I've read alot of the posts on rejeting and our infamous backfiring issue. Ive done some of the mods but still backfire about 20% of the time when upshifting from 2-3 and 3-4 gear and also when engine braking. Currently I have a sportster muffler (which I love), 1/2 spacer, 152.5 main and the mixture screw set at 2.25 turns out. I know it's set a bit rich because I don't need to use the choke to start it. Just hit the starter and she starts right up and doesn't die. Last I checked the spark plug it looked fine. No carbon. I don't think there are any exhaust leaks. I'd really like to eliminate the backfire completely. Should I try a bigger pilot jet? I see many here go with the #55. According to the model year there should be a #52.5 in it now.

sure its not a leak in ur exhaust??

when in doubt . weld it. haha

it will work.

911radioman wrote on 12/31/69 at 16:00:06:


over or under tightened. either way you will have a leak and a potential backfire.

The only way I got my bike to stop the backfire compleatly after the new #55 pilot was to turn out the mix screw. I reworked all the joints in the intake and the exhaust, turn my mix back and forth 8 thousand times and it still poped at the shift. Finally I got mad turned it out to 3 and no more pop. Since then I have been working it back in. It now sets comfortably at 2.75 turns out. But keep in mind I am running an open 1.75"OD pipe and the stock air filter in the stock air box with my 152.5 main, #55 pilot, and 1/2 spacer.

LANCER wrote on 12/31/69 at 16:00:06:


NO!

Okay- don't laugh, but here's a quick and dirty check for exhaust leaks. (Mine leaks)......
Start it up cold-  within the 1st minute of running plug the end of the exhaust pipe with your hand as it's idling. (The pipe shouldn't get too hot yet.) You will hear hissing/leaking noises from wherever the leak is; or, you'll hear nothing and your engine will slow down and die. That's how I found several pinhole rust leaks in the the stock muffler. Hope this helps.

To rich can also cause a throttle off backfire.  I often ride from 2600ft where I live, to 9000ft.  The thinner air up there causes a richer mix and a very noticable corresponding increase in backfires. I noticed the same thing with my yamaha 225 serow and cured it by decreasing jet size in it from 130# main to 127.5#.
Your needing no choke when cold is a tell that your on the rich side.
 I know this is against common logic, but it did work for me.  A lot of people don't have the advantage of a 7000ft elevation change to try this, but if you do it can be a useful tuning tool.

A properly done weld WILL seal that puppy right up ... permanently.   Whenever possible, I weld.