Originally Posted by lmsport
Was it a balanced assembly?
Scat rough balances all their cranks but the assembly must be balanced to the rod-piston-flywheel at some point.
Are you sure it's externally balanced? Assuming the crank, rods and pistons weren't balanced as an assembly? A 350 would be internally balanced OEM. You say you put on the old balancer, which should have been non-weighted for an internally balanced motor if it was stock, for an external balance you would have used a 400 balancer with the balancing weight adjustment "lump" cast into it. I'm assuming the flexplate or flywheel is externally balanced, has weights? Just curious if the sum of all the parts are compatible, and properly externally balanced. Odd that it's only vibrating within a 300 rpm range.