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.
