I am running my 52 with the same fuel, Thundr Bolt plug(R/C long), MS 11X7 prop. I would run the 12.25X3.75 execpt the prop would be under ground on my Global Lucky Stik. I had a problem with transition just like you are saying. The low end needle was set at 3/4 of a turn out. I leaned the low end a touch so I am about half way between 1/2 and 3/4 of a turn out. I can still pinch the fuel line and it will run for 4-5 seconds before it starts to change RPM.

I have found the needles to be very sensitive to adjustments. The low end, when changed from 3/4 to 1 full turn out will cause the engine to die when going form idle to full power. To me, it sounds like your low end is still a little rich. When you make changes make them in 1/16, 1/32 of a turn. I small adjustment can make big changes in the low end.

Dru.