Hanging rpm

[Frostbite]

Hi guys, I've got a problem with my Derbi (Senda R Xtreme 2066). When I'm riding it and just getting into the power band, and I let go of the throttle, it hangs in RPMs. It does not happen when in neutral. I've already checked for an air leak but couldn't find anything. I've tried new clutch plates because I thought it might be due to sticky plates, but that also didn't do anything..

Here is the setup and thins ive already done:
Nieuw arsal 50cc
Delorto 17.5mm
Main jet 80
Pilot jet 34
Nieuw manifold
Nieuw manifold rubber
Nieuw crank seals
Nieuw airfilter
Nieuw clutch plates
Nieuw ignition with Nieuw spark plug

Personally, I think it's the exhaust that's the problem. Feel free to give me ideas of what I can do, and I will tell you if I've already done it.