Just installed mine, haven't fired it up yet. I have the jack shaft which tends to push the motor out from the seat post. That being said I still needed to add a 1/4 spacer to the top "mostly for chain tension and to make the bottom mount tight to the frame". But it did help with clearance.
It was still very close at that point. The air filter was 1/8" from the frame. I learned my lesson with the sick bike air filter "that's to close". So I cut part of the rubber boot that attaches to the carb "about 1/4". It appears to be a little long and doesn't allow the air filter to rest tight against the carb. So you can get about a 1/4 from trimming that part and likely end up with a better seal.
Other then that, I've read that some people will make a 90 to attach the carb/air filter. Which should give you clearance, although does get in the way sometimes with pedaling.