Well uhh. I have swapped a shitty exhaust I have onto my 426 while i work on the good one. It doesn't have a gasket between the head pipe and the rear pipe. I was riding at night the other night and when I let off I looked down to see a foot of blue flames shooting forward towards the radiator. Im not kidding, a foot of blue fire. Now my bike backfires a lot...not sure if its supposed to. maybe its running too rich. Any of your bikes do that? I guarantee it would make it out the back if it was sealed.