Thanks for the interest guys.
I probably should have said clutch plates STICKING in the heading.
When I assembled these motors I gave them a turn over with the clutch cover off and worked the clutch to make sure all was well. Went through the gears. Just a general look for any problems.
This is something I do on every motor I rebuild.
All was well. I went for a ride and all was well.
Next time I went to start it I noticed that the plates were stuck together. [Was trying to find neutral ]
Pulled the cover off and dismantled the clutch.
Held the clutch plates in my hand and they stayed together in a lump.
Trick is that this is happening on 3 bikes one of which has a complete new clutch.
You can put the bike in any gear , pull the clutch in and rock the bike and the clutch will NOT let go.
Take it for a run and it,s ok until it cools down then back to the problem.
Huskibul appears to have the same problem.
Will try Husabergpete,s suggested oil type next.