Won't rev past 4k intermittently?

Hello, first time posting, so hopefully I’m doing this right.

I have a 1981 Yamaha XJ650 Maxim that I recently got back on the road. It has an intermittent issue where it sometimes won’t rev past about 4k RPM under load/in gear. In neutral it will rev past 4k, but it struggles and doesn’t rev as quickly or cleanly as it should.

What’s strange is that sometimes the issue will completely disappear mid-ride and then the bike will run perfectly for the rest of the ride (sometimes 5+ hours). Once it “fixes itself,” it keeps running fine until the bike sits for a while and cools down overnight. Then the next startup is unpredictable — sometimes it revs fine, and sometimes the problem is back.

Has anyone dealt with something similar or have ideas on what I should be looking at?
Thank you

 

First thing that comes to mind is perhaps some debris clogging a main jet. Sometimes little bits of gunk can float around and get sucked in to the jet, but then later might fall back into the bowl until things get sloshed around again. Unlikely it would be more than one cylinder though, so while it would cut power, I don't know if that would limit revs.

Another suggestion might be a coil with intermittent short or open that is affected by temperature. Harder to troubleshoot.

One other that bit me a while back is a leaky (split) vacuum hose on the petcock. In my case, the bike would run fine until all the sudden it didn't, usually resulting in a complete stall. Not quite the same symptoms as what you describe, but a smaller leak than the one I had could give more intermittent fuel starvation. Insufficient fuel flow across all cylinders could cause what you describe.