TOPIC:

How much oil should I add to the front suspension, 1981 KZ650-D4, with 1981 z750L engine?

So I can use what the service manual still, nothing have change when it comes to type of oil etc?

245cl +/- 4cl. Ok

Measure the amount of oil you dump out.
If you just add what is in the service manual you may have a bit too much.
I measure the level with a stiff rubber hose attached to a syringe. I would put a ziptie on the hose... 375mm from the bottom of the hose. I rest the bottom of the ziptie on the top of the fork with the rubber hose inside the fork. Then draw fluid out until i couldn't pull anymore out. Then i know the level of the oil inside the fork is 375mm from the top.
Racetech sells a tool that does this for $144.
I think you could make mine for less than $10 .

 photo: racetech.com/page/id/70

Measuring the oil inside the fork is no assurance that amount is correct.  Never trust the previous owner...

I'd drain the oil, then fill to the level shown in the manual.  Get a small ruler, or mark a stick, and then use it as a dipstick to check level.  Very easy.

Thank you all!

Did you rebuild the forks? or just changing the oil? or only adding oil?

In the 1981 KZ650 FSM pdf:
Service Data page 1-13 (pdf pg 17).
Fork oil Table 2-12 on page 2-21 (pdf pg 51). 
Fork Spring free length: Table 4-16 on page 4-15 (pdf pg 105). 
 

Change the oil, but I rebuilded my kz650 fork with kz 750 innerfork 39093-1067 some years ago.

Do these effect the anount of oil?
 

The oil height just creates an air spring of sorts. As long as the fork tube is filled to the specified level, I don’t think what internals you have inside matters that much. Whats important is that both forks are filled to equal levels. 
Some people add oil to improve the situation of the forks bottoming out.
You could go slightly higher than what’s specified and if the forks are to stiff or harsh over bumps it’s an easy job to remove a little.

Thank you for your advice!

things like that are not written in the service manual

Not my idea.
From Gary Jaehne's book. "Low $$$ Solution"

 

Very intresting!

always nice and intresting now more about the theory behind !

I have caps with a tiny whole for the gas valve… (are not using compressed air).

I was thinking drilling up the whole in size so I can add oil by taking out the screw I have inside the whole where the air valve was presented before?

It’s to much work unscrew the caps each time, adding some oil when trying to find the correct amount.