I recently rebuilt my 8hp Yamaha 2 stroke of the same vintage.
What I found was extensive corrosion
in the joint between the crankcase and lower unit, and also where the exhaust
manifold attaches to the crankcase. A previous owner had attempted a repair with about a 1lb of bondo to fill in the gaps. Several bolts were siezed and had broken off in the crankcase casting. The crankcase was a write-off.
However, all was not lost
. The crankcase casting was available for $350, so I bought a brand new one.
The other issue I found was that the lower crankshaft bearing was noisy and worn, and quite rusty looking. This one gets damp, I think, from proximity to the exhaust
More good news : the crankshaft bearings are pressed-on roller bearings. No machining required to change them, just a press.
As I was getting a new crankcase (with new honing), I changed the piston rings.
I took the opportunity to change the outboard
from a 6SF to 8SF, which means changing the carb and exhaust manifold.
If you have a lot of noise
, I would suspect either exhaust leaks
from the manifold / crankcase joint, or worn main bearings.
Total cost for crankcase, rings, every gasket
in the outboard, carburettor, exhaust manifold, thermostat, bearings, about $800.
I now have the only new 2 stroke in the US! It runs like an absolute champ now.