My uncle and I have about rebuilt this entire machine and it still will die when you are on the throttle. It will idle and run and 1/4 throttle all day long. We have replaced these parts: new jug and piston, rings, carb, intake tube, cdi box! We dont know what else to try. Any ideas would be greatly appreciated!

 

It sounds like a main jet problem with the carb and it's starving for fuel. Have you cleaned the tank, carb and put a fuel filter on it?

 

we have put new fuel lines and it has a brand new carb on it. could it be something else

 

forgot to say that when it is starting to die, if u let up on the throttle it will still die.

 

I may be wrong but it still sounds like a fuel delivery issue, whether that be trash in the tank restricting flow at the petcock or blocking the main or needle jet. I would think that if it were a needle jet or flow problem at the petcock issue, it would not idle for any long length of time but stall once the bowl was emptied due to lack of fuel supply to the carb. It stumbling between 1/4 and WOT leads me to believe it is a main jet issue, which is where the fuel is metered for delivery into the cylinder between those throttle points.
I would:
1) take the fuel line off at the carb and verify flow from the tank-put it (drained fuel) in a clear glass container and check for particulate
2) Remove the carb and clean the jets out; also check the float level
3) Check and clean the air filter-you don't mention this anywhere

 

we just pulled the carb off and cleaned it. took off the fuel filter and and put a new one on. adjusted the float and still nothin.

I forgot to mention that if u try and start it back up after it dies, forget it! You have to wait at least 5 to 10 minutes for it to start back up again.

Im about ready to ship it back to china!!!!

 

Hmmm... Have you checked for spark just after it dies? Curious to know if it is losing spark when it dies.

 

Yeah, it has spark with our test light but it doesnt appear to be very bright. We tried putting on a different coil also. That didnt help it either.

 

Ya know i just bought a RTK-250 reeeaaally cheap from a buddy cause he was having the same problems so I did all the things you have mentioned also. The thing that was perplexing was the bike only had about 25minutes or 2 rides into it...after massive poking and proding and head scratchin i found that the exhaust push-rod was ground off at the tip...almost an entire 1/4" was gone...weird since the bike was still brand new basically, coulda been some casting issue or some soft steel or something anyway im now trying to track down a couple pushrods and upper rocker arms should be good as new...nay better than it was new!!!WEIRD but like yours it ran perfect at idle and all the way to about 1/3 throttle after that it just bogged out like it was running out of fuel.