Re: noob to the motorized bicycle scene
50 mph is an unreasonable expectation. It isn't impossible but improbable to expect that kind of speed from a basically stock engine. 35 to 40 is about tops unless you want to spend some serious money and possibly compromise the engine's life.
These are motor assisted bicycles, not motorcycles.
As for your other problems, adjusting idle speed when hot, it sounds like you might have an air (vacuum) leak. Check your carburetor to intake manifold connection and the intake manifold gasket to cylinder. Once adjusted you shouldn't have to readjust the idle speed if everything else is good.
What fuel/oil ratio are you running? The clip position on the needle only effects fuel flow up to about 3/4 throttle. Anything above that, flow is controlled by main jet orifice size. The needle is raised out of the jet after 3/4 throttle. If your plug is black and oily I feel your oil ratio is probably too high. You shouldn't be higher than 24:1. If you are following the instructions that came with your engine and running 16:1, that's most of your problem.
Don't expect the engine to perform to its maximum potential until you have 2 to 300 miles on it. These engines are slow to break in and need some time to seat the rings and gain max compression.
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Over Youth and Skill & "Charlie Don't Ride"