I have a 99 sportster 883 with the 1200 upgrade kit. I got the bike last year and rode it only a few times but noticed exhaust didn't seem in sync. Took it out today and after it got warm it started backfiring from the rear cylinder. Like a caddy shack backfire. Its loud. Now I can listen to the exhaust and the front and rear don't sound even similar. Front is almost rhythmic but rear one is all over. It kinda sounds like there is a suction noise from pipe when running. It only has one coil so its not spark and I put 2 sets of plugs in. I only run high octane fuel and I stored it with seafoam. Took carb out and cleaned anyway. Still does it. I am thinking maybe I have a hung exhaust valve any ideas. Thanks for any info.

 

Update. I took rocker box off and put hundred psi into cylinder and it held so where do I look now?

 

how did you do this.... with a cylinder leak down tester? or just by putting pressure in the cylinder?

what does the plug look like vs the front?

Is the intake manifold leaking in air on that cylinder?

 

It was done with a leak down tester to measure the percentage difference. The plugs both looked good. Clean with very little soot or ash on them. Also I replaced them just to be on the safe side and the problem remained. I am also getting a new exhaust seal for the rear jug although I don't think that's the issue either. I also adjusted the timing with the module and no change and as it has an electronic ignition I also tested the sensor that measures engine vacuum. I am at a real loss here and I think I am just running in circles. I called the local harley dealer that put the 1200 kit in last year and they said to bring it in and pay the hourly for diagnosis. Thats not gonna happen.

 

has anyone done anything with the cams??if one of the rear cams is off a tooth it can cause this kind of weird stuff to happen....also, I don't know what exhaust you have but if the rear exhaust is shorter than the front or has a different type of baffle etc. it can cause some issues like you describe...

 

I was told it had a 1200 kit put in are cams changed for that? Also exhaust is after market and pipes are same length so the don't end at same distance . is there a way to check can timing other than total disassembled

 

What's changed since the last time you rode the bike?

 

Nothing changed. It did it last season too but I only rode it twice after I got it

 

To add I am also getting some fuel out of the breather. Its not a lot but certainly fuel after she warms up

 

May sound stupid, but make sure your battery cables are tight on both ends.