Every time I change gears and it went high RPM then back to normal. Hard to explain.
Just got new crab and it so much better!

That sounds like/was a vacuum leak

How will I know there a vacuum leak?

Spray carb cleaner on all the mating surfaces/potential vacuum leaks, the RPM's will go faster if you spray carb cleaner on a vac leak.

WS-40 works also and doesn't screw up the paint.

Slipping clutch?

ok I think find little leak and got a better plug vacuum plug. Anyway, Just got new crab. and it keep me get backfire once i speed up fast on the road then slow down and it cause backfire. I think it need crab adj ? any idea?

James:
If your getting a backfire just after you let off the gas, check your exhaust for cracks. It should be new since I got your old one but I had that problem with my 73 and it drove me crazy looking for it. Found the crack while pulling the motor, welded and backfire went away. Many other things it could be but something to check.

don't see any exhaust leak cuz it new. Is there something do with the new 30/31 crab.? It happen when i got new crab.

Sounds like to me the carb needs to be adjusted. I had a 34 Pic that would rev up when you let off the gas. Check the return spring and give the throttle cable just a hair of slack so it ain't super tight. As for the back fire it may be running a little fat id have to hear it.