I’m having a problem with keeping speed consistent it runs normally no problem but the minute I wanna gas it just hits a rev limit and won’t go faster and I’ve shifted to 5th and it still won’t go over 15-20kmph and it’s starting to drive me up the wall I’m 15 and really good with tinkering but this is starting to **** me off and I have little time to ride as I have school and other things that need my attention so I’m on the verge of selling it or just giving up on making it faster any ideas?

 

I'm not too well up on old Yams but I think they have a limiter for reverse. It seems like the reverse limiter is coming on in forward gears. Firstly, be sure it is a limiter rather than the carb bogging, the "feel" is totally different for the two faults. Is the reverse light working properly? As I would guess the limiter will be wired in the same circuit. If it is working, check the wires to the CDI as one of them will be giving the signal that it is in reverse. It could be the CDI itself faulty, but they are expensive, so best to get someone with auto electrical experience to check for other faults, before buying a CDI that it may not need.
Just had another thought, overcharging due to a faulty regulator can do strange things to CDIs, so check it is charging properly, no more than 14.5v at the battery with engine running, check at various RPM.

 

What merryman says also may want to try to run atv with air box lid off just for test if runs better the air snorkle has blockage maybe dirtdobber nest or rats nest by removing lid its let engine get more air for accelerations