CRF450X Carb problems?

[RoadRacerX]

2006 CRF450X with quickshot accelerator pump and FMF muffler can just stumbles terribly when I whack the throttle open. Rolling it open smoothly doesn't have this problem. I thought there might be dirt in the float bowl or gunk in the carb. I removed carb, took it apart, cleaned and re-assembled. Same result. Anybody else experience this? Any ideas?

 

[Damascus]

Sounds like your accelerator pump is not tuned correctly. Did you re-jet the bike when you installed your exhaust? Better-flowing, aftermarket exhaust systems almost always require jetting changes.
The accelerator pump cover shouldn't change your jetting, as the cover (if I remember correctly) doesn't alter the amount or duration of spray, it just holds more fuel, to keep the AP primed and ready at all times.
Does your 450 have a leak jet? Most 450's don't, that I have seen, which is a GOOD thing LOL, getting my WR250F dialed in was a PITA because of those leak jets!

Ride your bike around a few minutes, to get the carb fully primed, then stop and take it loose. Keep the fuel line connected, but pull the rear of the carb away from the airboot, so you can see inside it, looking at the airbox side of the slide. Blip the throttle quickly, you should see a jet of fuel squirt out several feet and last anywhere from .5 to a few seconds.

After you confirm you are getting a pump squirt, we'll go from there.