My son just inherited an 80cc Moto4 for his 10th birthday. My family has been passing this machine around for a few years, and we're glad to have it. It appears to be missing an air filter, and needs a battery. No big deal there. I removed the carburetor and hosed it out with some B12, but I'm pretty sure I still have some carb issues. I only disassembled it far enough to get it out of the machine, so I'm not sure what the condition of the internals are. There was a LOT of gunk on the output side of the carb.

1st issue - The machine starts fine when choked (have to jump due to the battery), but I can't keep it running at low idle. Right now I've got the idle turned up high enough that it will stay running, but I know that's not the way its supposed to be. The screw that I'm adjusting the idle with appears to have a rounded tip that meets a beveled surface on the bottom of the plunger. The further in the screw is turned, the higher the idle speed is. Right now I've got the idle set pretty high. It will occasionally idle down, especially when the engine is warm, and when it does it will often still die. I've tried adjusting what I THINK is the low idle mix (only other adjustment I can find, located right next to the screw I'm adjusting for the idle). I've run it every where from 1-1/4 turns, to 2-1/2 turns out and it doesn't seem to have any effect. I will throw this out there. While I would consider myself EXTREMELY mechanically oriented, I have VERY little experience adjusting carburetors. I've done it successfully a couple of times, but most of the time I struggle with it a lot. I just don't do it often enough (maybe 6 times in the last 15 years) to get good at it, or even remember what the heck I'm doing.

2nd issue - Once the kids take a couple of laps around the yard, the machine dies. Neither of them have any experience riding, so they are currently restricted to 1st gear. We have just over an acre, which also limits the need for more speed, but that's beside the point. When it starts cutting out, it will run a few more minutes if you back way off on the throttle. Not sure if this is a carb issue or if it needs a new spark plug, or it's something else. The engine is well beyond "warmed up" at this point, and I've never seen the carb cause such a problem after running this long. I have seen somewhat similar issues on my chain saws when the plugs are getting worn out.

Any suggestions would be appreciated!
Thanks!

 

Clean the carb.