clutch spring issues

A while back I blew a belt when the filter I have between the seats got dirty and the airflow was restricted causing it to over heat. It got hot enough that it melted the pieces of the belt right to the vent tubes.
Since then it has been hard to shift. I thought it might be the new belt so I replaced it again and still had the same problem.
I had it apart and noticed when I start the engine, when it gets to idle speed, the primary starts to move and engages enough to spin the secondary which makes it grind and hard to shift.
Could it have gotten hot enough to take some of the temper out of the primary spring and weaken it?
I turned a .200 thick shim on the lathe and put it behind the spring and the problem went away so I think I need a new spring (or maybe a clutch kit)
I notice that it takes a slight higher rpm to engage but it's barely noticeable.
I haven't had it out yet to see if my top end is affected.
Has anyone seen this issue before?