Yea if you sanded the center section your definatly gona need to true it up now. I dont know what kind of prop adapter your using but they one I am using is the stud that screws onto the shaft as mine is a short shaft engine. when it comes up tight against the flywheel if it is not concentric it will push the shaft of to one side so when it rotates it will look like the shaft has a wobble to it.

We found that merely chucking up the flywhel in a lathe and cutting the fins of and facing the center section did not ensure a concentric cut either. this is due to the fact that the back of the casting where is sets in the chuck is not concentric to the tapered hole.

Ok so what we did to get everything back in line was to take an old crankshaft and knock the counerweights off the shaft. what you have left is a shaft with the taper on one end where theflywheel mounts. install the flywheel on the shaft then chuck it up in the lathe. leave it sticking out far enough where you can face off the back side of the flywheel. once the backside is faced off the shaft can be removed and the flywheel chucked up in the lathe with the back up against the chuck face and all will be concentric when you face off the fins and center section of the flywheel.

I undestand you dont have a lathe. you still may need to have this done. check the local community college. they may have a machinist program where they can do that for you. usually it is no charge as they just need the practice.