Hi!
8500rpm is a bit low! You should get around 9500-10000rpm if the engine is Ok.
You said you haven't run the engine for 6 years...!? If this is correct I think I know what the problem is. Have you checked the ball bearings?
A hiatus of 6 years is not good for an engine which has been run on Castor oil. This usually means rust and lots of yellow/orange oxidized Castor oil in the ball bearings.
I would take the engine apart and have a look at the bearings if I were you.