The motorcycle can't accelerate between 4000 and 5000 revolutions.

It should still be the carburetor. In order to control emissions, the fuel needle of carburetor throttle is strictly controlled, which leads to the shortage of fuel in some cars, but when the speed is lower or higher, the fuel supply increases and they don't run. You can try to raise the oil needle slightly, for example, by adding a thin gasket under the oil needle. If there is improvement, you can try riding for a while, as long as there are no other abnormalities.