That's a lot of turns in my opinion, even with a Eddy/Carter style carb. I think you seem to be getting a handle on the timing, so i have to wonder how lean that carb is, for whatever reason. Let's see what some others think?
Totally agree with you Rick. I've had a lot of carters and Eddy's and usually never have to turn the idle mixture screws out more than 2-1/4~2-1/2 turns.
Straightlinespeed reset the timing to 13 degrees initial and verify it's 38 total. Then try re-adjusting the carb and see what happens. I assume when you adjusted it earlier you had the engine warmed up good? I always adjust the carb in gear as that seems to produce the smoothest idle, but in your case if it won't run in gear that may not be easy.
Have you tried pulling off each plug wire at a time to make sure all 8 are firing? Maybe something else to verify. A misfiring plug or wire could be the reason it's backfiring. The timing you have it set at now should never make an engine backfire. You have to be pretty far off to get them to backfire.
