Drain all you gas out into some clear milk jugs or something clear then titl the jug to the side an look at the bottom see if you can see water in it or dirt.
I'm pretty sure i know what your problem is because i'm running a setup similar to you, an i had the same prob happen it would only run holding it full throttle. I'm pretty sure this is correct but not gonna promise because i have the exact same setup. Your carb most likely got clogged runnin an open airbox, that's what happen to me dirt manages to make it through the filter an clogs your carb jets.
So pull your carb off an clean it.