Tired of waiting on the endless line of cars so I could turn out onto the road, I punched the gas. It hesitated and then backfired. Damn.

Recent changes are:

Went down in jet size from .113 primary and .107 secondary to .110 primary and .104 secondary. The reason for this was because my plugs were black, hesitation and bad gas mileage.

Also, put in a richer rod (.065x.035) to go into power mode without hestation. That worked fine until today. But today is the first time I had to really punch it and voila...a backfire.

Plugs gapped at .052. Ignition is an MSD Pro billet non cc HEI, 8mm wires and 6A box all about 6 moths old. Timing at 10*BTDC.

My questions are:

1. Can a gap too big (plugs are new) cause an exhaust backfire? If so, I'll re-gap to .045 and see what happens.
2. If not, am I too lean? (Did not backfire with the bigger jets). If so, I'll put the bigger jets back in. Maybe the bigger jets, .045 gap and .065x.035 rods will be the combo I need?
3. Am I too rich still? (No black smoke came out when it backfired).

Thank you!
All mods below:

IROC-Z

 

New plugs? Try getting hotter plugs. If the heat range is too low they'll run fine for a short time and then just give up--and would definitely cause a backfire, but will usually cause a hard start condition too. I would say your problem lies in the ignition anyway.

 

It sounds like more of a timing issue rather than fuel issue, might have too much timing advance.

 

i agree with max, i'd back the timing down a notch or two and go with 1 heat range colder plug gapped at .045. it's a damn shame i'm not vader or i'd know the real answer to this post.