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...........

First thing you may want to check is the position of the fuel tank wrt the carb. If the model is a taildragger, and the tank is level with the carb on the ground, it may go too rich in level flight. It could be that both your low end idle is too rich and your high end too rich in level flight. Hold the plane level at full throttle and point the nose up ~45 degrees. Does it speed up? Point the nose down ~45 degrees, does it go rich? If so, your high end is too rich. Set low end idle so it is reliable in level flight, limiting low end idle screw adjustments to 1/8 turn at a time. Remember that every time you set the low idle, you also must re-set the high.
Also, screw out the needle valve, make sure it is not bent at the tip.
I had one that not one of the engine gurus could idle. I finally closed the throttle, hooked up some fuel tubing so I could blow through the needle valve, and opened it until I could just hear the air start to leak out. The open 1/8 turn more. After that, I reset the high end and have good idle and top end since, and have never touched the low end screw. OS's seem to idle well under all conditions on one setting.. Make sure the top end is several hundred rpm rich from the max rpm setting.
The 70's I have are the most reliable, trouble free and hands off engines I have owned. After run oil is a must, never run lean, they will last a long time.
Good luck,

It's an easy sport 40 with tri-gear. The tank center line is just below the carb needle.

Try a new glow plug. OS "F" Good place to start. Also check for air leak in the fuel system.