[morzh]

I was told I do not need to install that hub by the mfr of the idler.

So, I put the flywheel in....does not go up to speed.
Wiped inner rim with alcohol, as I saw old traces of rubber on it.
Suddenly it works.

Now there is light knocking when the motor runs the stepped pulley, whether with flywheel disk or not. No knocking when the motor runs alone.

Time to borrow my wife's stethoscope (the automotive would be better as it has that long pin, but beggrars can't be choosers).