the engine runs good in nutral but in forward it is like skipping

Okay i took the 4 wheelers over to my gfs house lastnight and all the kids were riding the mini and having fun well after about 3 hours the mini started to skip and it only does it when you about half thorttle or more, lastnight it still was going pretty fast but just popping and stuff today i got off trailer and it doesnt seen to have any power, its got a linhai engine, what should i look at and check on it? if anyone knows it be great

thanks