hi always ran my 91fx on 10 or 15 % morgan omega pinkwith castor oil- no problem i do use a k& B 1L GLO PLUG-I WILL be trying a four cycle plug in the future-do think about changing the "o " ring that is on your needle valve that was my problem at one time it looked ok but when replaced the engine did start to not have air in the fuel line just before the carb at mid rangedo the pinch test just before the carb on the low end and the top endthe RPMS should increase when the line is pinched in both places if you are rich in both places