Re: throttle question
My guess would be that you drilled the hole too close to the end of the handlebar. That is allowing the twist grip to move in and out. The outer end of the grip should be just touching the end of the bar when you measure for where to drill the hole. Give it no more than 1/8" clearance then drill.
However you say you replaced the grip with one from a motorcycle. It might be too long.
As for your carburetor. "stock" doesn't tell me much. CNS? NT? Speed? RT? See? There are several carbs and they all have different throttle mechanisms. Again we need specific information, or a photo would help.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"