[Huron]

My '71 Cutlass with a 350 is backfiring through the carb. It's been my opinion that anytime an engine backfires through the carburetor that it's an ignition timing, valve timing, or valvetrain issue. Has that you been your experience?

I've only owned this 71 Cutlass for a couple of months. It was running pretty poorly since I got it. Now it runs great at wot, but if you accelerate modestly it tends to be really tentative, and then once you back off the throttle at cruise speed it will often backfire through the carb once or twice. (The backfire existed since I bought the car.) The other issue is that sometimes the manifold vacuum is ~16" h.g. at idle, other times its down to ~12" at idle. The car generally seems to idle fine. It starts as soon as you hit the key, whether the engine is hot or cold.

I've since installed a rebuilt distributor, and new cap/rotor/plugs/wires, set the dwell and timing. I also first rebuilt the 600 cfm Edelbrock that was on it, then I installed a Oldsmobile Quadrajet that I rebuilt myself. I've also capped off all of the vacuum lines to ensure that none of them are leaking. I'll be checking the intake manifold gasket and the carb to base gasket today.

Forgot to mention: it's got the Edelbrock Performer cam installed, and it's got a 3.73:1 rear axle.

Anyone want to hazard a guess as to what the problem is before I start checking the valvetrain?

Thanks!

Mike