Expert: Brad Sears - 7/20/2010

Question
QUESTION: hi i have a 1963 corvett 327 all # match and is all redone the timing is set at 10 btdc it runs perfect till it warms up then you can see the timing drop off to atdc and the car stalls the carb has been donetwice it is a carter afb we have put in another distributer it still dose the same. no vac leaks and the dist dose not move this is driving me nuts .ihave 40 years exp and have neverseen this can you think of any thing thanks laurie

ANSWER: Ok, a couple of questions to bring me up to speed. Are you disconnecting the vacuum line to the distributor when setting the timing? Is the vacuum line connected to ported or intake vacuum? What is the idle speed when old and running on the fast idle cam? What is the hot idle speed? If when the engine is hot and you bring the engine speed to the cold idle speed does the timing advance? Let me know.
Brad

---------- FOLLOW-UP ----------

QUESTION: yes the vac is disconnected   stock vac line is in place witch is at the frt lower carb middle idle speed is cold 1200 on choke off choke is 850 or so ,and it runs great all the way till then,and then it hits aqbout 195 --200 i can have the timing light on AND watch the timming fall to 4 atdc and stall you can hear it start to fall when it is at opp temp and you try to keep it running it has to be 1800 --2000  i am  takeing the timming cover off to check but doubt to find anything because it runs too good when it is below 200 any sugs.thanks Laurie

ANSWER: Does the car have the stock point ignition system? Are there any things in the vacuum line between the carburetor and the distributor? Is there any slop in the upper bushing of the distributor shaft? Let me know.

---------- FOLLOW-UP ----------

QUESTION: all of that is ok

Answer
If all of that is ok, then the problem has to be in the distributor. I have seen points bind on the pivot pin or a weak spring on the point set. I have also seen centrifugal advances do weird things. The exact temperature would indicate that there is a temp controlled switch in the vacuum line like in the later model cars that switched the distributor vacuum between cold and hot to improve driveability of the early emissions controlled engines. But you say that the vacuum line runs directly from the carb to the distributor vacuum advance. However I have also some where back in the dark ages seen one of the nylon timing gears worn to the point that the chain would climb the worn teeth and change timing. Let me know what you find under the timing cover.