89 KLR650 slow temp rise uphill
I bought a 89 KLR 650 with 963 miles on the odometer a couple of months ago, the bike fires right up and runs well but I noticed the temp slowly rising on up hill highway speed stretches in the first few days of owning it, but otherwise it ran great. I changed the oil (rotella t 15w 40), cleaned the air filter, checked the valves (in spec), did a compression test (100 psi), flushed the cooling system which had some rust in it assuming this was the problem I bought a radiator flush by Prestone ran this through then flushed again with distilled water which came out looking pretty clean I put engine ice in burped the system and continued my service. I replaced the t stat and I also installed a manual fan switch at this time. When I test rode it I headed for the nearest bit of slightly uphill freeway it ran perfectly all the way there (10 miles of twisty hwy) in fact more on the cool side but as soon as I hit freeway speeds it began to slowly head for the red I could only make it a couple miles before getting almost to the red but when I pull off and slow down the temp slowly decreases. I ride home downhill on the freeway and back down the highway without any issue. I try replacing the radiator (I have another klr 650 that acts a parts bike from time to time) I refill the coolant burp the system double check for circulation, seems fine but it drops a little I top it off and test ride it. Same thing. I come home and start looking for air leaks fix a tiny crack in the air box and cracked nipple and install an oversized main jet. This makes it where I can go about 15 miles on that uphill freeway stretch before it goes to about 3/4 of the way up, whether the fan is on or off. There’s no coolant or oil loss to speak of. It rides perfectly except in this one condition and I’m at a loss for what to try next. Any ideas would be appreciated.