New to me carbureted 660: Smothers down at about 1/2 throttle with air filter installed. I removed the filter and it scoots without it. As far as I know, the engine is stock. I cleaned and oiled the filter, then washed the oil out thinking there was too much oil. I've read posts about re-jetting the carb, but I can't understand why it would be necessary. Any advice and explanations sincerely appreciated.