Knife edge the butterfly and fill and smooth the screws in the butterfly shaft while you're at it.
nick
Don't.
I learnt the hard way. Scrappy hunting tomorrow for a new one 
As DH said, it wont make much difference if you knife edge it, yet it can cause so much trouble.
Yes if you do it badly and try on a spare one not one you actually need! I've done this to loads of carbs and 2 of my MKII golfs but always picked up a spare TB or carb and done it on that then fitted it. I need to pick up one for the current MKII to do.
nick
Turn a new one with a 30 or 45 degree angle on it
