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engine idle slows when hot

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TomFromStLouis

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I recently had my carbs rebuilt and the engine idle tuned to near perfection. Actually, it seemed to idle just a bit fast, but that is okay. This last time I drove the car though, it idled at lower revs and after driving a bit and getting things hot it actually died on idle. Daytime temps are a bit higher and I have in the past noticed this kind of dying when the engine is very hot and the car is at a stop. Classic overheating death, right?

1 - Why did my engine revs at idle decline at all? Does ambient temperature mean that much (90 degrees versus 75)?

2 - Will the revs at idle always slow down when the engine gets hot?

3 - Is the simple answer just tweaking those carb screws a bit?
 
What sort of needle and seats do you have in the float bowls? I had Grose jets originally and had the same issue. If it was hot enough outside to think about looking for a shady tree then the car would not idle well and would die if the clutch was pressed down. Once I had replaced the jets with rubber-tipped ones the engine no longer stalled when very hot although there is still a definite change in idle speed. I also lowered the float levels a little.

Assuming everything is in good mechanical condition, idle mixture is very much based on the ambient temp around the carbs. If it was set originally when the engine wasn't stinking hot then it'll be rich when stuck in traffic. Set it for hot running and it'll take a bit longer to get off the choke when cold. You can't win, only compromise to the most common driving conditions.

Try setting the mixture when it is hot enough to idle slow, you may find the nuts can go up two or three flats and it'll run better. There is a reason the SU carb spanner has a hole in the end to go on your keyring...

su_image-2823_original.jpg


Andy.
 
AFAIK. All the SU carbs I've seen locate the jet with a collar; the jet-centering tool is used instead of a needle to align the jet in its collar (I used one on my HD8s last rebuild).
 
Having recently done a pair of H4s using a jet centering tool I can say it is a far better method than using the needle.

The frustration occurs when using the needle if the nut has been tightened previously enough to imprint the copper washer into the aluminum housing in a non-concentric position. Then whenever it is tightened subsequently the jet tends to go back to that original position. The centering tool is robust enough to force the jet to stay concentric while tightening the nut.

That Moss carb kit looks like a pretty good deal.

Andy.
 
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