Unfortunately the TT factory never tightens screws and bolts much beyond finger tight. This results in air leaks, which results in erratic running.

Tighten the head, backplate, carb body, needle valve body, and everything else you can find on that engine. This will stop the air leaks and let the engine run normally. The TT 42GP is a great engine! I've got three of them that I'm using in combat and fun fly planes. With a mousse can pipe, they run even better.

I'd also suggest starting with an OS #8 plug. Once you get the engine dialed in, then try experimenting with other plugs.