XRAY - Model racing cars

I have just completed the break-In of an OS 12XZ, using the Hudy Break-In bench. This engine runs crisp and clean with no lag from 0 to 60% throttle. After 60% there seems to be drop off in the engine performance where the RPMs stay the same or even drop slightly. I've tried leaning the high end needle as well as richen. This engine also has a mid range which I have tried both lean and rich. Past 50% does the slide needle still play a part? Temps started at 200 and have dropped to between 160 -170 max (take in account of outside temp) but she is very thirsty. One 125cc tank on the Hudy stand runs 10 min at 50% throttle. For ventilation this is setup in my garage. The outside temp is 45 @ 35% humidity.

Can anyone help me here?

I'm in Maryland. It to cold to go out with temps off 30 degrees and lots of snow . All the running has been on the Hudy stand on the bench, never in the NT1. With the break-in stand the Air Filter is off, so the slide is visible. There is a manual linkage to the throttle. If the engine is rich, you're saying adjust only the High speed needle. What increments do you move the high end needle 1/8, 1/4 etc...? How long does the adjust take to show up?

Is there anyway the glow plug can be read to determine if she is too rich?
I'm sure the exterior temp also contributes to the settings, the outside temp where the bench is was around 45 degrees - yes or no?

Thanks for taking the time too help.