'82 XJ650J sudden loss of power

Been a while since I've been on here so sorry if I've posted incorrectly or forgotten anything etc. Anyways, last Wednesday I was on my way to a meeting. The bike started up, warmed up, and rode completely normally for almost a mile or so, until I came to a stoplight. After the light turned green and I started to go forward I immediately noticed something was off. The bike sounded slightly lower pitch, like a hint of a low growl or gurgle as I was moving, and I was suddenly way underpowered. Looking down I didn't see anything of concern and at this point I was going to be a bit late to my meeting so seeing as it stayed running and I wasn't too far, I just kept going. After my meeting the bike started up fine again, and got me home.

Anyways, for some reason my first thought was that perhaps I wasn't running on all cylinders, so I've checked for spark which all seem good (I did discover some rather black covered plugs but I already knew she was a bit rich). I also used a laser thermometer gun to measure each runner's temps and there was only a 3-4* difference between the coolest and the hottest. It warms up and idles just fine, and I can really only notice the problem when riding, probably after 2-3k rpms I'd guess. This makes me think that the main jet or something to do with it in at least one carb has become plugged somehow. The sad part is that I just fully rebuilt the carbs 1k miles/2 months or so ago and I was so sure that I got everything as perfectly clean as possible (this only slightly damages my confidence in my abilities haha).

So before I go and tear down my carbs again, any other ideas of what might be causing this?

 

Fuel starvation? Is filter clogged or the gas cap vent not functioning?

 

Well I do need to rebuild the gas cap (old dried out seals and whatnot) but I believe it still vents fine, and I have a clear in-line filter that looks like the fuel is still flowing as it should but those are both good points that I'll look into more closely. Thanks!

 

Easy way to test the gas cap vent is just open it up while it is misbehaving and see if that helps.