I would try the jet for sure. The jet on our stock dune 150 was a 114, and we were using a 120 with the uni filter 58.5 srp piston and the the stock muffler with a larger 3/4 inch inside diameter tail pipe.
When we replaced the engine back in 2009 the motor came complete with a carb and the main jet was a 103. I think that is okay for a scooter, and I did not use it I used the 114 main before I modifyed the engine.
I also tried to run my red buggie without the air filter when I was trying to fix the off idle bog that was a air leak on the intake manifold. The motor would not run properly without the intake tube that I made out of a radiator hose and 1.5 inch sink drain pipe.
I also found out the length of the tube between the carb and uni filter was best at 5 inches for my application.
I also remeber my red dune 150 that we bought used for $400.00 that had the intake leak took me two weeks to get running and our first trip to the desert it ran like junk.
I had to really use the pedal to keep it running, tried float adjustment, swapped out coils and the carb off the blue dune 150 that ran perfect.
I could not find the problem. I removed the cylinder head to port the head and found the intake valve was bent and the exhaust valve had a nick in it. I took the valve out of another head and used lapping compound on the seats and it ran awesome.
I hope this helps and I know how dissapointing it can get when you have tried almost everything. The bent intake valve was blowing back to the carb causing my full throttle power loss. I remeber it ran at light throttle but at full it sputtered so I would back off the gas a bit to keep it running. The bent valves was not visable, I found the problem when I was lapping the valves and it would spin out of round.
Johnny 5
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