Atc 110 help! Runability of machine

[Flipdawg08]

Hello was looking for some insight from those with more first hand experience with 110's than I. I have a 79 110 with a I believe it's a 83 110 motor installed . This thing is just going to be a beater for my nephew and friends to ride with me . So I'm finally working on getting it to ride. I got it to run but not very well. It will not go over maybe 10 mph in 4th gear . I believe the clutch is maybe slipping as well cause I have to wind it up a little to get it to finally move . It backfires and will stall if I just punch the throttle over just going easy on it then it won't.

- First things first I know I obviously have an exhaust leak right at the head as the exhaust I have installed is a home made , modified from a different model machine and I haven't figured out a solution to have the gasket seal up yet.
- I have a cheap ebay carb installed and at this moment not sure what it has for size jets inside as it was sold for an atc 110.
- no oem airbox just a filter right on the end of the carb.
- spark plug is old and came with the motor when I bought it so know that should be replaced as well.

[350for350]

It does sound like the clutch is slipping. Exhaust leak, no air box, and unknown jet sizes all sound like you're running lean to me. Probably by quite a bit.