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MG MGB Technical - Carb lifting pin

When tuning SU carbs (mine are HIFs ) I know that by raising the lifting you are able to determine if the mixture is correct by listening to how it changes the rpm.
Should you.. lift the pin and instantly release it...lift the pin and not release until you hear the rpm change then settle...or lift the pin and hold for a fixed period of time. How do you go about it?
Phil

There is some amount of free play in the pin. So push it up until you feel it contact the bottom of the piston. Then push it up a tiny amount more, say 1/16 inch. Hold it there while you assess what effect it has made on the engine. If the mixture is weak the revs slow down, if rich the revs speed up. If its about right, there is a slight rise in revs but it then settles back to where it was. Basically hold it up to see what the effect is. Adjust and try again.
Mike Howlett

Mike, talking of carbs..........

Is there anywhere/anybody you can recommend in the Ayrshire (or near) area who can tune and balance carbs? I've already been left very unimpressed by a MG "specialist"!

Phil, sorry to hijack your thread.

Thanks, Clark.
Clark

Hello Phil,

Another way to proceed in..., warm the engine, take out air cleaners. In this way with a small screwdriver you can lift pistons in succion chambers by the same amount as defined by Mike. This way, you get rid of the no acting coarse of the lifting pin, making maneuvre a bit easier.
Regards.
Renou

Unless you *have* to remove the air cleaners anyway, it is easy to tell when the pin has taken up all the free-play and starts to lift the piston. Until you get used to that unscrew the damper cap but leave it sitting there, and you will see it move when the piston starts to move. It's actually 1/32", but anywhere in that area should be OK. However I do find that the rev change on the HIFs on my V8 is noticeably less than that of the HSs on the roadster, it's almost subliminal. But in either case people often have trouble detecting the correct position when first tuning SUs, but you don't have to. Train your ear by richening the mixture until you *can* tell it is rich by the revs rising and staying up, then weaken it - counting the flats or 1/4-turns - until you *can* tell it wants to stall, then richen it again by half the flats or 1/4-turns you counted and you will be in the right area. With practice you will find you can gradually narrow that range and eventually be able to determine the actual sweet spot.

And when tuning twin SUs you must get the airflow balanced first. Having done the mixture on one carb move onto the next. When you've done that recheck the first, as they are inter-dependant and changing one will affect the other. You'll probably find a small adjustment is needed to the first carb again, then recheck the second and so on. But it should only need a couple of iterations to get both right. This should all be done with the interconnecting shaft clamps slackened, now tighten these, and check that just off-idle the airflows are still balanced - that is more important than getting it right at idle. Keep tweaking the clamps until it is balanced both at idle and off-idle.
Paul Hunt 2

Clark, I've never entrusted any MG work to a garage. I do everything myself.
Mike Howlett

Mike,

I don't blame you, but carbs are beyond me! Still trying to forget my last visit to a MG "expert"! Sometimes you credit people with more knowledge than they actually possess. Anyway, once bitten twice shy.

Clark.
Clark

This thread was discussed between 22/04/2008 and 24/04/2008

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