1978 Yamaha XS650 stumble at high RPM / full throttle?

I've got a 1978 Yamaha XS650 I just rebuilt. It runs nice at everything but full throttle, where it seems to start missing or stumbling. Backing off even slightly stops the stumble.

I have the timing and advance set up per the factory manual, and the advance looks like it's working correctly. Points are clean and not pitted, and properly gapped. New plugs.

I did not open the carbs during the rebuild. The bike averages around 50 MPG on my commute to work.

Any suggestions?

try it with the petcock in the reserve position. if that doesn't help raise the needles in the carbs, they should have 3 notches for the c clip on the needles, if yours doesn't have more than 1 notch use 1 or 2 aluminum pop rivet washers under the clip. if that helps but doesn't cure it you need to find about 5% bigger main jets or measure them & find or make a drill bit to bore them out. the jets are brass so a D bit made from the right size sewing needle will drill them. but measure what you have first then calculate only 5% more each time then try it out so you dont go too big. usually it only takes 1 or 2 times to get it right, too rich is better than too lean.
make sure your air cleaner is clean before you do anything else, if your running one.

this is usually associated with a fuel problem. when I had my bike I noticed that at wide open throttle I was actually floating the valves. I would pull the fuel shutoff valve and look at the screen inside of it. then work your way down to the carbs
adjust the valve lash they are floating
could also be choke partially engaged or sticking throttle slider
run high test gas
get rid of the stock air filter and switch to K&N's, air is restricted thru the stock ones
My guess would be that you need to make some adjustments to the carburetor. Used to have a '74 Yamaha 175 Enduro, and that was constantly giving me carb problems similar to what you described. Unfortunately, I can't prescribe any help, because I never learned how to maintain a carb. Sorry!

Good luck!
Fluffing at full throttle implies the main jets on the carbs are too small or the balance of the carbs is slightly out. Probably worth taking them to bits in case one of them has a sticking piston or diaphragm damage.
Dude! she's almost 30 years old. Give her a break, You rebuilt it? Check my 360 site to see my old XS650 special. It cost me major dough to keep that puppy in good condition. You need to get a Buell. XS650 Yammie. Yep, I would kill to be able to ride one again.
First bike. Broke my leg on it. Now I'm riding a hog.

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