strange loss of power down hill

so first off im running a 66 cc i bought off zoombicycle in the spring it performed well and im satisfied with it aside from the many many bugs i eventually worked out to many to even remember them all

but any way to the piont there is a hill a few miles away from my house and by the time i get to it my engine is warmed up so when i go down it at full throttle witch i normally dont do but to see how fast i can go i tried it twice in the last week and both times i made it down about 2/3rds of the way down then my bike slowed down
it almost felt like i was trying to use my motor to slow down like when engine braking but i was full throttle but probably only goin 30 or so the engine runs good and idles great but i dont seem to have the top end mostly just mid and high mid well medium high mid if that makes sense

ANY HELP AND ADVICE WOULD BE NICE oh and it is completly stock except for a poo poo pipe i didnt know about port matching when o got it although i did smooth the exaust manifold just didnt enlarge it i guess out of instinct or something i dont know i just felt like it should have been smoother the thought that making it larger never really crossed my mind

yes i know i should pull the clutch in but i felt compelled to see the top end out of this hill and after the bogg down i felt like trying it again


i dont think its the carb angle the hill is not super steep its just a long hill

it had plenty of fuel in the tank
the tuned pipe and billet intake sound great im gonna install them on my next build also with some moderate porting and hopefull thinking here but a crank truing and balenc

cheap mag huh maybe what do you man by control o assume cdi

also i am impressed with how well balenced the stock cramk is from what ive heard from others about the vibration they are getting but maybe thats cause im not getting the rpms they are but at to speed the vibration is thruoghout the bike but not to bad