Hi having problems , just put on new throttle tube , engine is reving  it's tits off  ,just a straight swap over checked everything in place , revs to high . Then slows down far to long to tick over , throttle is snapping back on forth ok but takes a while to come down to idle. Any ideas as not got a clue , new cable maybe ? 

Check the cabe at the top of the carb.

Air leak.

take cable out and start again. Ensure cable and plastic clip are where they should be in the slide. Hear the slide bang in the bottom of card. Ensure free play (small amount) on throttle.

Dont forget that if it was perfect before and you touched this and that, look for problem there.

Check to see if there is no molding ridge on the new plastic throttle tube at the pint were rests at off position against the clamp...right were the cable end fits...may need to clean up with a knife..