Long story long
i wrecked, prop strike etc. fixed plane and built back up exp. installed 10:1 pistons, new ignition harness and champ plugs. That is all I changed on engine. So I fly it the first time running it hard to seat the rings, and I hear/see a 50 rpm 'miss' on the tach. But it does it intermittently. This was in September. When it would do it, it didn't seem to have a specific time, in cruise 2350, climb...off and on. Then go away for say 30 min and do it a few more times. Didn't really think anything of it. So I'm on skis now and have approx 40 hrs since I got it back to together, with it still doing it off and on. One day, I land for 15 min, take off and the thing runs like crap for like 30 solid mins. Wouldn't let me get above like 2200. Backfiring, popping, tach jumping all over. It did seem to get a lil better when I got close to town, but still wasn't right. Pulled plugs, the front 2 bottom were black and wet, the others looked normal. Comp check one in high 60's, rest mid 70's. Tested the new harness, its good. Mag drop is perfect at 1700, and checked it at 2200ish with 75-100 drop.
Ive tried leaning it way out, running full rich, carb heat doesn't help. I strictly run prem cargas with marvel added religiously. I even put about 10 oz in a tank and almost a qt in the oil at one time to see if it would help, thinking it may be a sticking valve...nothing. And comp check says the valves are seated. Starts fine idles fine, taxis fine etc. but as soon as I break ground and gain rpm-2300 is starts Doin it's 'miss' again'. If I back off to 2000-2100 ish it goes away. engine is a b3b with approx 300hrs since overhaul, same on mags. I currently have mags off and are having them looked at and will install new plugs when I put mags back on(Bendix). It only has ran REALLY crappy one time, other than that it's the miss thing.im hoping mags/plugs will fix it. Im no mech and have been talking to several about the issues. Is it safe to say it's not getting good spark, to light up, then it does and boom backfires? There is black residue in my tailpipe. I've checked ALL the simple stuff. It ran fine before I wrecked and only thing I changed was pistons and the harness....have researched till my eyes have bled. Any ideas appreciated thanks