HARRI: I too have built a GT550 with mild porting, race pipes and a Neutronics ignition. I also had lots of trouble with fouling the left cylinder. Make sure you have the fuel valve turned to "ON" or "RESERVE" not "PRIME". The circle handle thing should be directly in front of the setting you want. My problem was caused by the valve being on prime and having bad needle and seat float valves which allowed fuel to flood the carb and cylinder. #110 main jets should not be too big. I have tried each size through 115 so far and still need to go larger. I also run K&N filters. I didn't know I needed to go richer until running on the track at 7000 to 8000 rpm for several laps. I tried advancing the timing from 24 deg. BTDC to 27 deg. BTDC. This seemed to give more power in low to mid range, but the bike seemed to run hotter. Since I was too lean anyway, I set timing back to 24 deg. Plug gap should be between 0.6 and 0.7 mm. I use a B7ES for start up and B8ES during the race. Check your needle valve and float level to stop that cylinder from flooding. Good Luck.