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SU Carbs

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mgbcca

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I have a '75 MGB with twin SUs fitted. It runs rather lumpy (it has a mild cam upgrade) and it smells as if it is running rich. The front two plugs look OK. The back two are white and burnt. I'm thinking the carbs are not set up correctly - front rich, back lean.

Can someone suggest a novice's guide to getting the basic carb settings right - or at least close?

Thanks all
 
I have tuned my own carbs with some amount of success, but it's not something I want to attempt to explain to someone else; I'm sure I would screw it up.

There are tuning procedures layout in both the Haynes and Bentley manuals. You also may find this article of interest: Tuning Your S.U. Carbs.
 
when you were adjusting, did you start with the rear carb - you should've
 
Hello all,
Certainly set everything up to the book, and it should be close. But if there are modifications to the engine then it may not be right throught the rev range so some needle changing may be required.
I have never heard anyone on this forum mention a simple device that is available in the UK. it is made by Gunson's and is called Colourtune. It is a spark plug with a small window and you can watch the combustion and adjust the mixture until it is correct, basing on the range that yellow is RICH to blue is correct and colourless is weak.
For multi carburettors install multiple Colourtunes.

Tony,
would you explain why you say the rear carburettor should be adjusted first?

Alec
thirsty.gif
 
piman....Colortune is way to go when everything is finished (for fine tuning is how I use mine)...I always do the rear one first as that's the closest in the fuel delivery system & it needs to be right for the front one to operate properly
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by tony barnhill:
piman....Colortune is way to go when everything is finished (for fine tuning is how I use mine)...I always do the rear one first as that's the closest in the fuel delivery system & it needs to be right for the front one to operate properly<hr></blockquote>

Hmmmmmmm, on mine the gas is delivered to the front carb first. I just assumed the other MGs were the same. Personally, I would think if there is enough flow there to keep both bowls full during operation, that it would have no effect which one you did first.
Bob
 
I'm curious about this "closest in fuel delivery..." too. Gas on my car goes to a tee fitting which splits off to the front float bowl and the rear float bowl. There's less tubing to travel through to the front, but both float bowls are kept fairly full of gas as the engine's running and fuel pump is running, at least full enough so they can both let the same amount of gas out as the carb wants it (provided they're adjusted the same). Gas doesn't really go to each carb so much as it goes to a reservoir that is kept filled so each carb has an adequate supply of gas available as it needs it.
 
Hmmm. Now I'm curious too. I've always done the front carb first, probalby because I'm left handed, then done the rear, and back to the front. This makes me wonder if I could save one step. Worth a try in the interest of science!
Then again, the fuel delivery on everything I have worked on for awhile has also had fuel delivery to the front carb.
 
Spray WD-40 around all the vacuum connections - you'll know if one is leaking the minute the WD-40 hits it
 
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