Man, old topic.
Unless a bike has grips like the leather ones all my builds get BMX foam grips like these.
That is enough to do 3 bikes for $12 at my local bike shop.
There is a trick to doing the throttle with them:
1. Get the white throttle barrel with the crap hard plastic grip out of the throttle body and cut the grip off, a box cutter works great.
2. Cut your BMX grip to the length of the throttle barrel and stick it in some warm soapy water.
3. Put the throttle barrel vertically on the floor and with a dry wash rag milk the foam grip down on to the barrel with both hands, one hand pulling from the front, the other pushing from the back.
4. With a Q-tip put a light smear of light grease in the throttle housing where the plastic cable stay of the the throttle barrel will rub against it and a nice coating on the handlebar where it will turn.
Bottom line is, when you let go of the throttle it should snap back instantly to no throttle every time, if not then in this shop anyway it still needs work.
When out riding on my 66cc I'll be going along at a gentle slow speed then a ghost takes over and the throttle turns all by its self speeding up and I have to twist it back hard to make it not do so.
That could just be poor fuel flow as when your gas/air mix get leaner like when you turn your gas off while it is idling it revs up before it dies but not likely.
I suggest you just undue any throttle zip ties so your cable is flopping around loose and check all the adjustment points as if your cable is too slack the end of the outer casing can catch on the outside of the adjuster fittings on either end and keep it engaged.
NOTE: Never put your throttle cable under the gas tank mount plates!
There should be just a tiny bit of slack in the cable with the throttle off so the idle adjustment has the final say of throttle.
If your throttle does not snap back when released read the above, when it does check it as you zip tie the cables back in place.
I hope that helps.