150cc snowmobile

[darrm1]

I know this site is for scooters, however, I bought a chinese snowmobile for my son that I believe has the 150cc GY6 motor in it. I have a few questions that I believe could be answered here.

To start:
I bought this sled used from a guy who said it ran good but the battery was dead, I know, never take someone for their word. After getting it home and unable to get it to run properly, I turned to the forums to diagnose. Turns out mine, like most, didn't have the vacum lines hooked up to the fuel pump. Once I T'd the line and hooked that up, the machine fired right up and ran great. Which leads me to believe, that the machine has had 0 miles put on it since it coundn't have ran for the guy I bought it from without the line hooked up.

Heres my issue:
I have to assume the guy I bought it from tried lots of things to get it running, but couldn't, and obviously I have no idea what that all was. When I got it, the muffler was not hooked up, nuts were missing. I didn't realize that at first, I just thought the machine was loud. When I hooked up the muffler, the motor runs very poorly, almost won't. We had a mild winter last year, so I ran it without. I know, not good!!! My question is obvious, why won't it run with muffler installed. Possibly the last guy tried different jets maybe? Muffler wrong size? valves?

Any help would be very much appreciated
thanks in advance
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