Sometimes when I go to shut the car off it backfires and makes a puff sound out of what sounds from carb and exhaust. Runs great under hard accelerations. Hates stop and go traffic or just crawling along. Its a 305 with a new vac advance distributor and Edelbrock 1406 600 cfm carb. Timing is set to 6 degrees as in between the 6 and 8 notch on timing indicator.
BACKFIRE DURING SHUTOFF
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It sounds like you have too much advance timing, retard it a bit and try it. Although 6 degrees is not a lot.
How does it start when hot? It sounds like you pulling full vacuum at low speeds and little at WOT. get a vacuum guage and check it at carb ... or just disconect vacuum advance and plug start car and test drive it see if that makes a difference. (NOT ALL SET-UPS ARE THIS WAY)You should have little to no vacuum at idle and full vacuum at wot. The other thing to check is see if you canister works that way get a vacuum pump hook it up and check you timing then pull the vacuum at idle see if the timing advances. good luck
Pretty sure that's wrong. Almost positive it doesn't really matter if vac advance is ported or not, but you should have 0 vacuum under WOT and at part throttle is where the vacuum advance works to your advantage. However I have mine setup for ported vacuum.
84ss said:
when you set the timing did you have the vacuum line to the distributor removed and plugged?
actually it'd be running lean, the excessive heat in the intake causes ignition when the airflow slows down enough.
Is this the carb that was idleing way too fast until you mashed the throttle? What caused that? Was the throttle sticking open from interference, or was the choke holding it open? I ask because if the throttle is sticking open at all, it will try to run on (dieseling) due to the tiny amount of mixture sneaking past the butterflys. That whoosh out the carb would be exploding mixture sneaking past an open intake valve. A very rich mixture to begin with would allow unburned fuel to linger in the intake, and exhaust, contributing to the backfire condition.
What RPM at idle? If it's too high and you turn the ignition off you'll get a dieseling effect.
Also recheck your timing with a different timing light than the one you are used to–-and make sure the timing marks on your balancer are correct. Sometimes, depending on the balancer and how old it is, the outer part can turn, messing up your timing reading.
Also recheck your timing with a different timing light than the one you are used to–-and make sure the timing marks on your balancer are correct. Sometimes, depending on the balancer and how old it is, the outer part can turn, messing up your timing reading.
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