Update

I got to run the bike this weekend on a wet cold welsh day. Bike started ok however it kept dying in the morning no matter how many revs i tried to give it. After adjusting the air screw out quite a bit possibly up to 4 turns out and adjusted the idle to a little quicker the bike ran well all afternoon. This spark plug was brand new when i started the day. Does anyone have any idea if this is a normal or good colour plug after a days trialing

Idle speed is too high,plug colour looks slightly weak.4turns out on the mixture screw seems a lot ,if everything else is OK 2 turns or so is close.These engines have a lot of flywheel weight so they slow down slower than a GasGas or Sherco,carburetion on the Beta when new wasn't as good as you would like.Think they fitted a different carb on the later ones which improved them ,might be worth investigating 

 Have you ever pulled the pilot jet out and cleaned it. It should be at least a 27.5.