With no idea what year, or how many miles are on your 318, we are just guessing.
Backfire at idle with a stumble, and low power could all be caused by a vacuum leak. A 600 Holley on a 318 means at least the intake has been changed, and 600 Holley's are known to have warped cab bases. Start by looking for a vacuum leak. Base of the carb, throttle shaft bushings, intake to head gaskets.
Maybe it just needs a new carb.
No where am I reading where your ignition timing is set. Its not unheard of to have the balanced slip on high mileage 318s, try bumping the ignition timing up a bit and see if that helps. Sometimes a little extra ingition timing helps a lot, and sometimes it doesn't make any difference or it may even make it worse, but its something easy to try.
The next thing for a high mileage 318 could be a loose timing chain, it may need a new timing chain and gears. Gene
It is a 79 engine but I am not sure of the mileage on it. It does have an aftermarket intake and headers as well, I'm not sure if the cam was changed but it does sound like it. I checked all the vacuum lines and then checked the carb and entire base of the intake for leaks as well but I couldn't notice any difference in the idle while doing that (carb cleaner while it was running). I haven't gotten a timing light on it yet but I put the old distrubutor on it and adjust the timing back and forth until it was in the middle of impacting the way the engine ran. I will get the timing set and then check the timing chain. With not knowing what the engine has had done to it or the condition of the internal workings I am considering pulling it anyways. I wanted to wait a bit longer before doing that but it may get shifted up higher on the list. Thanks Gene,
Jeremy
