do i have to have the fly wheel set at a serten posion to adjust timing chain on a py90?

Answer 1

I do not know what a py90 is, but, I assume that it is a single cylinder engine. What you want is a position in which the adjuster is against the slack side of the chain. You can usually do this by setting the flywheel to TDC (IE piston is at top dead center compression) at which point there will be no tension on the cam chain and the adjuster will take up the slack.

Answer 2

I'm in the same boat as geo46er - I don't know what a py90 is.
If the cam chain tensioner is automatic, then the position of the flywheel is only needed for cam timing (if you had the cam chain off of the sprocket).
If you just want to adjust the cam chain tensioner -
--Remove the tensioner - will it spring out, but not in (until you de-activate it)? If so, it's automatic and no adjustment procedure required (except when re-installing).
--Figure out the procedure to collapse the tensioner and install it.
--Some have a ratchet mechanism that can be temporarily locked, while installing the tensioner.
--Some have an inspection cap at the end. Remove it and wind up the tensioner's spring with a screw driver. Install the tensioner and remove the screw driver.
--Some are semi-automatic - they spring in and out, unless a lock nut is holding it.
If this is what your bike has -
Start the engine and let it idle.
Loosen the lock nut.
When the noise is lowest, tighten the lock nut.