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:

:hail:IROC-Z

 

Start by thinking WHY a backfire happens- raw, unburnt A/F mix makes it into the exhaust due to being unburnt in the cylinder. A nearby cylinder fires and throws flaming/really hot gasses into the exhaust. POW! The unburnt stuff ignites.

I think .052 is excessive gap. I wouldn't use any more than .045", but that's not what caused the problem.

I'm runnign from memory here, which gets fuzzy sometimes, but I think your carb originally came with .071x.043 rods and, of course, a 113 jet.

Try putting the PRIMARY jet back to stock (.113) but with a rod that is .068x.035, if such a rod or similar is available. That will fatten up the WOT mix like your engiene obviously wants, but keep your part throttle at near-stock calibration. This will give you the richer WOT mix your engine obviously wants but keep your plugs from loading up.

Few tips here......

If the engine stumbles after idling for a while and then gettin on it this is often caused by too rich an idle mix, which fouls up the plugs, and then you get a hesitation the next time you get on it until RPMs rise, combustion temps rise and the buildup burns off.

A stumble followed by an exhaust pop, if not ignition related, is ALWAYS due to a too-lean mix. If it were too fat it would simply feel like you put you foot down into a bucket of cold oatmeal. No power, but no pop either.

Your plug's coloration, in a street-driven engine is determined 99.99% by the idle and part throttle A/F ratio. WOT mixture has almost no effect on their coloration because you only spend .01% of your time there. Don't use plug coloration to firgure out if your WOT mix is correct. That's race car stuff that doesn't translate into the street-driven engine world.

 

I'll do that and post how it works. Thanks for taking the time to explain.

:hail:IROC-Z
:hail: Damon