I too have this problem. Running 5% nitro, 16% pure castor, rest Methanol. Dont want to up the Nitro, too expensive here. Started with the not being able to shut off problem and cutting at full throttle. Sealed the carb, fixed the cut off. Put the engine cuts down to not fully run in/overheating. Indeed after a couple more flights got it to run well. Then suddenly (after removing a shim to get more power), it ran like a pig, after peaking, it would cut if held vertical of splutter if ran a little rich. Also the main needle could be turned +- 1/4 turn without a noticeable difference. Fuel system checks out ok, no bubbles in the feed and no leakes under pressure. Just tried using some silicone tubing to further seal the needle (the o rings were lookin a bit worn, esp the outer one which doesnt get much lube). HAvent run it yet to see if it helped. I was also thinking maybe a cooler plug to compensate for the increased compression until I read this thread.

I shall investigate the holes lining up, and fix it if so (without drilling any nipples) and report back in a couple of days.