The power piston is staked at the top so that it doesn't come out. Use an awl to scratch the staking off, then flick the pisto
A couple of things come to mind.
Did you test the float?
You probably have the nitrophyl float which should be 14.5 grams.
Here is what I suspect.
The float is absorbing fuel.
When you had it apart, the float dried out.
It slowly absorbed fuel again.
The carburetor may also have gotten dirt in the fuel.
When you take the top off, look at the bottom of the bowl for any sign of dirt.
If there is dirt, then make sure the fuel filter is replaced.
The gas tank could also be dirty.