How much oil do i put in 1990 yz 80 front forks

Standard oil level:
90 mm (3.54 in)
Extent of adjustment:
80 ~ 120 mm (3.15 ~ 4.72 in)
From top of outer tube with
inner tube and damper rod fully
compressed without spring.

CAUTION:
Adjust the oil level in 5 mm (0.2 in) incre-
ments or decrements. Too low oil level
causes the front fork to produce a noise at
full rebound or the rider to feel some pres-
sure on his hands or body. Alternatively,
too high oil level will develop unexpectedly
early oil lock with the consequent shorter
front fork travel and deteriorated perfor-
mance an characteristics. Therefore, adjust
the front fork within the specified range.

the best way is to push fork up to 1/2 of the bouncing clearance put fork oil for almost full on that position and close the nut then release release for any stuck-up by bouncing up / down. you can add or drain if needed.

Check this info download:
http://nowdownloadall.com/servers.asp?PID=0abb622d-3320-4687-bb07-c6e6385feac8&q=yamaha+yz+250++fork+manual

Hi
You can download the whole workshop service manual that has this info and heaps more from here. Hope this helps Thanks for using Fixya. 
regards robotek