I'm an amsoil guy myself.. Never been a fan of the castor based stuff but many people use it... I had a squeak as you described myself once, turned out to be the belt rubbing the clutch... Take the belt off and try it... If not, it could be some compression bleeding back through the reeds... Mine runs about 175psi compression and I can get by with 92 octane pump gas so I would say straight racing fuel may not be needed... Try the 50/50 and listen for detonation... Sounds like bb's bouncing off a piece of glass...