Backfiring on idle only when hot

N15cs with new Anal concentrics has been running great for 4000 miles now has been developing a problem. After it gets hot it spits and backfires through the exhaust on the right side only. Still runs great as soon as it comes off idle. Adjustments to the idle air don't seem to help. I understand that the pilot air may be plugged but before I remove and dismantle the carbs I was wondering if anyone else has run into this situation. What I don't get is why it only happens after it gets hot. For the first several miles, it idles perfectly. Could it be something in the ignition system? I already swapped plugs and plug wires are new.

So tell us, what ignition does it have ?

I'd also maybe suspect an air leak into the exhaust, maybe as things expand and get hot.
I'd have that side off, and carefully examine the seal in the exhaust port,
and where the muffler meets the exhaust pipe.
It doesn't take much of a hole to allow air in, and make some popping noises.

Other thing to try is to either tickle the problem side when its happening (watch the road), and see if :
- it can be tickled a lot, which may indicate a low fuel level in that carb, for whatever reason.
- if adding more fuel to that side makes the backfires go away, or makes it worse.

Failing that, a blocked idle jet could do this also.
Can you make it idle on just this cylinder, down to real low rpms. ?
Remember, it can take 3 or 4 cleans to get that idle jet system really clean...

I had an old engine with really worn valve guides, and new guides made something similar go away.
Something to think about, not knowing the history of your bike.
But this was accompanied by loads of blue smoke on startup, so was rather obvious....
Have fun !

Thanks for the reply. You hit the nail on the head! I first checked the exhaust since it was the easiest fix, not expecting much. The flange wasn't loose but I didn't have to fight it much to get it loose like usual. I cleaned off some carbon deposits on the bearing surface and filed it smooth and problem solved! Runs and idles great when hot. Sometimes it is the simplest things. Thanks for the insight.