Make sure the butterfly is parallel to the air flow at wide open. If you go over center that will do it. Second, is the high needle set properly? Open it up a little and see if the problem goes away, might it be leaning out? Be sure your gaskets on the intake are good. Although an air leak sould not mess with the top end as much as the idle it is worth confirming. Ignition, set it for 28 DBTDC. Be sure you are finding TDC accurately by either the dead stop method or a dial indicator methods. Since it is runnig you could play with the timing as see what the tach has to say about it. Could your spark plug gap be too wide? IF it is missing over a certain RPM close the gap a bit. Your batteries are fully charged?

Only after it is running right mess with the radio. You will have to set the end points and throws. You will be loosing the idle screw, it serves no purpose in a model installation. Keep the return spring. If it is to powerfull weaken it. You want it though. It acts to give you zero linkage play. It does not have anyting to do with whether the engines runs properly or not.

My homie 25 runs fine with the larger carb and the smaller one too. It runs smoother with the 11 mm carb.