It sounds exactly like the problem I was having with my carb. MY quad only had a DG pipe on it, but it turned out the low speed jet was way undersized. It would no idle or rev. If you could catch it in the higher RPM's to get the thing to run on the main jet, it would be fine. I would try going up a few sizes in your jet and see what happens. I ultimately fixed all my carb problems, and got a performance boost, by switching to a 28mm flat slide carb.

 

What I did was get the first quad (QUAD A) running, and put the pipe on it. It ran good.

I then took (QUAD A) carb to my place where (QUAD B) was and put it on that quad. It ran fine just the same, pretty good.

I then took the screwy carb from (QUAD B) back to my other place, rebuilt it like the first time and put it onto the quad at home (QUAD A).

So my dillema is, if it ran fine with the pipe on it and the carb rebuilt, why would it run any different with this one? Same exact carb, Same exact quad. (they are twins)

 

I'd start looking into engine problems. Cam jumped a tooth or something, valves leaking/out of adjustment, ect.