smaller throttle butterfly
 

I am going to machine throttle body to 62mm, and will keep the old butterfly. so butterfly is 2mm smaller. I know the idle rpm goes higher, and will need to tune ecu. question is anything go wrong more than that?

Why would not get a new butterfly?

You'll need a new butterflies to match the bore. Otherwise, you'll have a shitload of headaches. You'll have a 1mm gap around the old butterflies. No way the tune that huge air leak out.

The idle will be like 4k haha