62 cadillac with 4GC, 60,000 miles.
Runs good when warming up, I can even drop the choke open off the fast idle cam and it continues to run fine. That means it's off the cam, off the choke completely, everything seems good. Then, and this happens very suddenly, after about 25 minutes of running when it's fully warmed up (just over the quarter mark), the bottom just falls out from under the idle and it drops to near stall speed. If I put it in gear, it will either stall or barely run with a flicker from the gen and oil light.
Is this the hot idle bypass valve I hear about? I've spent hours with it in this state trying to dial in the mixture screws, and turning out the air bypass valve, nothing gets it to idle up enough. It also starts to run terribly in this condition, where it wont even start and lean pops out of the exhaust, but will also sometimes flood itself where I have to crank it with my foot to the floor to clear it. Massive stumbling when driving it around to, just not running on all 8. Sometimes with a few cracks of the throttle it will return to good idle and 20 in vacuum steady, but slowly starts to die out.
I got it to somewhat stay running but in gear with the foot on the brake it has a bad miss. I need to get someone to hold it in gear for me so I can turn the screws more, but I think I should be able to get the idle higher. At some point turning out the air bypass doesn't make it any better. About 3.5 turns out is as high as she'll go, and it sounds too low. I've started from scratch with the turns out on the idle screws and bypass according to the manual so many times!
I can't find any vacuum leaks doing the carb spray around everything. And I did notice the bowl was starting to overflow, perhaps the needle is getting stuck in both directions; flooding it sometimes, or not opening enough other times. It only happens when the carb gets up to temp it seems. I'm suspecting it has something to do with the hot idle compensator, but I'm not sure how to check for that. I have not rebuilt the carb yet - just took the bowl cover off to check the basic operation. Fuel is clean in the filter.
This poor running condition occurs very suddenly - it will run fully warmed up on the temp gauge for a fair bit of time before it just falls on its face. Any help would be appreciated.
i think you are having an issue with the carb fuel bowl overflowing —( as you said you see ? ) and flooding the intake manifold and cylinders to the point of fouling the plugs
Replacing the carbs float needles and seats ——and adjusting the float height —should fix that
Adjust the top of the float height—- to not rise above the the fuel bowls top lip when incoming fuel is shut off
Be sure you do not have a leak in a float - it will have gas in it - should only have air in it
The fact it returns to an idle —-after a few cracks of the throttle means it is burning off the excess gas in the engine for a short time until it returns
First you need to be sure your points ( ign and parts ) and timing are on spec always as a begging base line to any engine trouble shooting
The Timing setting will effect the engines idle speed —-always
The fuel filter can look good and not be —determine when changed last —as there is a lot of debris in the gas tank of these old cars to clog it
You are playing with the carb adjustments far to much —without instruments and a complete tune up prior to any carb adjustments
The engine is responding as it is overloading and overloaded with gasoline - -flooding as you drive ( a leaking needle and seat ) and flooded when you try to restart with foot to the floor
To much gas is causing a very rough running engine
Your plug color the smell and being wet with gas — I am sure will show a very rich running engine
A carb rebuild may be needed
the fuel line was kinking off just above my inline filter. Noticed the filter was empty when it died after getting slower and slower - squeezed the line open and WOOSH fuel went to the carb and it picked up idle again.
Then I noticed the fuel was boiling in the line. Double whammy. The way they have it hugging the block, what were they thinking. I took it out of it's holder-inner-things and zip-tied it more straight and away from the block. Haven't road tested it yet.
Got it idling decently now even when screaming hot. Float was not the issue, neither was the bimetal hot compensator.
Just glad you found the problem