I don't quite know where the needles are set at exactly but here is where to start from. First thing you need is a tach. Use the tach to set the high speed needle at wide open to the highest rpm setting that you can get. You have to turn the needle in in very small amounts. Let the engine set for about 10 seconds after each setting change. Once you get the highest RPM setting back off the needle about 400 RPM's ( about 4 or 5 clicks). The way to get the low speed needle set is to unhook your onboard glow. Use a regular glow starter and start you engine. When you pull the glow starter off does it quit. If it does lean the low speed needle in small amounts until you can take the glow starter off and the engine will still run. Once you get to this point you are really close to having it right. It will be right when you can pull the glow starter off and it does not loose any RPM. Once you get the engine to this point rapidly advance the throttle to wide open. The engine should transition smoothly. If it bogs it is too rich. Adjust the low speed needle in very small amounts( 1/16th of a turn). Once you get the low speed set right reset the high speed needle. The low speed needles on these engines are set very rich from the factory and have to be leaned alot. It sounds like your engine is running lean on the high speed needle. If it has good power when you take off and then looses power after a couple minutes it is running lean. I had the same problem with mine also and just had to tinker with it until I got it right. Once you get it right you will love it. If you haven't checked the valves yet do it now. I just set my valve clearance for the first time. This engine rocks now and runs excellent and makes great power. You need to run a least a gallon of fuel through it before it starts to get good. These engines take some break in time but run great once broken in.

Rick