I dont think pinching the exhaust nozzles is the best way for feeding an twostroke engine. Twostroke engines needs to breathe and restricting the muffler would cause overheating and loss of power. However some engines seems to handle more backpressure than others which is good for fueldraw but on all engines the needle settings becomes more narrow, only very very small increments are needed.

Your tachometer readings are good so your muffler is probably fine. Maybe only some low end needle adjustments are needed?