Well, if you did the top end correctly, stop thinking the issue is there. Have you done 4 stroke top ends/valves before??
It is possible that you set your rings wrong, either oil or compression - it happens.
But compression is compression, and timing is timing. Nothing inbetween should be the issue.
I run a 175 to 182 main jet in my 39mm carb on my 350X, I run almost a 190 on my 452X with a 41mm carb.
Oil fouling isn't going to make the engine cut out, it would run horrible within the entire throttle range.
If it ran well before, and you didn't TOUCH the carb before these symptoms, you can rule out the carb. If you did touch the carb, is it possible you made a mistake in there? Or need to change some jetting??
It's easy to go up or down a few sizes, see what that does.
If the carb overall could possibly be a variable, why not buy yourself a 400EX carb for under 75$, toss it on and see what it does. At the very least, you will have upgraded your carb and if you have an AM exhaust and some holes or no air box lid, you will want that carb anyhow.
I'm not here as much anymore, but I'm glad to try and help...
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country