I have a OS 70 four stroke that is having some problems with the top end performance. I am running a 12-8 prop with 15% omega fuel with good idle and transition. On the ground it seems to run normal but in the air it seems to bog down at 2/3 to full throttle. When I climb hard it seems to regain the power like it is running on the rich side but straight and level it bogs down past 2/3 throttle. I don't know what type of plug it has . Looks like a fox plug. What is the problem...........