Brewbrother wrote on 12/31/69 at 16:00:05:Last weekend was a spacer mod, iridium plug, sportster muffler and plug door. This weekend will be finding the exhaust leak that is causing a backfire when shifting from 2nd to 3rd. I am also going to do a carb experiment. which I may post about later this weekend.
Good luck on the backfire thing. I'm still tracking what I would call a 5% backfire. In other words, I've got 95% of it gone.
Yesterday, I decided to pull the entire exhaust off and replace the header/engine exhaust gasket. I've gone through the series of tighten, run it, tighten, run it, etc.
Rode about 200 miles today and darned if I
still don't have that same 5% backfire!!!

I've been through the carburetor, and that thing is a clean as a pin with no gunk anywhere, it looks brand spanking new on the inside. Floats aren't sticking, jetting is right.
The only thing I can figure is that it is next to impossible to remove
all the backfire from it. Heck, I'm even running a tick on the rich side and I'm still getting that little annoying backfire.
Anybody else got any tips to check on? While I had the exhaust off, I checked the muffler to make sure it is tight, it hadn't loosened up any.
Now, I will say this. I had the bike running pretty darn rich on the idle mixture, where the bike would actually idle slow because the screw wasn't in its happy medium. I've since put it in its happy medium but no difference.