G'day Mate,
How much running has this motor had, sounds to me like it needs a lot more. Will it run without the pump or muffler pressure on the ground, & in level flight? If not it needs more running.
As the motor runs in, the seal between the piston & the sleeve improves, this increasing crankcase suction which will draw more fuel/air mix into the engine, that is why, as engines run in, they go richer at idle & need to be leaned out some to run smoothly.
By the way, muffler pressure & back pressure are not the same thing, muffler pressure is used to pressurise the fuel tank to allow more consistant engine tuning during aerobatics.
But back pressure is caused by the shape of the muffler, it allows the exhaust gasses to be more efficiently removed from the engine.
4 strokes don't need back pressure, thats why they have small straight through exhaust pipes.