What is wrong with my Honda z50R?

I have had my dirt bike since I was a kid and I am 19 now. It is a 50cc(well 49) with a 4 stroke and 3 speed 94 z50R 94 model. It never gave me trouble until earlier in the year. It would run very well then occasionally would give up at full throttle. This happened more and more and progressed to the point were I would turn it on and ANY gas would kill it. This happened over about a 2 weeks of 5-10 min rides. At this point it will only idle momentarily with full choke. I cleaned the spark plug with wd40 some of the time and it seemed to work briefly, but ended up doing the same thing. I totally took the carburetor off for about a month(as well as emptied the gas), but did nothing with it. So I put the carb back on yesterday, put the only gas we had(which was mix for the blower) in it, and it started up no problem. It astounded me, I was so happy. I ran it for about 5 min but after that it would just stop firing at full throttle. I can run it at about 70 or 80 percent. If I pick up enough speed I can open her up all the way, but it's just not the same. It will struggle and accualy try and slow down if I manage to go full throttle with already achieved speed. Keep in mined that it had no trouble opening all the way even from a dead stop 5 min before. What is up?

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