[MuscleBlock81]

Ok, i have completely new valve train, ignition, and carboration/intake systems on my 1974 454 chevy big block....

It has PLENTY of power and torque in my 81 camino, but when i'm coming down from a high revolutions it wants to backfire through the exhaust... it does it when i'm coming down an off ramps from the hwy.

I have an HEI distributor with a holley 700 Double Pumper. Where should the vacuum lines for the vac. advance go for the distributor? The intake or carb? if the carb... then which nipple? i don;t want to play guessing games if at all possible.. the hei w/ holley dp is a pretty well known set up, if someone can help me out maybe this will solve the backfires through the exhaust?

THANKS.

i still have the old springs, valves, and retainers in the valvetrain... everything else is new.

 

[wildman926]

Are you running rich? Any exhaust leaks? This would cause backfiring when letting off the gas.

 

[MuscleBlock81]

I leaned out the mixture a little bit more today and it seems to help but only by 10%... maybe it was just my imagination thinking it helped....

YES i do have exhaust leaks... I have a fully welded system with dual flowmasters and hooker super comp heards BUT i couldn't get on of the headers to bolts up all the way in the back due to chassie clearance and i can see another cylinder has a leak along the header from black carbon build up showing up on my chrvy orange head...

will leaks at the heads/headers cause the back firing? if so, why?

 

[rifraf]

Because you are giving a volatile mixture enough air to cause a mini explosion,I believe thats the best way to explain it,too lean will also cause it,not enough back pressure will cause it and a distributer curved wrong,or hooked up wrong will cause it. There's some stuff for ya to chew on a bit.

 

[wildman926]

rifraf is right on the money. Maybe you should seal up the exhaust period. Hopefully it is something that easy and you do not have multiple issues.