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MG MGA - High Idling RPM

Something that has bugged me for years is that when I am on a long run and usually on a hot day the idling RPM creeps up to about 1500 from its normal 800. When stationary I have tried putting my foot under the pedal and pulling back to no available. But what does work every time is a quick blip of the throttle and back she comes.

I have investigated to my wits end including increasing the return spring pressure on the throttle spindle; spraying the spindle for wear (idle variation) - none; had the pots off many times during routine work and checked all there is to check; both pistons always close with a resounding clunk. Other things that have made absolutely no difference are new jets and needles and a new throttle cable.

On another thread a noticed reference to a different suction chamber spring. I am wondering if a stronger spring may make a difference just in case one of the pistons has a bit of stiction occasionally when thoroughly heat soaked? I had a look through the Moss catalogue and they list the red spring, perhaps suggesting there is an alternative. However, I did note their diagram showed a washer below the springs (used with original tapered springs only). Mine are that type but I don't believe I have the washers in situ. Would that make a difference?

Any thoughts on what I have missed most welcome. This is more of an annoyance than a major issue as the car otherwise performs consistently well and never skips a beat.

Steve

PS. In the beautifully warm sunshine yesterday of 24 in the shade (hot for the UK this time of the year) she sat at 180 all day, both free running and queuing. Never budged a needle width. I swear by that asymmetric fan. So much puff comes out of it.
Steve Gyles

Steve,
Your symptom suggests that there is some kind of ratchet event going on which would seem to be confirmed by the fact that it clears if you blip the throttle. Before you dive into the black magic and cost of randomly replacing things on the carb go back to basics. engine speed is governed by the butterfly valves so check that they are fully closing first by unlinking the carbs from each other, back off the adjusting screw and give them a 'snap'. You should hear the butterfly valve click against carb body. You may need to disconnect the main return spring and do it with your fingers so you have maximum opening of the throttles.

Hope this helps
Alan
AR Terry

Alan

Many thanks. They did operate correctly last time I checked the basics but of course when you balance the carbs with the interconnect disconnected you set up a slight variation.

On the springs point, the photo shows one of the uncompressed springs. It compresses to 1.5" which suggests it is a red although no colour showing on the coil. There is no sign of the washer so the question is should there be one and what is its relevance?

Steve




Steve Gyles

I have looked at the Burlens site: http://sucarb.co.uk/piston-springs.html and cannot tie my spring length (free and compressed) to the one recommended for the car. I will clarify with them when they are open next week unless there is an expert out there who can educate me.

Steve
Steve Gyles

Hi Steve

I don't know what distributor you run or its age but I would look at the auto advance mechanism. This is known to stick giving a slight advance causing high tickover, by blipping the throttle it gives the mechanism a bit more energy to return to norm. The effect can be temperature sensitive as the mechanism expands and sticks, broken springs can also be a problem.
Hope this helps.

Barry
B Bridgens

Hi Steve, I had the same problem a few years ago and it was due to a strand of wire breaking on the inner throttle cable at the pedal end, this was catching on the outer cable shield. So maybe check both ends for any breaks.
I Hazeldine

Barry. I have changed distributors for other reasons over the years; also the flame tube; all to no effect on this random issue.

IH. Changed the throttle cable.

Steve
Steve Gyles

This may not help Steve but when I ran a 3-brg 1850cc enine with H4s and a pertronix electronic distributor, it had exactly the same problem.
The tick-over would gradually become faster, particularly on hotter days and blipping the throttle would usually slow it down a little.

I now run a 5-brg 1950cc engine which has another type of electronic distributor with a twin-choke WEBER DCOE carb and the tickover still tends to speed up in the same conditions.
(Both distributors had no vacuum advance fitted)

So it must just be a B-Series thing, I will think about this one a bit more Steve.

Colyn

Colyn Firth

I too have experienced the same symptoms lately as the weather has warmed, with the idle staying high after a run, but it takes a little longer sometimes to come down to normal even after a blip of the throttle. Currently running with the original rebuilt distributor with a new vacuum advance. I am suspecting the mechanical advance sticking and/or the vacuum advance delaying release due to a slight blockage in the tube from the carb or from the slight kink in the tube at the rear clip on the head under the cylinder head nut. It twisted a bit when torqued because I could not get it to stay put so might be slow to release vacuum when hot? Would be curious if you fix your problem to see what works. Also if anyone has a good solution to keeping the clip straight when torquing the rear cylinder head nut I would like to know. I put the clip between washers under the nut with anti seize but still no luck.

Matt
M Grover

One or two things. 1. Butterfly spindles not returning to the stops. When it is idling at it's higher rate, switch off and see if the spindles are off the stops. .2. Mechanical advance sticking. This could be the two part spindle not moving freely due to rust or just lack of lubrication.. Grab hold of the rotor arm and wind the centrifugal springs up and see if the mechanism springs back fully. Springs could also be wrong, weak, broken or unhooked. 3. Piston(s) are not returning to the bridge when things warm up a bit.. Clean out the suction chambers, piston and damper bore with clean petrol, (no abrasives!!)and pay great attention to cleaning the seating area of the suction chamber, then check their un-damped return to the bridge. On the assumption that the pistons might have been mismatched with the chambers in the past, swap them over and see what happens. Check that the damper piston is assembled the right way round. Check the jet is properly centred. Don't change the piston springs for anything other than Red type A. Check the ones you've got with a set of electronic kitchen scales. Your springs when compressed to 2.635" should read 4.5 oz. (Fold a piece of paper, cut to the right length, to form a "V", stand it in the spring and compress the spring level with the top of the paper).
Allan Reeling

Wear on the throttle shaft, where it runs in the throttle body, can allow the throttle shaft to run off center. This may have the throttle plate touching the carburetor throat so it will stick just before it gets back to rest position. This gives inconsistent idle speed. A blip on the throttle will (sometimes) clear the condition.

Without rebuilding the carburetors, the quicker cure is to loosen the screws holding the throttle plate in the throttle shaft. (Remove the carburetors for access. Back off the idle screw until the the throttle plate goes completely closed. Jiggle the parts a bit until the throttle plate will self-center, touching all around with no light getting past the plate. Tighten the screws to hold the plate in that position, and reset the idle screw.

Reinstall the carbs and give it the standard adjustment for synchronizing air flow and setting idle speed. Fine tune the fuel mixture if needed.
barneymg

Allan

All that and much more I have done so many times. Over the years I have been unable to spot anything untoward. Measured the spring. Spot on.

Will have a look at the points Barney makes.

Steve
Steve Gyles

Of course another way of looking at the problem is to assume something is wrong at idle, which improves and increases the idle speed!!! i.e., Vacuum unit starts doing it's job, mixture ratio improves, plugs clear!
Allan Reeling

Yes, other than that annoying idle everything is tickerty boo. never had the mixture running so good since I replaced the jets and needles 6 months ago. the carbs were a 'new' reconditioned set when I rebuilt the car 20 years ago. i had always struggled to lean her out. Probably that is why I have had the carbs out so much for fine fettling over the years, cleaning this and that; checking the movement of that; smoothness of that etc. Turned out I don't believe the supplier changed the jet assembly during the refurb. Once I did that and instantly tuned everything out to perfection I thought the world was wonderful........then this yesterday, an age old recurring issue.

Steve
Steve Gyles

Stev,
Looking from the standpoint of "something goes wrong". A doubling of idle speed can only be down to; increased mixture for whatever reason, or ignition advancing,
From "something improves to increase idle"; Spark improves, timing improves, mixture improves, valve seating improves i.e., compression improves.
Odd timing can be caused by; the base plate sticking and the vacuum unit can't pull it round. Sometimes the earth lead can jam it. A slack timing chain will cause all manner of timing fluctuations until the engine speed climbs; If you put a strobe on the timing mark is it stable at idle and does it advance when you connect the vacuum pipe?
Are there any variations if you successively remove plug leads?
I suppose a sticking valve might give your symptoms too.
Allan Reeling

Allan

That was all new when I rebuilt an 1800V in 2000. Had the same issue in the old 1500. Tried 3 different refurbished and new distributors; old type contact breakers; 2 different electronic ignitions; 2 different vacuum tubes; 2 different coils You name it and I've tried the lot over the years. Ignition always timed on my dynamic timer; vacuum advance then ok when attached. Always very stable on the light be it at 3500 RPM or idle. As I've said, the performance is excellent. It will see the pants off most stock MGAs on acceleration and top speed. Had it over the ton and that was before the sports screen. just the annoying idle creep from time to time.

Steve
Steve Gyles

I had this problem with my MG TD and finally it was a slight slack in the carburetor shaft that only manifested at high temperature.
Gabriel Martínez

I'm going to pull the carbs (again) for a closer inspection, taking on board the comments to date. I have just been inspecting them again for the umpteenth time. Shafts look okay but I will inspect more closely on the bench. As an aside and likely a red herring to this issue, the choke operating brass rods look a bit worn in places. For instance I noted that when I pulled the choke fully out there was so much play in the choke cam operating rod lower hole that the cam itself barely rotated a millimetre. The rod just seems to move laterally but not up and down to rotate the cam.

Is there a good diagram somewhere of the assembly all connected correctly that I can use as a reference? I have the exploded diagrams in the likes of the Moss catalogue and workshop manual.

Steve
Steve Gyles

Yeah, it's nuts that there is no mention of choke adjustment in he Workshop Manual. See here:
http://mgaguru.com/mgtech/carbs/cb102.htm
barneymg

Steve,
I had similar on my Twin Cam carbs; albeit slightly different design.

A rod connects from the choke lever to the fast idle cam and when the choke was released, the fast idle cam did not fully return even when there was slack in the choke cable and the jets had fully returned.

I fitted an additional return spring, just on the cam, and all is now well. I'll try and upload a photo of It when I get the chance.

Accepting that the choke levers must have apparently worn holes to permit the geometry to work, It sounds like you have wear at each end of your fast idle rod set-up. If I recall, the rod is fastened by star washers.

I'll try and upload a photo of It when I get the chance.

Regards
Colin
Colin Manley

I forgot to mention that this was occurring (still) after I had swapped out my existing Petronix distributor for a 123 distributor, neither of which have any vacuum input as the Twin Cam does not have vacuum advance.

Regards
Colin
Colin Manley

This has got all the symptoms of the idle mixture being way too lean
It needs to be at least 13:1 or preferably 12.5:1 --in there somewhere is good and will give stable idle
It's a mistake made by many adjusting the idle mixture to achieve max idle speed for a given throttle position--but in reality correct idle mixture is on the richer side of max idle speed
Also ,is there a touch of slack in the throttle cable at rest---It's a commonly made mistake to adjust it up with no slack, thinking that it needs to be tight to get full throttle--It's more important to have a touch of free movement so the throttles can return freely-and so the throttle pedal can bottom out just on or before full throttle as this stops the cable being overloaded at full throttle

willy
William Revit

Steve
I used to suffer varying tick over speeds but not know. Unfortunately I don’t know which of many changes might have been relevant, sorry.

Paul
Paul Dean

Colin

I am going to pull the carbs to do an overall inspection. I don't believe the choke cam plays a part in my creeping idle because it essentially never moves due to wear in the connecting rod lower pivot joints. At rest (where it always is) there is clearance with the idling arm. I just operated the choke in the better light this morning and you can see the the cam rod twisting in the pivot arm and the pivot arm also twisting in the brass operating rod resulting in no up and down movement of the cam rod.

Once I have done a full inspection I may take a pleasant trip (complete with gas mask and NBC kit from my RAF days) down to Salisbury and chat it through with Burlens.

This does make me wonder about the quality of the exchange carb overhaul when I did the car rebuild 20 years ago. The car always ran a tadge rich despite my investigations over the years. The jet assembly change 6 months ago cured it instantly. It was one of the items I never thought to be a cause as I assumed it would have been a new item. I now have my doubts it was ever renewed during the refurb. It now looks like the choke levers were also likely not replaced. For the little the choke gets used there seems an extraordinary amount of wear and play.

Steve
Steve Gyles

Carbs on the bench and had a good look. Wear is relatively minor for 20 years. I detected a very slight movement on the throttle spindles. I put my dial on them and measured 2 thou play at the interconnecting ends of each shaft so does not seem much when translated to the central business area. Otherwise everything looks good with solid clunking of the butterflies.

The main pivot holes on the choke brass operating arms look a bit ovoid with 0.5mm difference in diameters. All that extra play adds up in an otherwise slack connecting system resulting in very little movement of the choke cam so I will replace them while I have the bits on the bench

One step at a time. Will rebuild when the new bits arrive and do a test. Take it from there.

Steve
Steve Gyles

One thing I forgot to mention is that I have a vacuum gauge plumbed into the rear carb through a tapping in the thick black spacer gasket (also similar tapping on front - Normally blanked off and airtight). At normal idle the gauge sits happily at 20" of Hg (the acceptable range on the gauge is 17 to 22). If I swap the vacuum pipe over to the front carb I get the same. Air leaks would therefore not seem top of the list. However, when I get the car on a run again I will make a point of observing the vacuum gauge when the fast idle occurs.

Steve
Steve Gyles

I am still awaiting spares and so with the carbs out I have been having a general poke around.

Something that I noticed just now concerns the accelerator pedal (right hand drive). With the throttle cable currently disconnected I have noticed that the pedal does not return fully home. i.e. the return spring under the pedal is a smidge slack. I cannot decide if this may be important to my issue as the cable is normally strongly tensioned with the carb throttle shaft springs. I am sort of guessing that any additional hanging weight on the tensioned cable could slow/stop full butterfly closure from time to time?

However, could someone confirm if this spring should be powerful enough to hold the accelerator pedal fully closed.

Steve
Steve Gyles

Further to my last, with the pedal at rest with just the spring holding it, the top of the pedal at the spring anchor sits 5/16" open. The spring length is 7cm. Can anyone confirm correct length?

Steve
Steve Gyles

Might have cracked it, but all subject to a good run. I noticed that the front idle arm did not always close fully to the idle screw. The problem seemed to be in the interlinking shaft. When I balance the carbs i usually just slacken the rear of the 2 shaft couplers. never touched the front coupler for this purpose. I slackened it all off, gave it all a good wiggle and rotations. Did the balancing again and tightened up. Hey presto, worked beautifully. The logic of it defies me but it seems to have solved it.

I used the SU balancing rods and tubes in the pots. So easy. Balancing took about 30 seconds. Once done I could then use the rod shafts to raise each piston in turn by 1/32" to 1/16" ish to check the richness/leaness. Again, so simple.

I also found a small issue with my vacuum pipe. I had snipped off the end as my distributor vacuum is not tapped, just a tube. I have a piece of plastic tubing connecting the 2 parts. The tubing was resting across the heater valve. When it gets hot it collapses a bit, reducing the diameter. I also noticed that I should have cut the pipe rather than snip it. Again, the hole diameter was much reduced - probably halved. I guess that would have affected the rate of the vacuum advance working, both on acceleration and retardation.

Lessons learned. I hope the next run will confirm my findings.

Steve
Steve Gyles

I have found it is a lot easier to work on the carbies and check all the linkages if the inlet manifold, heat shield and carbies are removed (installed) as an assembled unit. Great for setting up the mixture levers so they both start operating at same time, looking in the carby throats to see if butterflies seat correctly, etc

(I also made a new fast idle connecting link to the cam out of a 2.5 mm welding rod to remove all the slack in my linkages)

Mike
Mike Ellsmore

Like Mike, I also always remove the whole carb assembly as a unit (minus the outer air filter cover, filters and mesh). Much faster to do and saves all the fiddling to set up again after reinstallation
Dominic Clancy

Think you will find it's a combination of heat and the ethanol and whatever else is in our petrol now . Very much doubt you will cure it but will keep you occupied.
A J Dee

Mike and Dominic.

I agree (in hindsight) that doing it that way is best, certainly when replacing the choke assembly.

I replaced the lot including the clevis pins. I expected to find the holes in the brass levers had worn and this was confirmed when measuring against the new ones. But what surprised me was how much the clevis pins had worn, especially the ones that formed the main pivot points - linked to the zinc coated struts. Once installed, the fast idle cam movement was mega in comparison to previous.

As the replacement brass rods were identical to the old ones I found no adjustment was required to synchronise the jet pull.

Incidentally, while leaning into the engine compartment tuning the carbs I was very pleasantly surprised with the volume of air from the asymmetric fan blasting past the carbs. Whilst of course it is warmer air than a bilge fan, it is not that hot. Correct MGA radiator and shear volume of air seems to be more than sufficient to prevent fuel vapour locking in slow/stationery traffic. In fact in my experience to date the temp seems to drop if anything in slow traffic and shows no or little sign of any significant increase when stationary.

Steve
Steve Gyles

Perfecto. Long run, replicated last run in similar temperatures and idle spot on at all times.

Looking back through the posts Barney's advice closest to sorting it - no surprise there then. I did not go the hole hog and reset the idle arms on the throttle shaft, mainly because I did not have a small enough Allen key at hand. But what appears to have solved it is loosening off both throttle shaft clamps and giving it all a good jiggle as Barney describes; then rebalance and tighten.

May be the answer for others with the creeping idle.

Steve
Steve Gyles

Thanks for the feedback and follow-up Steve...too often that gets left off the end of the conversation.

Gene
Gene Gillam

Great to hear you found a solution to the issue! I will give the same a try. Thanks for the thoughtful approach and documentation along the way.

Matt
M Grover

I meant to add earlier that with the SU balancing kit I could, rather surprisingly, balance the carbs by feel, although I also confirmed it visually by looking at the rods. As a follow-up I checked the vacuum of each carb on my test equipment just to be 100% certain I had done it correctly. I am now very confident that this is the easiest way for me to balance my carbs.

With the engine running I was able to push each piston down to its closed position by pushing on the rods, comparing the distance travelled by each piston both by feel and observation. I was actually quite surprised at the amount of force required - equating to about 18 inches of mercury on my vacuum gauge that had to be overcome. It was a bit like the finger pressure required when checking a slightly deflated bicycle tyre.

For those interested this is the kit and instructions I have on my website: http://www.mgaroadster.co.uk/su_carburettor_tool.htm

Steve
Steve Gyles

Steve,
Does your third wire fit into the third rod (the squashed one used for fuel float setting). Mine doesn’t, not a problem for twin carbs, but something that I noticed on my set.
Mike
Mike Ellsmore

Mike

Two of my tubes have the squashed end for the wires which are fine for our twin carbs. My 3 tube is cylindrical at both ends. I use that one for my float setting. I guess those with triple SUs have to either squash the tube a tadge or the looped end of one of the wires.

Steve
Steve Gyles

This thread was discussed between 06/05/2018 and 15/05/2018

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