What wt oil should I use 5,10 ,15 20 ?I have used 10 but forks seem to rebound fast

I think you need to revise the damping mechanism. I knew someone who had one when they were current and even 80w oil did not give good damping in the standard forks.

I assume you have the forks set correctly in the yokes? You should have about 1/2 inch protruding through the top yoke, if not it will steer and rebound like a Harley. If the springs are good use either 10 or 15 wt and about 150cc a leg.

Tony

The book says 126 cc, so you add more to help damping?