Why does my...

Hi Ted,

Looks like I'll start the "could be this, could be that" list.  As you know, there is a list of usual suspects for backfiring, even subtle as yours seems.  Sounds like you have timing issues covered:  

Hole(s) in exhaust system

Running lean (intake manifold leak)

Idle too fast

Low octane fuel

Incorrect valve adjustment

Best of luck.  I'm sure others will chime in with other potential causes.

On Jim K's list re:low octane fuel to eliminate that add an octane booster like '104+' at about $10, found most everywhere, this will treat about 20 gals of fuel or for one of the best on the market and hard to find, made in Australia 'NF Octane Booster Racing Formula', about $35, you only use about a 3% mixture per gal of fuel...this stuff works very well from experience.

When I had a very hot cam and dual 4bbls in my '55 Chevys 383 I often would have a similar symptom to what you describe, my timing and carb tune was right on also. A friend recommended adding an octane booster...that cured the problem up with my engine in no time...when I tried the 'NF' wow! did it ever wake that engine up...but it took almost a full bottle of 'NF' at $35 a bottle mixed with 16 gal of fuel.... that equals one expensive fill up

What I have done in the past is get the back end up, start the car, put a rag in the exhaust, then feel around the exhaust gaskets. I bet you'll find your leak.

Good morning Ted! How the heck are you? If the engine ran fine on the available gas before, I don't see it "suddenly" needing higher octane gas- something has changed. As Justin said, take the wrap off the headers. There's no faster way to destroy a good set of pipes! If a little rich, maybe the wrap is holding enough heat in to create an afterburn in the pipes you're only hearing immediately after it's turned off, and when it hits some oxygen... Time to invest in some carb jets. Or are you hearing the carbs burbling? And Rusty's trick is always a great one. Al

That's an "oops!", right there........