After putting headers (Headman Elite) on my '74 360 I cranked it up and idled it for about 30 minutes (headers only - no exhaust hooked up). During that time I had several miniature backfires. It all seemed to even out the longer it ran. This morning I was driving it in to get my pipes reconnected and I noticed it again as I decellerated. When I'm on the gas I don't feel or notice it. The 360 runs a Holley 4bbl intake with a Holley 4bbl 650.
It was not doing this before the header install. Is this all due to a lack of backpressure with the headers alone?

 

Is there a header exhaust leak, or the flange on header to head gasket leak?

 

No leaks to the best of my knowledge. Ran a tube from each port to my ear and found one that needed tightening. Still no difference. I'll be able to hear better after my pipes/Flowmasters are hooked back up.

 

This is normal with open headers. It should not do that once the exhaust is hooked up. If you still notice it, then check for leaks.

Marty

 

Just picked the truck up. Backfires are gone but now I have a rough/fast idle. Acts like a vacuum leak. Couldn't find out where on the spot. My exhaust guy tried to help me find it - in fact, pointed it out to me. None of the obvious spots were the problem.

 

You may have to readjust the idle mixture on the carb now that it's breathing better.

 

I found my problem. The manifold vacuum line had come off the vacuum canister. What an idiot. Now I need to drive it with the vacuum working right to check for any carb adjustments.
Even with the vacuum problem it really gained in power.

 

Those miniature backfires is usually caused by fresh colder air entering the exhaust, thru some type of air leakage. Glad you posted and found the problem.