G'day Mate,
I would try running it without closing off the exhaust tube, & see if it runs ok, It should not need more muffler pressure, than the muffler was designed for.
Be aware, muffler pressure & back pressure are not the same thing, muffler pressure, adds slight pressure to the fuel tank, for aerobatic maneuvers, motor should run on the ground without it, & back pressure is a function of the exhaust system to help get exhaust gasses out of the combustion chamber more efficiently.
If you look at a 4 stroke motor, they don't need back pressure, that is why you can run them with a straight through pipe, but 2 strokes need BACK pressure, that is what a tuned pipe does, & on a 2 stroke motor cycle they have expansion chambers, similar to our normal mufflers.