WB R4 muffler backfiring

I have a friend who has an R4 muffler on his 01YZ426, it is otherwise stock including the header. We are in CO, 5000 ft ASL, 70-100 degrees F. My stock 01YZ426 runs great with 155 main, stock needle, 40 pilot, 1.25 turns out on the fuel screw. I had severe plug fouling problems this winter but a new CDI fixed it.

His bike runs strong in mid and top but it has bad decelleration backfiring. Throttle response is not great at low throttle openings. Usually this would mean too lean, but when we go up on the pilot or fuel screw the backfiring gets worse. He has swapped his CDI, no big change. His air filter is pretty dry, pretty sure its not the filter oil in the carb issue. I can't find any carb boot air leaks. We took out his accel pump diaphragm and cleaned it, no visible dirt. His plugs are sooty black everywhere, and he occasionally fouls one. He is a fast rider on an open MX track, the bike is not being lugged at low RPM.

Right now the bike has a 152 main, one clip position up on the stock needle, and a 38 pilot. We have tried a 45, 42, 40 and 38 pilot with various fuel positions, no improvement. He thinks it runs best with the 38. I am bugging him to put back on the stock muffler, he is reluctant but we will do this next time.

Any ideas? Could an air leak in the muffler to header cause this problem? Anyone else with an R4 having this problem?

I have a power core IV on my 01 and it was backfiring tons when letting off the gas but I had no sluggish problems down low either. I put back the stock muffler ;my seat of the paints dyno tells me it is actually quicker stock. It is amazing how all these bikes run diffrent. I run down at Berthoud and go trail riding up at Rampart (sp)so I know all about the elevation problem here in CO.

sleekfreak: What pilot jet and fuel screw turns out are you running? If you turn your fuel screw out more, what happens? Does this help the backfiring (it does on my stock 426)?

To be honest I dont have a freaking clue. When I bought the bike the dealer said they already adjusted it for elevation and for some dumb reason I never asked what they set it too and I have never looked( and wouldnt really know if I did) . I have lots of seat time and never had any real problems with fouling plugs or anything else so I never bothered with it. Just changed the sprockets and chain and let her rip switching back and forth between the PC IV and the stock unit. Between maintance on 2 street bikes and two dirt bikes a person runs out of time messing with things when they are fine you know? I really need to pull the carb and clean it. I will get to that next week and let you know. We are riding Rampart tomorrow and I hate to get into something and not have it back togther in time.

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