Sorry to butt in again but it seems to be temperature related and you say don't sound quite right at half throuttle, which reminds me of a new engine I serviced a few months ago.

The problem was tracked down to a air leak between the chambers in the carby base, which only became evident as the engine temp rose.

Refacing the carby base, insulator block and backplate cured the problem.

Maybe its worth a try.

Team Crossman