This may be an odd reply but what type of propellor are you using. Is it constuction of wood or composite? The rotational weight of the propellor acts as a groscope. The airframe is attached to this gyroscope and its natural reaction is to reacts in the opposite reaction. This is the reason why engine offset is set into the firewall. Switching to a lighter propellor will decrease the rotational mass and lessen the adverse reaction.