I had a real puzzle recently when I replaced a clutch for someone in an engine he brought loose to me. When he put it back in the bike it was near impossible to change gear. Obviously something I'd done wrong. So he brought it back, I stripped and checked it. Seemed perfect. Tried again, just the same. Between the two of us, we farted on for a week or two.
Then the post on the gearchange swivel snapped off!
He's such a meticulous cleaner that there was no lubrication on it whatsoever and it had partially seized.
I felt a bit silly
What I'm saying is try the easy things first before removing the motor and stripping it down......twice.