Have recently encountered surging with throttle off either at idle or backing off at speed. It seems to be getting worse by the day (have to keep my finger close to the kill switch). It starts O.K. and will idle after warmed up. Have checked venting, fuel flow, float level, throttle cable, spark plug (good color). This bike was running fine until recently. It is if a shot of gas is getting past the carb. after backing off the throttle. Have checked for vacuum leaks on carb. but nothing found. The bike will not start when hot until I pull out the choke completely and push it back in. Any clues?