Hey guys I had some problems with my OS91 FX. It cuts off in WOT. I have a pitts muffler. What I did was replace the fuel line pick up line with a bigger line and a bigger clunk. I also replace the fuel line from the tank to the needle housing with a bigger line and left the other line alone thats going to the carb. I use to plug one of the exhaust coming out of the muffler so I have enough back pressure to the fuel tank. Since I replace the fuel line that fixed my fuel starbation at WOT and I removed the plug on the exhaust port also and richen my needle valve. Right now I got 4 turns from fully close position on the needle velve and I'm running 15x7 APC. I tach it at around 9200 rpm. I'm happy with it.