Did you check/set your float level to specs?
Can't remember exact setting-16mm or 22mm?

Put sharpie marks on your drive clutch sheaves,see how they are shifting out.

Is it smoking a lot like its flooding or not burning the fuel?

Is it possible its up shifting too fast and your always in 4th gear when you should be in 1st gear and the Engine does not have enough torque for the load to ever power out of it and rev out, take pictures of your drive and driven clutches and post, I have seen a few springs break in the stock drive clutch and cause it to be in too high of a gear and never shift out.

When I had the motor (( Internal Combustion Engine ? )) apart I visually inspected the drive clutch and it seemed to be in-tact - but I dono. Im starting to doubt the life left in the drive belt. Its a Mitsuboshi belt and its kinda old so I cant see the numbers on it. Compared to my FL350's Comet system this FL250 drive belt has quite a bit of slack front and back.

Now, I did pick up a Honda part number 16012-358-034 partial build kit for the stock carb. It came with a brand new Honda needle Jet and jet needle as well as a few other pieces. It appeared to be one of the last available from EBay. Visually the needle jet part that screws into the carb looked a bit smaller than what came out, so I'm sure its going to lean it out some. The needle looked the same as the cheap aftermarket rebuild needles.

I put all the parts in and then set the cart up on rear jackstands to test out some free-revs and examination of the belt system and other stuff while running and mid/high throttle. The Engine was still billowing smoke so I dropped the main jet from 120-118-115 and the 115 makes the best result. The Engine revs a bit harder than it did before now. Clip on the middle of the needle. The clutches are both working as they both move in/out.

So I'm ready to test drive and tune next trip but maybe I should get a replacement belt and try it the same day? I think a belt can be replaced without too many hand tools.

I took the belt off last night and it was pretty worn out. The spec say the stock belt should be 15/16 wide, but this one was measuring about 13/16 inch. It was also very much cracked and looked like it was going to rip apart soon anyway; so I ordered a Dayco HP3025 which is 31/32 wide and just over 41" long. I'll report back how it feels after replacement.

The new replacement belt fits a heck of a lot better than the old one. I hope to have this cart's issue's solved but I just don't have the time to really tell just yet.