Your engine is loading with fuel at low idle, thats why it only stumbles after long periods of low idle. The fix is simply turning in the low needle until it will idle low for a minute or more with only a minimum of stumbling when the throttle is opened. The other part of your problem is that the carb bore is too large for the rpm you are turning. Find a smaller carb or sleeve down the stock one until the peak power comes down 100 rpm or more, at this point muffler pressure will not be needed.