I am in a long straight and am running 40+ mph. It acts like it want to shut down. It backfires and will finally die. After a couple of cranks it runs normal again. Doesn't do this every time, so it is hard to pin down.
Anyone else had this problem.

 

I am having a similar problem, usually on whoopty doo's
have been tinkering & the last thing I have done was to adjust the float height.
I cannot confirm that I have it fixed yet, will keep posting updates.

 

I wonder if your fuel tank isnt venting properly, and when you are running it at high speed for a long period of time there is too much vaccum in the tank and fuel cannot leave anymore, maybe trying running with the fuel cap loose or off and see if this helps. I know on my 3 wheeler i had this problem and it took a while to pinpoint it

 

Peteyz24 has a good point. Loosen the gas cap and try it. If problem gone you have a vent problem. If it doesn't fix it you need to check fuel flow into carb. With cap tight, pull fuel line from carb and let the fuel run into some container. If after a while fuel flow slows the problem is tank fuel pick up or vent. Now try it with the cap loose, if ok vent is bad. If not you have a carb problem, partially plugged needle and seat or something like that. I suspect there is a problem with the vent or tank pick up.

Bear

 

That was the first thing I did the first time it happened,
boy that would have been an easy fix, but I'm not that lucky!

 

Had a friend pick up his brand new 650 last week with the same problem right off the truck. It would rev fine in nuetral but soon as he tried to accelerate in gear it would stumble and die. Took back to dealer with 1 mile on speedo and they tried everything- changed jets, screwed with floats,vents etc. Finally changed dual carbs from another machine and presto, problem solved. I heard this from somewhere else as well. Who knows, perhaps a bad batch of carbs or something. Point is, you might try lots of things to fix the problem but won't. Thats what the big two year warrenty is for. Good luck.