Too much backfire on the overrun

[ceejayjay]

My 454 Chevy has always produced a pleasing belch of flame under hardbacking off of the throttle but now pops a bit more than I would like even under gentle throttle off. No discernible change in running or performance, and my best guess is either a dodgy plug going slowly on the blink or dud wire. The plugs ara a total bastard to get at!!! - need three different bits of cobbled tools to do!! so don't want to go that route if its being caused by something else.
Thoughts?
Carb untouched by human hand and has been dead reliable - holley 750 double pumper.

Thansk
Chris

[Clarkson]

Sounds like you have a exhaust air leak??

[ceejayjay]

There's a thought. If so maybe most likley where the manifold enter the 4 into one I'd guess. Both are in A1 nick but maybe not where they join up? Worth a look.
Cheers

[Clarkson]

If it's not right there it will definitively make your sidepipes backfire.

[ceejayjay]

thats tomorrow morning sorted then
cheers

[conrod]

Chris,

From recent personal experience () it could also be due to a leaking inlet manifold gasket. I had almost cured my "popping on the overrun" problem by replacing an exhaust manifold gasket. Then the engine developed a very loud screeching sound due to a vacuum leak. Replacing the inlet manifold gasket cured that one, but also finally eliminated the overrun popping/banging problem.

Hope this helps.

[427scr]

Any air leak into the induction system will cause a backfire.

my car runs a pair of SU's and used to belch flame every time I let off.

I serviced the carbs, including new throttle spindle rubbers and it stopped doing it!