The previous owner described this issue to me. I drove it for about 2k miles with no issue, and then after doing a bunch of maintenance and repairs a few weeks back, I started having the same issue he had talked about.
After about 20 minutes riding on the highway at high RPM in 5th gear, the bike suddenly loses power, slows down, and won't rev over 2500 rpm. Continues riding just fine, and I can make it home at 35mph, but if I rev over 2500 rpm, it bogs down and starts lugging the same way as if you slip the clutch too quickly. Choking it gets a little more power out, but not much.
The previous owner said that he would dump a whole can of seafoam in there and it would stop for a while. For me, the times it has happened, I disassembled the carb, cleaned it, reassembled and everything runs fine for a while until it doesn't. I did notice a little bit of mechanical wear/shiny spot across one side of the needle and slide.
1999 KLR650 13k miles running 91 octane + 1/3 bottle of seafoam upper cylinder lubricant per tank
Previous owner said that his mechanic(?) buddy rebuilt the carb(?) and removed all (some) of the CA evaporative emissions system to try and solve the issue to no avail
Things I did before the issue appeared were
-valve shim adjustment
-10 cent carb mod
-new radiator and coolant flush
-fixed the idle mixture screw which was unscrewed so far that it was hanging out of the carb
-re-attached the slip on exhaust, which was not connected correctly and blowing 80% of the exhaust out the side
-oil change and new filter
-cleaned air filter
My only real guesses for what could be happening are
- the carb diaphragm isn't seating properly for some reason and that the vaccum from sustained high rpm is causing it to suddenly pop loose. I don't think seafoam would have fixed that though
-Rubber boot from the airbox or to the engine is vibrating loose and leaking
-No fuel filter causing clogging
-Some kind of fucked up vaccum leak from the chop job on the emissions
-Water seperating from gas and sitting on top of the tank with all the fuel addatives and years of built up tank gunk getting sucked into my carb
I know this is long, rambling, and all over the place, but if someone could point me in the right direction for diagnostics or most likely issues to check for, that would be amazing.