Silver '65

Have '65 NOM 350 with an Edelbrock performer carb.

Just in the last few weeks it has started (what sounds to me like ) backfiring through the carb ever so slightly while driving. I can't repeat it when idleing in the the drive way. It's driving me crazy that I can't figure out what the problem is.

Assuming this is what's going on, then isn't this indicative of a lean condition? Isn't that fixed through the air/fuel mixture screws at the front of the carb?

If so - How would I adjust this? Something like turning them 3 turns and then 1/8 more or something?

If that isn't it, I have a suspicion that there may be a slight exhaust manifold leak - can that create the same sort of problem?

Thanks for any insight!

AZDoug

Lean.

Idle screws only work at idle. Jets take care of everything else, though you could have a dirty transition circuit depending on what throttle postion is.

Exhaust leaks only affect exhuast backfires. Do you have manifold or other vacuum leak, like a split hose or stuck PCV valve that is causing your problem?

Doug

Silver '65

You're right about the idle screw, that's air/fuel jets is what I meant. (Listen to what I mean, not what I say!!her. ) Isn't the general adjustment three full turns & then another 1/8, or am I making that up?

I checked the vac hoses & they seem to be all right. Unless there's a leak between the carb & manifold, I don't think there's anything there, either.

Would an exhaust leak also make a sound similar to a slight carb backfire? Like I say, it only occurs when I'm driving and take my foot off the gas.

Grey Ghost

I had a similar problem. Lots of good info in this thread:

http://forums.corvetteforum.com/show...um_id=4&arch=1

cheesecake

Are you driving steady throttle or are you hitting the throttle accelerating? At what RPM? If you drive at a higher RPM does it pop back thru the carb?

Vetterodder

To someome that isn't familar with the difference, yes, an exhaust backfire could sound like a carb backfire. That your sound occurs when you let off the gas suggests that you're hearing an exhaust sound instead of a carb sound. Some of the possible causes are a rich carb mixture, fouled plug, bad ignition wire, bad plug connection, not enough ignition advance. Any of those could result in excess unburnt fuel in the exhaust that ignites when you let off the throttle. Another possibility is a leak in your exhaust system.

Jack60

Starting point on stock motor is 1 1/2 turns Not 3+

JohnZ

You don't adjust idle mixture by "turns"; you adjust those screws with a vacuum gauge, and adjust them for the highest steady vacuum reading - every car is different.

When you take your foot off the gas while driving down the road, you are instantly feeding fuel to the engine from the idle system (not from the main metering system); if the idle mixture adjustment is too lean, you'll get afterfiring.