifold. I did notice that with the car on a flat floor that the Front carb is almost 1cm higher on a spirit level than the Rear. Could that produce a difference? I raised the rear as described, but not the front and now get equal fuel levels in both jet.
M Hyde

I'd be trying it out the road as you have it and see how your plug colour goes----If the jet fuel levels are matching your job is done
The carby code is a weird one but picking bits out of it it's 1956 so definately MGA 1500 era---wouldn't mind seeing a pic of that code if you can get a shot at it sometime

willy
William Revit

Ran into something similar. Try the bolts from the intake maifold to the cylinder head There was just enough to create vacuum leakage. Tightened up the bolts to the head and good to go.

Cheers
Gary
Gary Hansen

Willy, here are the stampings on the front carb at the mounting point. Hope this helps in identification.

Thanks also Gary. I am pretty sure those bolts are tight enough as I just refitted them with a new manifold gasket. I am fitting new heat shield soon and will recheck.


M Hyde

Bit weird--looks like the stamp has jumped or something
but i'm happy enough to stick my neck out and say-
---September 1956 so yeah, most likely 1500 MGA as you have mentioned

willy
these little Easter eggs are nice--yum yum
William Revit

I will let you know how I go when my new solenoid arrives. Great appreciation for your perseverance. Willy.
M Hyde

Still running rough Willy. Starts but not even near smooth. I am sure that the fuel levels at the jets are very close to correct and the fuel levels in the bowls much better. Front 2 plugs look fine with some tan even at idle. Rears are sooty and #4 even damp. So the rear carb seems still the problem? Enough for today. I did test the fuel pumps (newish electric ones) and they are showing 4.5 and even close to 5 psi. That seems too much?
M Hyde

When it blew the welsh plug out, did it get hot, might have done a headgasket-?------compression test maybe
William Revit

Looks like too much fuel pressure to me. 2.5 to 3 lbs. of pressure is plenty under normal driving conditions. PJ
PJ Jennings

Agreed Paul--asked earlier about pressure and it was 3 1/2 psi now it's 5
William Revit

Thanks PJ and Willy. I now have two pressure testers (one is new) and on the old one it was around 3.5psi. The new one shows 4.5psi. Which to believe? I have ordered a lower pressure fuel pump to try as well. These existing pumps (I have 2 that are switchable) have done many miles all over our East Coast and I don't suspect them really.

When the expansion/welsch plug blew it did get hot for a short time. I tested for head gasket damage, tested compression and all readings were strong thank goodness. Between 155 and 165 and no subsequent coolant or oil fouling. The car did run well for a time and no signs of head gasket issues. The only issue has been getting the carbs right. For some reason I am stuck on these, even after many MGA/B carb tunings over the last 30 years!
M Hyde

It is a bit weird i must say.
I'm running out of ideas a bit but interested to know what's doing this
A bit hard not being hands on, easy to suggest stuff from miles away but really at this stage it does have all the signs of too much fuel pressure, but why, when nothings been changed------would it be worth swapping the needle and seats over front-rear and see if the issue follows---?
William Revit

My lower pressure fuel pump should arrive today Willy. I will test it out before fitting and get back.
M Hyde

Willy, the newest pump to arrive is 4psi, despite claiming to be 2.5 to 4.0. I suppose this is normal. I retested the existing fuel pumps again and I am getting 3.5psi now with the glass filter attached. So I think the pumps are out of the equation. I have ordered a bunch of new parts from SU MIDEL in Sydney today after talking with the tech there. Floats, saddles/forks, spindles, needle and seats, and gasket kits. I will get new needles as soon as I can identify mine. New solenoid also got fitted.
M Hyde

Just to clarify--The fuel pump is mounted down the back on the chassis---it hasn't been moved up front by chance
The low pressure pumps that mount in the engine bay like TC's and MGY are rated at 1 1/2 lb and if they get a HP pump that is normally down the back you get 3lb-ish
What I'm getting at is that if you have your 3+ lb pump up under the bonnet---it'll be too much and flood

willy
William Revit

Yes Willy, the pumps are mounted under the car, in parallel, in a protected area down the back with filters just after the pump in each case. I have a switch under the dash, to swap between the R and L pumps, if ever needed.

It seems more difficult to obtain genuinely low pressure fuel pumps. I have two original SU pumps as spares but they need rebuilding and voltage reversal as the car is -ve Earth.
M Hyde

All good -- just trying to eliminate possible causes for your little problem------thinking - thinking
William Revit

Me too Willy. Your forebearance and persistence is appreciated. It will be a couple of days as I finish up a welch plug issue.
M Hyde

I have a TD with front mounted fuel pump. When the main part of the housing, ( field coils) stopped the pump from working I installed a used MGB field coil assembly, onto the TD base and top of the original pump, reversed the polairity, installed a Bi polar TVS diode. works fine with the higher fuel pressure!
Len
Len Fanelli

What pressure does that combo give Len-
William Revit

William 4.5 PSI on a new Mac tools fuel pressure gauge / USG gauge +- 3% accuracy> 5 psi.
Len
Len Fanelli

BTW Fuel pump modification was done July 2008
Len
Len Fanelli

From my experience with pump changes, pressure differences of the amounts discussed had no detrimental effect in my car.

Also, may have missed it along the way, but do both pistons rise together by the same amount when you slowly lift the throttle?
David
David Padgett

Thanks for that Len.
This issue of Merv's is really weird all up, I'm starting to think there must be some sort of wear/groove in the top of the float fork that's tricking the needle/seat---I'd love to see it all in the flesh, but in saying that, Merv has been around SU's for quite a while---
It's strange that it still runs rich with the fuel levels down the jet tubes now equal-------it's weird and will turn out to be something simple
Cheers
willy
William Revit

Merv--it's really strange that with the fuel levels equalled up that one carb is running richer than the other---it can't be an air leak as that would show lean- could it be that the aircleaner gasket on that rear carb might be upside down or the wrong type with no holes ,blocking the damper vent holes between the carb and airfilter---it's something that normally gets checked during assembly as you would know but may have been accidentally overlooked
William Revit

I was initially thinking that fuel pump was over powering the needle and seat. But that seems not be the case, if just less than 4 psi is OK?.

I have just fitted new float control forks to both bowls and set then at more than 7/16" to get a decent height of fuel in the main jet tube. I also got new floats (the Viton types). I also replaced the gaskets to the air cleaners )they were not good) and the ones to the manifold. Also new fibre washers for the bowl bases. I can see on the documentary history of the car that there has been a problem in the past of the rear carb being too rich and that it was not responding to mixture adjustment. SU MiDEL rebuilt them in Sydney reportedly. I have also ordered a couple of new leaner needles (GL), to try at some stage. Both are currently GJ's.
M Hyde

OK --It's sounding more and more like the top jet tube seal is leaking, it's about the only place it can suck more fuel from---rare as the proverbial rocking horse poo but if it's had this problem before it might have a damaged seal- OR the little tapered washer that holds the seal in against the tube 'might' have been fitted upside down with the flat side against the cork which would prevent the washer from doing it's job of holding the cork in against the jet tube----Then while you've had the carbs off the cork probably dried out leaving you with a renewed leak--maybe

I had a look at the Midel pic of a H carb and it doesn't even mention the upper gland seal just a bottom one------there are 2, upper and lower

Found this wobbly old u tube, the upper washer and seal are shown correctly at 2min/30

https://www.youtube.com/watch?v=QbZUk2Dfics
William Revit

That is an excellent video. One of the best I have seen. I already have new rebuild kit for the Rear carb main jet Willy and will get on to it tomorrow. The video really helps. I also have a few new needles coming early next week. This really helps.
M Hyde

It'll be interesting what you find in there--
couple of points---the cork seals need a good soak in light engine oil for at least an hour to get a bit of oil into them--even if they're the 'O' ring type they still need lubing up-
Also, in that video he left the spring off the mixture adjuster when he assembled it, he was going to put it on but it looked like nerves took over when he was starting the nut and he removed the spring----you need the spring on from the start and screw the nut right up or just about-----the reason being, if you haven't got that bottom spring on there you have to pull the jet tube out again to fit it and then you risk the chance of damaging the new seals poking it back in-----As you can see in the vid. the new seals are a light stretch on fit when new and flexible and with the tapered washer and spring in place the seal is pushed inwards so removing the jet tube and poking him back in without damaging the new cork is---playing with danger, easy to nick a piece out of the cork-
i'm looking forward to seeing what you find in there
William Revit

Well Willy I have found that one of the brass cup washers was missing and a flat copper one missing also. The bottom cork one was very flat! Soaked all the new cork gaskets overnight in oil.

I realised too late that the bottom spring HAD to be on, but carefully removed the jet again and reinstalled with some oiling. The problem is that when the bottom mixture adjustment nut is wound right up, the jet does not quite reach the level of the jet holding tube at the bridge? Is that because of the thicker cork gasket/washer on the base? I am reluctant to over tighten the 21mm nut on that.
M Hyde

Bit of a bugger with the spring , but moving right along--
Having the odd H4 or two I plucked a random one and took some measurements-
With the 21mm nut done up tight as it should be and the mixture adjusting nut right up the jet tube is 'just' short of the top of the top brass guide which in turn is below the deck height of the bridge---measuring from the bridge it's .025" to the top of the jet tube-
Now the 21mm nut---It has to be tight, not overtight but tight enough to compress the cork enough to allow everything inside to bottom out--when it's done up this outer nut assy pinches the upper and lower jet guides and their copper washers together and holds them in position after centering the jet to the needle, If it's not tight/bottomed out the jet holder will be able to move out of place and get out of true with the needle----The cork gasket should compress inside that stepped holder/washer----I just measured between the top of the stepped washer and the cab. body with it all tightened up and this one measures .035" there
The 21mm nut should feel like it's bottomed out-solid
William Revit

Thanks for doing this Willy. As you advised, I tightened the 21mm nut to the point it felt bottomed out. Now the inner jet tube does indeed measure to 0.025" to the bridge. Very close in measurement to fully wound up front carb also.

I measured between the top of the stepped washer and the carb. body with it tightened up and it measures .030" there. Looks on track then!

Centering seems OK as well.

What depth do you recommend from the bridge should I set the mixtures for initial start up?

I am waiting on a few more bits and pieces tomorrow or Tuesday, including new GJ needles (with some leaner GLs as back up).
M Hyde

Ahh, good stuff
Don't forget to recheck the centering with your new needles---just in case.
As far as jet depth goes, SU recommend 2 turns down from flush at the bridge but that's impossible as we've found with the jet not getting up to flush, The nut is 26tpi so 2 turns give .077" but that's usually too rich anyway
I normally set them to .065" and even then they usually need leaning a flat or so which would be .060"
So basically as long as they're both around there and the same and when you adjust them do both exactly the same all should be good
William Revit

'Thanks Willy. A few of the racers here in the US did find that the BEST jet height for fuel atomization IS .0625 +- .0025" below the bridge PEROID! (And never touching the mixture adjustment nut again!) If needed find another needle OR if too rich at idle drop the needle .015" or so.
Fuel level in the jet tube should be between .1875"-.245" below the bridge. The easiest way is to drop the fuel level too far, then raise it to a nominal .220" below the bridge. I found, in my case, that the fork set at 15/32 or 12 MM will = .220".
Part throttle response, at any RPM, is MUCH better than above ,065" or below .060"! By the way I set my jets @ .06250"! Also for damper oil use 3 in 1 oil OR 15-50 Mobil 1 synthetic motor oil.
Len Fanelli

Really helpful gentlemen. Currently my float forks are set very close to the point Len describes. I should have some new needles very soon. I ordered both standard and leaner as above. I will fit the carbs in the meantime and have a look at fuel levels in the adjusted jet tubes today.

Hopefully this will help others as well. It has certainly helped me.
M Hyde

While waiting for the new needles, I reassembled the carbs and checked the piston fall again. The Rear one was not closing completely, with the piston sticking somewhat at about 0.5cm above the bridge. Subsequent checking showed a slight bend and some wear in this needle, possibly also contributing to the rich mixture outcome experienced.
M Hyde

Merv,
You've got to be getting close to a solution now.

With the piston sitting up 0.5cm at idle, the mixture for that carb has to be rich compared with the other. Recall the mixture adjustment 'tweak' of raising the piston a fraction and noting if the engine speed drops off (lean), stays the the same (ideal), or speeds up (rich).

This also would account for the mixture imbalance, ie.nut settings, you've experienced between the carbs. Matching the idle adjustment for each carb would also be tricky.

Keep going!

David


David Padgett

Thanks David. Just waiting on new jet needles now. This rear carb was a mix of several problems it seems. Amazing that it worked at all.
M Hyde

Merv-- don't forget that you can centre the jet holder up to line up with the needle--best to wait till you get your new needles, If your old needle is badly worn it's probably been out of centre for ages--and the jet itself might be worn as well--?
My method of centering up is to screw the mixture adjusting nut right up to as far as it'll go, loosen the 21mm nut probably half a turn then get a philips head screwdriver down the damper hole and hold the piston down firmly, give the 21mm nut a little tap with something to help centre the jet up then tighten the 21mm nut---try the piston for a good clunk return , if no good go again until you've got a good free return clunk--then when you drop the jet down for the correct idle mixture you'll know it's all nice and free
William Revit

True Willy. I was just testing with the old needle and didn't keep trying as I don't want to score the new jet body I had installed. I like your approach on centering. I used one of the inserts ("Tools") that come in the kits from various suppliers. I seem to spend most time waiting for parts!
M Hyde

This is what happens when you don’t put enough torque on the jet needle lock screw. Don’t be as timid when tightening it as I was!

Kirk


Kirk Trigg

Wow! It dropped down into a cylinder.
M Hyde

Kirk, that image looks scarily familiar. This image shows the threaded rod that held the air cleaner cover on my TD. It got sucked in and made one helluva noise. It left the same mark on the top of the piston as in your picture. OK, back to M. Hyde's issues. Jud


J K Chapin

My needles arrived but I got 2 GL (weak needles ordered as a back up) and GS needles when I was sure I ordered GJ ones. More waiting ...
M Hyde

Jud,

The learning curve on these cars seems fairly steep and sometimes expensive.
Kirk Trigg

Jud,

The learning curve on these cars can be really steep and sometimes expensive.
Kirk Trigg

I am finally getting back to the carbs. Fitted the new mixture needles and associated tubes and new washers, float forks, etc. Set upon floats and then checked the fuel level from the bridge in the tubes with by lowering the inner jets around 3/8" with the choke. Looks good and I can see the fuel levels there and there is no flooding when In reset them at 0.065" from the bridge The needles are well centred and drop nice and smoothly. I have not yet added oil in the dampers. I was thinking to use a lighter oil than the SU Damper Oil?

However, I am having an annoying problem with the rear dashpot and its original piston. I get a nice clunk and progressive drop on the front with the dashpot screwed down, but the rear hangs up about 3 mm from the bridge unless I loosen the dashpot side bolts a little and wiggle it. Springs are in. I am sure the needle centering is not the issue. I can't work out why?
M Hyde

It's needle/jet centering, you need to have the dashpot screws tightened in normal running condition when you're centering the jet up
To prove it you can take the needle out, reassemble, and then test without a needle if it drops ok like that it's definitely jet centering causing it-
With the damper oil, SU damper oil is just that--SU damper oil but having said that I tend to use atf only because i use it in strombergs as well with the 'rubber' diaphrams so use it in su's as well just because----atf is rated as roughly 5w10 so yeah just a bit lighter
Don't go light if you don't have a specific reason to do so though as the dampening effect is working like the accelerator pump of a normal carb, so too light can cause flat spots/dead acceleration
I'd think normal SU oil or light engine oil would be fine in your warm climate up there

SU damper oil is a straight 20 grade oil

Don't forget you need the jet screwed up as high as it'll go when you're centering the jet up just so you'll know it'll have clearance when you end up with jet at the correct height.
William Revit

Right as usual Willy. I had centred the Rear carb with only the piston inserted (and held down) and had not screwed down the dashpot ... I did that and the piston drops nicely to the base, without and with damper oil.

I still have a little appearance of fuel in the rear jet at 0.065" even with the float set well down. Front is fine. I will need to investigate that. Bruce Smith is kindly sending me a whole bunch of new float forks to choose among. Mine are not matched and are tricky to manipulate.
M Hyde

Merv, I'm having trouble with terminology here. I assume when you said you loosened the dashpot side bolts a little and wiggled it, you meant the suction chamber.

So, since the piston drops properly without the (dashpot) damper, but not when the damper is tightened down have you checked that the damper rod is not bent and consequently limiting the piston from dropping properly?

Sorry if I'm confusing you.
David Padgett

Further to my last, you could try swapping the dashpot damper between front and rear carbs and see if there is a change. It would at least tell you if the damper was influencing the drop of the piston.
David Padgett

Thanks David. I did swap them over at one stage also. I did the front carb needle centering with the dashpot tightened down but for some reason, maybe tiredness, I only pushed down on the piston when centering the Rear. Mistake as Willy observed. I should have fitted the dashpot. All good now - nice 'thunks' on falling on both with damper oil added also. You are right and I was also suspicious of the damper rod at one stage. But it seems fine.

I also seem to have addressed the difficulty with the fuel level in the float bowl and jet tube on one carb. The float bowl lids were mismatched some time in the past. I picked up a good used 1161 (front) top, that should be better with the new float levers I now have.
M Hyde

Merv
There's been a fair bit going on with these carbies all up---makes you wonder a bit about their history
Hopefully all will turn out as you deserve for sticking with it---
willy
William Revit

Merv
Because of the unknown origin/history of your carburetors, I've been trying to think of anything else that could possibly be out of plumb with them--
Something that might need looking at/checking is the actual dashpot covers them selves and the screw in damper pistons
I've put up a couple of pics of the covers but in fact there are 3 and two types of damper pistons

Pic #1 is the external vent type of cover and has to have a screw in damper piston assy with an external vent hole in the top cap

Pic #2 is the internally vented type and has a damper piston with no vent hole

There's a trap--
There's another version of the cover that has the diagonal casting on the outside of it and looks like #2 but isn't drilled through inside
This type has to have the vent hole in the damper piston cap

Might pay to check, just in case--

willy






William Revit

Thanks for pursuing this Willy. It's been a challenge, so far. They are early 1500 MGA H4/SU's. However a mixed bag of parts parts has been thrown at these carbs for some time.

The 'pots look like the second one with the small bracket (as in the photo; there may be one in the reverse of the first photo)) near the spout of the pot - but with no vent in that bracket. There is a vent in the damper screw cap in both pots. The damper screw caps are black ones. I am not sure if this is correct? The screws in the base of the dash pot seem to fit well.

The Rear float bowl cover has raised part # 1160 and the Front cover has no marking. It should be 1161 I believe. This one has a higher 'tower' internally for insertion of the spindle and float lever and needed a second, lower hole to be drilled to be at the same level as the 1160. I have a 1161 on the way to me.

I will be pleased if it starts and runs, as it did before.
M Hyde

I would have thought that MGA carbs would be the later "Dustfree" version with the buttress drilled for a vent.
The TD Mark II's H4's have a buttress but are not drilled and uses a vented damper cap
The TF uses vented bells and a non vented cap.
I had fun with a mates Mark II until I realised it.
Ray TF 2884
Ray Lee

Interesting Ray. I did find this link which describes the variants. It is somewhat confusing, however.

https://mgaguru.com/mgtech/carbs/cb127.htm

My carbs have been on this car for decades and have worked well over many, many miles. Clearly they have had various parts replaced, not always original I suspect they are a mix of MGA and TF. For example getting the best float bowl cap levers is a challenge right now as the the fuel bowl covers on mine are different. The dashpots are not vented but the damper caps are. They are stamped as "Z".
M Hyde

Too slow on the update ..

The bowls are both 3495 and 3496. The stamping on the carb body where it attaches to the manifold are hard to read but 'appear' to be "N" and a single number (1956?). There are no lifting pins or 'tickers'. Steel screws for securing the dashpots to the body and brass retaining nuts on the float bowl covers. They have the single banjo bolt attaching the float bowls to the carb, but with the (often leaky?) rubber spacers deleted and replaced by a fibre washer.

It would be good to see some float levers for this 1955-6 set up.
M Hyde

Merv
jumped into the cupboard and dug out a 1161 lid, it has a tickler but anyway here are some pics----if it helps





William Revit

In my way of thinking, what i usually do is pack the valve assy with suitable size fibre/copper washers until the lever is sitting horizontal when the valve is closed then adjust the outer part of the arm to get the correct 11mm float level under the fingers with a drill bit taking care to get both fingers at the same height





William Revit

Willy, the 1161 lid I found - was one of 3, two had ticklers and one didn't (matches my 1160). At least the 'tower' under the lid for the spindle and fork will be correct and match the 1160. I remember back in the day, adding thin fibre washers under needle and seat assemblies but many decry that process these days for some reason. I have no issues with it.

I should have a big box of new float forks coming from Bruce tomorrow, from which I can select two correct, matching ones, and send the rest back. The one I got from MIDEL was so wrong. I believe they should be AUC1980.
M Hyde

Yeah-not sure about the part nos of the levers, can't say i've looked for numbers on them--I have floated some silver solder into them in the past to correct worn surfaces where the needle had worn a groove/hole
William Revit

I am getting there I believe. Just waiting weeks and even over a month for some new parts (e.g., an NOS 1161 float bowl cover as mine was unstamped and different).

Getting the right float levers is a challenge as my current one that sits best is 35mm total length (image 1) It sits nice and flat and the needle shut off operates well. It may be too long however and not centre on the float itself, causing tilt. I will need to check that.

The other float lever is an ally one and a little shorter, but set at the same level (around 11mm/7/16"). I should have more in a day or so, hopefully.

To check, I looked at the fuel level in the main jet (with choke down 3/8"/9.5mm) and it is just visible on that mark. With the choke up, I have 0.065" from the jet to the bridge on both carbs.
M Hyde

Fuel level below the bridge should b a nominal.220”
Recommend range is .1875” to .245”
Len Fanelli

Thanks Len. I maybe a little too low then. I can reset the float levers to get around 0.2" (6mm).
M Hyde

About 15/32” should do it.
Again set the jets to .0625” +- .0025”
and never touch the mixture nut again.
Change the needles and or drop the needles as needed
Safety Faster
Len Fanelli

Len, I like the idea of 'never touching them again"! I am at about 15/32" on the lever clearance now. I have the jets at 0.65".

However the float levers are a variable. It is very hard to know WHICH float levers to get. Even SU MIDEL can't tell me here. Mine are both around 35mm (1 3/8") total length.

Merv
M Hyde

Merv,

There are only two types of lever, early and late, both 1160 and 1161 use the T2 early type. If you go on the manufactures website there is a host of information. www.sucarb.co.uk Maybe you have a fork from a T1 or T3 carb.


B W Wood

Great and thanks, one of mine is your first image. My second is the second image and the third is the one that SU MIDEL sent me. It was wrong I felt.

Merv
M Hyde

Merv
just measured up some levers and all the levers I have for this size float bowl are between 33-35mm long depending on how they've been bent, but all of them have the highest point of the curl lining up at half way accross the bowl, level with the centre bolt hole as you would expect--Measured some from the smaller float bowl and they were about 30mm ish
If you have the 1160-1161 lids which you have, your bowls are size 2 (2 3/16 o.d.)so you need the 33-35mm long ones
And--all due respect to Len, SU designed these carbs to have the step in the needle dead level with the bottom of the piston----If you have them level and leave them level you can swap needles easily without mucking up the idle mixture
And--them later tin float levers will swap in ok but the early type fit heaps nicer--even with new pins the tin ones wobble around a fair bit.
It really pi***s me off when someone like Mid** sets themselves up as the experts but doesn't have a full handle on their product and you have to spoon feed them to get the right bits out of them
end of rant
willy
William Revit

A worthy 'rant' Willy and good advice, confirming what I have suspected. Thank you for doing this.

The circa 35mm lever is correct and the one that Midel sent (the 3rd in the image BW Wood provides above) is not helpful and wobbled excessively. I managed to find 2 more T2 AUC1980 levers and should have them next week. I should also have the NOS 1161 cover/lid then, to replace than the Jag (2284) one that was on the car, with its higher pillar and altered lever action.
M Hyde

The correct 1161 fuel bowl cover arrived. It will be much better than the Jaguar one! $30 well spent.


M Hyde

I am pleased to report that the carbs are set according to the directions given by Willy (and Len) for fuel level at the float bowl and also at the bridge. It started straight up and ran smoothly with no hesitation.

Many many thanks for all this learning journey and the perseverance shown.

Unfortunately I now have a leaking welch/core plug under the exhaust manifold. This was replaced not long ago with a good copper one and I thought installed correctly. I will post separately as now I have to pull the carbs and manifolds off again!

At least the carbs seem spot on.
M Hyde

William Revit, Tasmania
Trying to think out of the box
Aluminium pistons---are they both the same--weight
Early pre-spring pistons were heavier and had a steel ring insert around in the outer edge---it'd be really strange to have an odd pair but it's been done in the past by unsuspecting suspects
might just pay to check when you're fitting your new springs

Posted 11 May 2023 at 02:13:11 BST Up arrow Down arrow
M Hyde, Queensland, Australia
They are matching numbered ones Willy. And have the same measurements. I also fitted a fuel pressure regulator set to max at 3 psi just in case. Those new springs should come today but I am going to Cape York tomorrow for some fishing.
M Hyde

Back from the wilderness of Cape York (or 'Pakinka' in the local clan language).

Fitted the TF blue dashpot springs after testing them against the existing black (?) ones. They certainly have less compression resistance than the black ones previously in there. Carb pistons are the same weight. The car starts easily and idles very smoothly. On a drive, it pulls well with no hesitation or reluctance at full throttle and higher rpm.

The two rear plugs are still a little sooty but I will check again after a sustained drive at decent rpm. It may be that the choke linkage bar was not properly adjusted. I will try Tony S's AFR gauge next week when I can get a threaded bung attached to a suitable stainless pipe. That is an omnibus measurement however.
M Hyde

Cape York--Can't get much further up than that
I've never made it that far but have a mate that hangs out up at Mapoon for a few months each year. He loves it there----and the fishing, How'd you go , catch anything
How far do you have to travel to get up there from your place-
I've been told that following the old telegraph line up on a trail bike is a magic trip--"IF" I ever get there that's what I'd be doing I think
William Revit

Beautiful place Willy - around 3000kms from here - by vehicle, plane, helicopter and boats. Followed the old telegraph line via Moreton Station. Lots of fish and crocks around 16' long. No swimming. Barra and other fish everywhere and Cassowaries that were 2 metres tall! Great trip to do and the Torres Strait islanders are lovely people. Saw this bike at one of the Telegraph Stations (note the P plate and the map taped to the fuel tank.)

M Hyde

Thanks for your ongoing and valuable help Willy. Image from above here.


M Hyde

That's the way to travel--i think I'd have my pack up front on the tank though---where you can see it. and a bit wider handlebars, he/she would have been fighting that with them little bars.
2Mtr cassowarries---I knew they grew big but crikey they'd be dangerous that size- angry birds with a mighty kick i believe, I've heard that you don't look them in the eye or they have a go at you, -is that right.

we'll probably get growled at for not much MG content but oh well.
cheers
willy
William Revit

True but ...

The first car to reach Cape York was a Baby Austin.

In 1928 Hector Macquarrie and Dick Mathews drove a tiny Austin 7 on a heroic journey through the bush and over numerous tropical rivers to be the first vehicle to reach Cape York.

https://www.capeyorkaustralia.com/austin-7-to-cape-york.html
M Hyde

Willy - if I may one last question. I was looking at my Damper caps today. They have plastic caps with a vent hole (brass would look nicer). However, I can also see a vent hole in the tube of the dashpot, through the web, and into the inside top of the dash pot. I thought that there should be one vent and not two? That is, the damper rod caps should be unvented/drilled?

Many thanks Merv
M Hyde

Hi Merv
Basically NO you shouldn't have both--that combo 'can' sometimes cause a tiny bit of a rich mixture by acting as an air bleed for the damper piston causing it to be slow responding---a bit like having thick damper oil.
The worst combo is not having holes in either the housing or cap, this causes the piston to hardly move at all and if it does get up it's very slow to drop-
So - one or the other really, --If you're 100% sure yours are drilled right through-(get them off for a good check or poke a piece of wire through the hole to check)- then you can fit the solid style caps---As you say brass looks the business-----If you're going off looking for brass ones, check the length suits your carbs-too long and you won't get full travel, and also there's two types--A hex shape or round with a knurled edge---Your TF should have the hex type.
Cheers
willy
William Revit

An often overlooked detail on SU's is the height of the slide at rest above the bridge, called the 'air gap'
There is a small 'plastic' bumper in the bottom of the slide that prevents contact of the slide and bridge.
On many of the SU's I have rebuilt this 'bumper' was hammered flat and one carburetor slide would sit higher than the other at rest. The slides should sit slightly and equally above the bridge at rest.
(There are different opinions on the fixed height of the air gap, somewhere between .005" and .020" seems to work)
FWIW!
J Stone

Thanks Willy. I will check they are drilled right through and plug the holes in my existing dampers, until I can find/buy some unvented brass ones. These seem right: https://www.sumidel.com/shop/item/auc8102hb-mgtf-hex-brass-damper-mgtf-h4

Not sure how I missed this. Concentrating on other aspects of the mixture of parts in those carbs. Running well by the way!

JS-I am not exactly sure what you mean by 'slides' but I assume it is the attached piece that aligns in the grove of the piston vertically- in particular the base.

I will check that too.

Many thanks!

M Hyde

I mean the piston...
apologies
J Stone

Edit to add: The 'bumper' is in the bottom of the piston and holds it up off the bridge
J Stone

I fitted new (same length) brass topped, unvented dampers. Car runs well through the range and seems less rich at idle.
M Hyde

Merv.--Impressed that you stuck with it--some don't and never get the results-
In congratulating you I'm sending a picture of my favorite little mate -----------lol
Just on another issue, A friend here has just replaced the air filters on his mga t/c equipped with the Vokes filters and it ran as rich as---Turned that the new filter inserts were saturated with some sort of waxy/oil stuff and had to be washed out before it would run properly--something for you to keep an eye out for when you get your new ones

willy



William Revit

It was a long journey Willy. Your experience was the key element is my development of a more detailed understanding of these particular carburettors. As someone who has been ‘tuning’ SUs on MGs and Zenith and Solex on old Porsches over many years, I found these SUs a real challenge. Your willingness to stay the course was pivotal. With great appreciation Merv.
M Hyde

This thread was discussed between 03/04/2023 and 18/07/2023

MG TD TF 1500 index

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