Highway de acceleration backfire

[jibbers66]

I have a 72 barracuda 440/727. Holley carb/Headers. Car runs great but I do get some backfire under de acceleration on the highway.
Is this normal if it’s performance tuned?
Would it e fuel or timing related.?
Thanks in Advance Guys…

 

[Builderguy]

FWIW from an old mechanic named "Bart":

I've seen a few things that can cause this...

a. Exhaust leak at header flange. This allows air to be sucked in and the oxygen causes the leftover unburned gases to "pop off".

b. Excessively lean fuel mixture combined with insufficient spark advance on deceleration causing part of the combustion process to be occurring in the header. Most common cause is use of ported (timed spark) vacuum for spark advance.

c. Too much (exhaust) valve preload.

d. Too much movement of the distributor breaker plate on high vacuum causing a rotor/trigger phasing issue...... that'd be my wild hair guess..

I'd try richening up the idle mixture a 1/2 turn at a time to see if that makes a difference one way or the other... will give you a direction to go in, anyway.

 

[jibbers66]

FWIW from an old mechanic named "Bart":

I've seen a few things that can cause this...

a. Exhaust leak at header flange. This allows air to be sucked in and the oxygen causes the leftover unburned gases to "pop off".

b. Excessively lean fuel mixture combined with insufficient spark advance on deceleration causing part of the combustion process to be occurring in the header. Most common cause is use of ported (timed spark) vacuum for spark advance.

c. Too much (exhaust) valve preload.

d. Too much movement of the distributor breaker plate on high vacuum causing a rotor/trigger phasing issue...... that'd be my wild hair guess..

I'd try richening up the idle mixture a 1/2 turn at a time to see if that makes a difference one way or the other... will give you a direction to go in, anyway.

Than you for the info Ray. I did notice a small crack in the right side header today, I’ll try tweaking the carb a bit.

 

[heminut]

It's been my experience that A is usually the cause.

 

[Xcudame]

You just need some better exhaust gaskets (copper or remflex) or machine the headers flat so they fit tight against the cylinder heads. From the factory Mopar used NO exhaust gaskets on big blocks.