I bought a 50cc Yamaha Zuma off of craigslist, and it's had some aftermarket parts put on it (exhaust, 70cc big bore kit, new carb). I cleaned the carb and converted the throttle to a one cable set-up when I bought it.
I'm still having trouble getting it started reliably. Once and a while it won't start unless I remove the air filter (accessible through the same panel that covers the battery) and cover the air-intake with my hand. Then, I can slowly remove my hand while holding the starter and it will run fine.
I believe I'm effectively 'manually' choking the carb. The carb has two adjustment screws: 1) controls the minimum clearance of the slide (I believe this is the 'Idle Mixture Screw'), 2) controls the fuel/air mixture.
I have tried various combinations of these screws and find that letting screw #1 out almost all the way (so that the slide leaves around 1.5mm of clearance when resting), and keeping #2 about two turns from tight has the best results––even still, it still sometimes won't start without a 'manual choke'.
The air filter is a canister and is also aftermarket. My thoughts are that maybe the air filter is letting in too much air? Another thought is that the oil is too thick. Anyone have any better ideas?