Sounds like a lean condition. A lean condition is certain death for a 2 stroke engine. Check for leaks with a can of carb cleaner by spraying around areas suspected of a leak. Intake, Carb, Head gasket, intake at cylinder.. Spray in those areas and see how it affects your idle. If there is a leak, it will slow the motor down as it sucks in the Carb cleaner. IF you find a leak or leaks, fix it right away or you will burn your motor up. If you find no leaks, then you need to raise you carb needle 2 slots. Hope this helps... Enjoy the ride...
For those who haven't had much experience with two cycle engines or motorcycles that have them...........they are quite radically effected by air leaks. A four cycle engine isn't effected quite the same.
It's a common scene when you are around a cycle event where 2 cycle engines
are common to hear the things start running away with themselves like the carb is stuck wide open. An air leak somewhere will do it almost every time.
It is something to become familiar with if you are younger builder/rider as sooner or later you'll experience this situation. Or you'll be around a paniced owner that this is happening to for the first time. But ya gotta locate the leak and shut the engine off in order to tighten things up. (and beware of tightening things on a hot aluminum engine as it doesn't take too much to do the trick) With so many of the motorizing kits coming in from China being two cycle it would make sense to learn about this as soon as possible after you get your bike together.
Where exactly is it leaking? On a couple of mine, the cuts in the carb were too deep and when I slid the carb on, it went on until it stopped, but the cuts were still allowing air to be sucked in. I ground them off with a belt sander. If you have to do that, you can use a file, dremel or ??? to do it. No silicone RTV needed.