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?