Hello all. Maybe someone can help me. I've got an issue with backfire when getting on the gas. Even happens when revving sitting still. Will almost stall out when revving, idle will come back with a slower idle sorta then return normal. I have a stock carb not sure the year but the slide needle is fixed, main jet was 145 and pilot was 52.5. Per the manual it looks like the 2nd carb they started using. I've changed the main jet to 155, pilot the same ( think I might want to go to a 55 ) Put an extra washer ( 2 times the size of original ) plus the original on the slide. I've got the mixture screw out about 3 turns. I think it's running lean. Haven't done a plug chop or looked at the spark plug yet, but it seems lean. The super trapp doesn't have a baffle, it's not wide open completely but it's missing something in the middle, at least that's what a buddy told me. I'm thinking maybe up the needle a little more and maybe the pilot size and main jet size. I can't really get on the gas when riding and the backfire is ridiculously loud and annoying.

Thanks Batman!

If the engine is popping and backfiring when you are opening the throttle....then you have an ignition of fuel delivery problem.  There really is no reason your bike won't run well with a #150 main and a #52.5 pilot jet with the stock air box and air filter.  Removing the white spacer on the needle and using two or three #3 machine screw washers raises the needle a bit, and it corrects for a slightly lean condition that can occur at low throttle settings....the engine tends to surge a little bit when you use light throttle settings.  (The white spacer is on top of the needle and making it thinner raises the needle and richens the mixture).  

If the backfiring occurs when you have closed the throttle and the engine is slowing down........you are forcing your engine into a situation where it will always be noisy and backfire.  This really isn't an engine that like to be revved like a fuel injected sport bike.  The backfire is not nearly as noticeable with the stock muffler.....it is far more noticeable when you put on a less restrictive and louder muffler.  A Supertrapp works fine on this bike when it is complete - if somebody has taken off the back plate and turned this muffler into a "glass pack", that is not an ideal situation for this engine and it may be impossible to get the carb tuned properly.

When the engine is at higher rpm and you shut the throttle completely - the butterfly closes, the slide and needle drops - so you have closed off all of the fuel supply except for the small idle circuit.  The engine creates a high vacuum - and it pulls a lot of air past the closed slide and closed butterfly....so the mixture goes really lean the fuel/air mixture cannot be ignited by the spark plug.  The unburned fuel goes into the exhaust system where it can be ignited by the hot header/muffler....the result is a lot of popping and banging.  (The Transient Enrichment Valve (TEV) is supposed to provide additional fuel to correct for this lean fuel/air condition

You should jet your carb for how the engine runs when you are opening the throttle - not for how much noise it makes when you shut the throttle.   There really is no good reason for you to be using #155 or #55 jets.......something is wrong with your carb (did you put the spring on the bottom and the spacer on the top of the needle when you put it together)? If you carb is correct than it sounds like there is something wrong with your ignition or your muffler setup.

The situation where the bike stalls....or almost stalls when going back to idle is a continuation of the "too lean mixture" as the engine is slowing down from being revved.....and it can also be a sign of an idle speed that is set too low - raising the idle speed a bit can help correct this condition.

Andrmon ,some owners at sea level have gone to a 55pilot and a 155 main jet ,but it sounds like the muffler being to open should be the next thing to deal before trying larger jets.

Had some aluminum laying around. Cut and drilled it. Started with 3 holes but it created too much back pressure. Still popping/backfiring but not as loud now. I've thoroughly cleaned the carb including that TEV, diaphragm was good. Kinda thinking the diaphragm is worn on the carb and maybe a 55 pilot would help. Better now but definitely excessive. Pic of the mod I did.

21 plates, can't get them off because one of the bolts just spins, another broke when trying to take off. I really do believe it's the carb and not so much the exhaust. I have a stock muffler, put it on and was still getting the same results except you couldn't hear it as much. I think the diaphragm is beat on the slide. No noticeable holes but it's rather worn and stretched. I think it isn't lifting high enough when accelerating or giving throttle when sitting.

I have no experience with the supertrapp  , but if Vercy said it will run with six plates and a flat plate as a cap, then the 21 plates and 9 holes you're trying to run is like a straight pipe. We know that doesn't work . I'd follow his advise.

Thanks for the help guys, I'm over the supertrapp, I found a cheap replacement that definitely works much better and is baffled but still not normal popping going on. When I decide to do the valve adjustment I'll dig into the carb again. Something isn't right and haven't figured out what just yet.