I had a carb rebuilt and it starts great idles great real responsive while it's in park but when you go to drive it it has a terrible dead spot at about 2000-2500 rpm that will kill the engine if you don't let off it and it deads out again at about 4000 rpm. I know very little about carbs so any ideas what it could be? it's a vacume secondary 750.

Do you have a vacuum gauge on it?
changed the sec vac spring?
What power valve is in it?
acc pump cam?squirter?
have you adjusted any of the above?

I'd start by adjusting the float level(s) and check that the accelerator pump squirt amt/duration appears to be adequate

Did you check to see if anyone put the sec spring back into the vac pot? Also if the older model check ball is removed it will bog with a real light spring.

Id start by contacting the guy who built it before messing with anyrhing