Just bought a '93 K75S, very clean and well kept with 23k miles on the clock. Overall seems to run well except that acceleration and throttle response seem a bit flat, and I notice significant roughness or engine vibration at full throttle at high rpm (above 6000 rpm or so), sort of a growly roughness as it revs above 6000 rpm at full throttle -- not overwhelming, but definitely there.
I should add that this is my second K75S, but I sold the first one some time ago so I'm not sure I'm remembering correctly. What I seem to remember is stronger acceleration than what I see on the new one, and I don't recall any kind of growly roughness at high rpm. Am I wrong about that? Is increased vibration common at high rpm under power? My other bike is a Japanese 4-cylinder naked bike that is fast and smooth all the way to redline, so perhaps I'm judging the K75S a bit harshly. But I really don't remember my former K75S being anything other than extremely smooth anywhere in the rpm range -- would appreciate a reality check on this.
If the rougness at high rpm is not normal, what could it be? I changed the plugs for good measure (originals looked past their prime, but no blatant signs of abnormality) and the engine starts easily and runs smoothly, it's only under hard acceleration that it seems a bit flat and a bit rough. I'm wondering if it could be the throttle bodies out of sync, or perhaps timing is off. I do notice that the fan comes on more often than I think it should, but I'm not quite sure what to expect. If I ride around residential streets in 2nd gear, 20 - 35 mph, temps from 70F to 85F the fan will go on and off -- is that as one would expect?
Any suggestions on what I'm seeing? How to check if there's a real problem or am just imagining things? How to diagnose or fix?
Any thoughts or insights appreciated -- thanks.