One of the reasons why a fork is partially disassembled is so all the old oil can be purged from the cartridge and also the surrounding area.
With the fork cap and spring removed, the cartridge can be worked like a churn to pump all the oil out of it.
Without doing that, you will always have a quantity of contaminated oil inside.
Also, since a certain quantity of oil remains, you can't be sure you're adding the correct amount of oil back, and with the spring installed, your level is unknown as well, so the air space is also inaccurate.
The main reason fork oil is replaced is due to fine metal particles that are shaved off the fork tube wall when the spring bows and rubs against it.
The metal particles eventually settle at the bottom of the fork and accumulate around the compression valve.
Not only do they interfere with valve shim operation, they can (if bad enough) get impacted into the valve orifices.
There have been reports of old bikes that have never, ever had their fork oil replaced, and the metal particles had completed blocked the compression valves to the extent the valves had to be replaced.
The particles had been pounded into the valve orifices to where they couldn't be chiseled out without destroying the valves themselves.
Since the RC51 has and attached compression valve assembly rather than an internal one on most forks, it is possible to remove the valve assembly and possibly drain the oil.
There are two small O-rings for each assembly. Don't lose them as Honda doesn't sell them.
Then you can pump the fork to push the oil out from the cartridge and fork area.
If you try this, I suggest placing the fork in some kind of pan or such as it will be real messy.
Then you can reattach the compression valve assembly, and then unscrew the fork cap and add the prescribed quantity of oil, then pump the fork to try and work the oil back into the cartridge.
You will have to pump a while then let sit for 5 minutes, then pump again, several times.
There will be no way to know if the air space is accurate, but it will be quick and dirty.
Last edited by SubSailor; 09-11-2012 at 10:06 AM.