What causes this? At a/f ratio > 12.5:1, I get pops in the exhaust, and my a/f
gradually gets leaner...

At a/f ratio < 12.5:1, the idle is steady, except for intermittent thumps.. which
causes the rpm to drop momentarily..

 

Fuel afterburn in the manifold causes this. It's way to rich at idle- should be 14.7-15.0

 

i'm getting popping in my muffler too

 

i shoot 5 foot flames inbetween shifts.

 

does it idle like this: "bop bop bop bop"

if it does, then its probably misfiring. get new plugs

 

no, it's irregular and not loud at all

 

 

it is probably running lean depending on the weather you are in like if it is cold out it will run more lean.

 

I'll bet its your timing.