GP200 Clutch issues

[Dave5000]

I've recently bought an "off the peg" factory built gp200 engine
After much tinkering with the clutch cable I'm still having problems
When I get the bike up to around 50mph in 4th (so the Speedo says but your guess is as good as mine as to actual speed) the is a note change in the engine, sound like it's back in 3rd. I think it's clutch slip but I can't slacker the clutch off any more without getting drag. The bike doesn't want to go any faster.
I had an old series 1 150 engine before which was a built from scratch and it burnt it's clutch out after about 150 miles
Have I got a crap clutch or have I another problem. Any help much appreciated

[hullygully]

probs just the Indibum clutch plates , change to surflex or 'last you a life time' reedspeed reds.

[Fast n Furious]

Whilst you've got he chaincase side off, check the cush drive for potential problems.