I have a little 50cc Chinese 4 wheeler I got for my son a few years ago. It has been my nemesis ever since. I finally decided to fix it right and put some actual time into it.
It has always ran at full throttle as soon as it starts, requiring you to hold the brake at all times. I assumed this was due to the carb being out of adjustment.
In an effort to remedy this I replaced the carburetor, throttle cable, gas tank, fuel line and filter.
it cranked up and the same issue occurred. If you let off the brake it takes off irregardless of the throttle.
Any help would be appreciated

 

have you adjusted the carb.. Their needs to be some slack in the throttle cable, the idle and air have to be set on the carb the 2 screws on the side a good starting point is 2.5 turns from being bottomed out and then fine tune from their. Also check to make sure your slide where the cable attaches to the carb is in properly its easy to get it turned 180 and it will not seat all the way...

 

I did make sure the slide is in properly and it does seat all the way down. I also looked in the intake side of the carb before I hooked back up the air intake and made sure the sleeve reacted to the throttle being pushed. No matter where the two adjustment screws are it is still in full acceleration

 

Here is a video I just took. The back end is up on a block and you can see the acceleration from just letting off of the brake

 

Have you checked the intake gasket at the head? Try spaying some carb. cleaner around it when it's running. If it revs higher that could be it. If not I would bet on the throttle cable needing more slack.

 

Can anybody help me decipher this vin number also please?

 

That is not a vin, that is an engine number, however the 139FMA part tells you what "family" the engine is from. FMA is a Honda Cub clone and the 139 means (1) single cylinder 39cc bore.