Odd Stalling Problem

[Ebuddy]

My Roketa MC-30-150 (B GY6 variant) starts and idles awesome. When on the kick stand, I can rev it up to 7,000 RPM up and down sustained and quickly release and it goes back to idle without a hitch.

When I ride it, it runs and accelerates great. After, say, 1 minute or two at higher sustained speeds (35mph +), when I slow down (like for a stoplight), it slows down and then dies. Following this, it's VERY difficult to re-start it, almost as if it's flooded. Once it does restart, it seems fussy for a short time and then the problem repeats.

When I get it back to my garage and start it up, it starts and idles fine and I can play with the throttle with no issue.

It's almost like it's flooding...but not quite. I don't know what to make of it. I presume the carb is fine since it otherwise runs and idles with no problems but it SEEMS like a fuel delivery problem. Valves were adjusted just a couple hundred miles ago and the issue's gotten progressively worse.

At first I thought that somehow the belt/clutch/variator was somehow getting caught but I replaced the belt and installed new rollers (yea, sliders would have been better, but it only needed to get me until the spring). Everything seemed to look and operate normally.

Any other ideas? I'm going to store it for the winter soon, but want to be ready in the spring.

Thanks! =)