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MG Midget and Sprite Technical - SU carbs damper springs

Hi

I am in the process of fitting twin 1 1/2 SU's to my 1275 engine. They are HS4's on a maniflow manifold. Should I use red or blue springs? Engine spec is 1293,S head 10.5 CR with kent 246 cam.
]Special tuning suggest red for 1098 engines but nothing for 1275??

I am using HS4's so I can retain the crankcase breather system
Bob Beaumont

Hi Bob,
Somewhere "out there" is a spreadsheet based programme that you enter your A series engine specifications into and it tells you which combination of carb parts - jets, needles and springs - works best. I used to have a copy of the spreadsheet, but cannot at the moment find it. I think it must be on an older laptop.

Sorry, this contribution isn't much help but maybe someone else has a link to it?
GuyW

Bob,

Do you mean it does not specify which spring for a 1275, or do you mean it specifies that no springs be used? I think the only time I have seen no springs were the weighted piston type.

I have HS4s on a 1275 +0.040 inch, ported and cammed, and I used the same springs that were in the HS4s when they were on the MGB. I never did have it on a dyno to see if that was ideal or not.

Charley
C R Huff

Hi Charley it doesn't specify which spring for the 1275 Spridget. The MGB uses red springs The special tuning H4's fitted to mini's use red, but the twin HS4's in later ST mini's use blue!
Bob Beaumont

mintylamb SU program is what you want. My A series was very happy on standard MGB HIF versions of twin 1 1/2s
Rob Armstrong

Thanks Rob

Great site thanks but it doesn't cover springs. I assume you are using red springs in your set up?

Bob Beaumont

Bob
My vote goes for red
If special tuning use red on a 1098 then they should be even more suited to your engine
I'm thinking std. springs (red) could be borderline for a std. 1275 and blue (lighter) springs could be worth a try, but with your larger 1293 plus camshaft engine, red would be my choice specially if you have a decent free flowing exhaust

red---4 1/2 oz
blue--2 1/2 oz

I don't know how far you have looked into the subject of damper springs but I guess that's behind your question
The correct spring choice should allow the damper piston to reach full lift just before the point where the engine develops it's maximum power
Some racers tend to go for the lighter spring choice to make sure they get full travel a bit earlier, but keep in mind that these guys are tuning for outright performance and would have their mixture on the rich side to accomodate the lighter springs

willy
William Revit

Considering your over bored at +.10 and you have a maniflow intake and the head is gas flowed ... breathing is not an issue... in fact id say to much air is going to be a problem

something id be conserned about is that the carbs are going to be to large for your set up . Im thinking your going to lose alot of constriction ... which is okay for driving above 5500 5o all out but at 2000 i think your going to suffer in both performance and gas milage

to much air and not enough capacity

what might be better is a single HS4

are you also using 1.5:1 roller rockers or the 1.25:1 valve rockers...id go the 1.5 before i would the duel hs4 carbs other wise all your flow and easy breathing will get bottle necked at the valve openings


prop
prop

Something else to consider ... your running 10.5 CR

which is okay for .... today as your fuel is a much better quality then ours is in the usa. But as the EU continues with ethinol your fuel quality will continue to suffer in a downward spiral just as ours has over the past 10 to 15 years

When that happens its a night mare to tune the ignition to a lower standard of fuel with a high CR .... so the easy option is to lower the CR for better tunability
prop

I genuinely don't know Bob. They'd be the springs that were in the MGB carbs, they came from a 1979 MGBGT. If there's an easy way to tell what they are let me know; the engine is in dad's car now :)
Rob Armstrong

Bob not on a spread sheet as such, but the Haynes SU Carburettor manual gives a set of specs in the back for standard engines by make and size

Part 3
Appendix 1 - Applications list

In the section for MG and Wolseley it lists the 1300 as

1275cc 1967/68 AUD186 HS4
Rich needle BQ Std DZ Weak CF Piston spring Red

1300 Auto 1967/8 AUD271 HS4
Rich needle BQ Std DZ Weak CF Piston spring Red


Later where it lists the Blue as an alternate it also lists the needles as different although in the main 1300 engines seem to have the springs and needles shown

None of the 1300 engines gets a Yellow Spring listed
Bill sdgpM

Thanks all for your advice and suggestions. I'll think I will go for RED as a starter. The reason I am changing the carbs is the HS2's are just not breathing enough. On the rollers we managed to get a needle which optimises the engine and gave a good balance between top end power and driveability but the tuner felt it was wanted to breath more. I was not really looking for loads of grunt and only changed the cam as the original one was worn out! The 246 is the mildest Kent do but it has more bite than standard. Before I changed it the HS2's were just about OK.

I like twins rather than a big single hence the route I am taking. Interestingly I discovered ST offered HS4's for spridgets (C-AUD709) with special manifold (C-AJJ 4040) but can't find any more details.This was part of the Stage 2 tune which essentially was bigger carbs better exhaust and a flowed head with 1.4 inlets. Apparently 0-60 arrived in 10 secs and top speed 104MPH!
Bob Beaumont

Before you go that direction... what about a lighter weight oil in the dashpots

perhaps 3 in 1 or sewing machine oil

prop
prop

Bill
I think the needles etc you show there might be for a single carb setup
Bob
I have just found an old list that I made of the specs of a mate's midget that we did some mods to for Targa Tasmania a few years ago The car ran hard and fast for the whole week of competition

1275- plus .030"
28-68 camshaft
10.25:1 comp. ported head with oversize stainless valves
LCB manifold with 2" freeflow exhaust

Very similar to your car by the sound of it

After lots of testing we ended up with red springs and BG needles

Needle choices vary from car to car especially with different exhaust systems, air flters etc, but,in saying that, this car was on ram tubes with no filters and a very free flowing exhaust. Therefore I would reckon the BG needles and red springs would be a safe starting point for you------

Cheers
willy

When you get it on the rollers, get your man (or woman)to check that you are getting full damper piston travel up in the revs -say at 4000-5000- and if not you might have to go to blue springs, but if that is the case, which I doubt, be carefull as the mixture will lean out through the whole range meaning richer needles will be required
William Revit

Ah Willy buggerit 'cos it looks like you might be right :(

But even then Red looks like the best start place

Oh well, that's another comic for the Charity Shop…
Bill sdgpM

William thanks for the further advice. I was going to go for AAU needles (spring loaded)on the advise of the tuner but will also consider the BG. The cam I am using is not as hot as you had and I am running the standard bore exhaust system albeit with an LCB. I am not looking for a top end performance just a well balanced road car. As I say it was fine until I had to change the Cam!
Bob Beaumont

Bob

what is the attraction to the dual hs4 set up... over the standard duel hs2

i think your going to have to much carb from your discriptions when all is said and done
prop

Hi Prop

I made reference in an earlier post to the ST kit produced by BMC which used HS4's on a very similar set up I have. Certainly when we had it on the rollers it felt the engine wanted to breath more. The HS2's are getting near their limit when the BHP goes over 70 and we were getting about 75. We have had to use a compromise needle that is really running too rich at around 1500rpm to ensure we get reasonable pick up and decent mid range. The tuner felt that larger carbs would provide more flexibility and given ST offered the option originally I think its worth a try.
Bob Beaumont

Im far from an expert..

but i just dont see how your suffering from a lack of air. your engine spec are just to mild

ive got to think its your cam being to mild or just not enough lift in the valves... or maybe even an ineffective air filter thats making to much of a 90 degree turn going into the venture of the carb... or the carb pistons not getting full travel under accelleration ... just a bunch of other areas your not considering

For what your going to spend on duel carbs on maniflow intake id seriously consider a 1.5: 1 rocker set up 1st ...that alone will is going to get you very close to the 1/2 inch of lift ... aka sweet spot for maxuim EFFECTIVE air flow at the valve heads ... from what i gather currently your at 1/3 rd of an inch of valve lift... that alone will deliver serious HP and torque ... any thing beyound 1/2 inch lift will be a waste

another thought is worn valve sewt angles and air escaping during compression or even the cam timing to retarded can have a huge effect on where air is sucked in as it relates to the postion of the valves ... if to retarded the air can be slamming into a partially open valve and giving the apperance that the engine is not getting enoug air... which in therory its not but not becuase of capacity but because of blockage

i know im the lone gunman on this ... so that should be considard

at the very least have peter burguss reflow your head and see if there are any obstrustions

Another possiable area would be to see if the ports to the valves are more squared angular instead of a gentle sweeping curve.... a very common mistake from diy newbie head porters

another area to test inside the ports is the divider wall that creates the siemiese if thats carved up to much and not balanced that can adversly effect the flow of air more to one cly then to another

also... do you have the centering rings in the intake to head connection if your off by even an 1/8inch that can have serious effects on air flow ... just as an example

i understand what your saying and can agree thwt you do have an air problem but i dont think based on the specs that its an air capacity problem... i really think you have an air flow problem.. the flow is getting hung up some where between the airfilter element and the dish in the piston and getting bottle necked


Id take the whole head off with the carbs and intake mounted and maybe even the exhaust manifold and have the whole unit tested for air flow restrictions...just take off one element at a time and see if you get pressure incresses and drops

I know my thought that its an air obstruction issue and not a capcity problem sounds extreme but if there is an air flow problem then duel hs4 carbs wont fix your problem in the slightest

the engine will only suck in the air it needs based on how much it can handle and that is dependant on flow and capacity

Think of a garden hose with a kink... making the dia of the hose bigger wont deliver more water past the kink it will still be the same amount of volume past the kink as was the smaller dia hose

i just think you need to do more testing before you settle of you need bigger carbs

lets face it... duel new /fresh rebuilt hs4 carbs on a maniflow intake is easily $1000 usa money on a good day and could well exceed $1600 buying from the usaual suspects ... the orginal maniflow intake that vizard used is about $500 new by its self in the usa and have sold below $350 durint the rarity that they come up on ebay which is about as common as 12 hens teeth in a pile of rocking horse poo

prop
prop

Prop,

Keep in mind that Bob is not making wild guesses. He is making educated guesses based on rolling road sessions, and I assume he will roll the 1-1/2 when he gets them installed.

Also, IIRC, you have a pair of 1-1/2 that you planned to try on your car. I suggest you try them. It might make a believer out of you. I have them on my warmed up 1275 on an MGB manifold, and have always thought they did well. I have not driven it on the road because I built it for autocross, but when driving around the paddock it seems well mannered enough.

Remember that 1275 sits near the borderline since the 1500 Midget/Triumph uses 1-1/2 carbs. Also, while the 1800cc MGB uses two 1-1/2 carbs, the 1800cc Volvo B-18 uses two 1-3/4 carbs. So, there is room for opinion.

Charley


C R Huff

Hi prop

Thanks for your words of caution. Just to explain, the tuner is Slark Racing (offshoot of Downton) who supplied the flowed and worked head originally. They are a very well known tuning outfit and prepare and race mini's. We spent a day on the rollers to reach this conclusion. I had already got the HS4's anyway so no massive investment.
Bob Beaumont

Its an interesting debate

charley is correct i do have a set of hs4 on a modified mgb manifold somewhat ready to go

but im very impressed with what the hs2 can do.. im sure there is some benifit to going the extra ccf perticularly at the top end ... which i dont drive alot in

my set up is 1330 9.4CR ported head with the cooper S valve sizes paul ivy stainless steel racing valves 3 to 1 exhause header aluminum fly and 1.5 :1 roller rocker swift tune sw5 07 cam advanced to 103 degrees with issky performance performace springs

and im running basically stock hs2... sure i cleaned up the intake and matched the pieces togather got rid of the sharp edges so it flows alot better...

but i just cant imagine it running much better then it does... i fear that bigger carbs will tqke away from the lower rpm ad loose alot of torque and grunt ... which is were alot of fun is at... yes ive driven in excess of 100 mph ... but those moments are few and far between...

i just think something has got to be off if your suffer that bad at the top end of the rpm range to need to go to the bigger carbs...

I dont think you mentioned but im assuming you got rhe dissy recurved after the rebuild

any way... ill shut up...you have my opinion but i do look forward to watching your project and i want to see it be what your looking for... and wish for the best for you.

prop
prop

This thread was discussed between 07/04/2016 and 11/04/2016

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