Yamaha motorcycle stiff clutch

Disconnect the cable at the engine. Does the cable move freely when disconnected? If it does, remove the left side cover then check the operation of the clutch arm. Does it move freely and spring back freely? If not then clean it up. If it does move freely, remove the clutch push rod from the center case. Roll it on a flat surface. Does it roll smoothly or does it sort of lope along. If bent, replace it. If rolling smoothly then put it back in the center-case and remount the side casing. Lube the clutch cable and remount it also. The adjuster #23 should be screwed in until it is snug then back off the screw 1/4 turn.

Assuming no one has tinkered with the actual clutch and it was operating properly a few months ago or when it was put up for the winter, now is the time for an oil change. Dirty, gritty oil can stiffen a clutch big time. Drain the oil and refill with 10w40 motor oil. No synthetics or oils with "special" additives or friction reducers. No oil marked ECII. Just normal motor oil. Shell, Mobil, Pennzoil, etc., the major brands. Ride the bike for an hour or so then change the oil again. Doing this will flush most all of the grit from the gearbox. Hey, oil ain't that expensive. This should take care of the problem.

A “very helpful” rating on this answer? Thanks!

I was going to try to answer this but the first guy, tombones49, pretty well answered it already. His answer was right on. Why didn't you give him a very helpful rating for the answer?