Fork oil documentation conflict

1981 CM400A. The FSM says 4.9 oz of oil, which is what I put in after a fork seal change. However, I was just looking at my owner’s manual, and it says 6.4 oz. any idea which is correct? What happens if I put in too little or too much? The bike is back together, but I’m waiting for the wheels to come back from getting tires installed, so I haven’t tried it yet.

The spring is the primary suspension, the fork oil controls the damping.....
Usually, the greater listed amount is for instances when the forks were disassembled and are completely devoid of oil (like seal changes),
and the lessor amount for a "drain and change/refresh" scenario.......
It is more important to have even amounts than a specific volume, and many of us "tune" the suspension by adjusting oil volume , viscosity (weight) or both.......

Ok, that brings up another question. I have both 5w and 15w Kendal fork oil. I used 5w initially for the 4.9 oz. Instead of tearing everything apart again, I was thinking of using a plastic syringe to inject another 1.5 oz into the drains hole, and was considering using the 15w oil assuming it would firm up the suspension a little. Do you foresee any problems with this plan?

The 1981 CM400 T-A-C-E addendum on page 26-9 of the FSM specifies 6.4 oz. or 190cc.
The original fork oil specified was ATF which is equivalent to 7W. I use 10W on all of my bikes to give me a little bit stiffer fork action.
Trying to force another 1.9 ounces of oil into the forks thru the drain will be tough since the air in the forks has to be displaced to accommodate it. It'll be easier to just support the bike and pop the caps off.

I thought of that. i was thinking of having an assistant hold down the Schrader valve used to put air in the front suspension while I was injecting the oil (or just remove it temporarily). I’ll probably give it a shot and see what happens. Popping the caps in this bike means removing the air hose between the forks. And that means removing the handlebars and one of the instrument cluster screws to get to the fittings to remove the air hose. If I don’t make a nasty mess, this will be a new way to change the fork oil on an air assisted fork assembly ;-)

It's worth a shot, worse case is make a mess and have to do it the hard way.

Know anyone who does air conditioner servicing? Borrow his vacuum pump and attach it to the the air filler port(schrader valve removed) and pull vacuum on it while you remove the drain screw, the oil won't run out with the pump going. Get a small plastic hose and put one end against the drain hole and the other end in a jar of oil.

That’s an interesting idea. I happen to have such a pump.

I'm wondering if the suction won't cause fork oil to get pulled into the vacuum pump.

The air bubbles will just rise up thru the fork oil, supposedly the fork oil will have anti-foaming additives anyway.

I will definitely let you know. Out riding ATVs this weekend, but will try it when I get back.

Well, the results are in. The plastic syringe method was a messy failure. The oil is just too thin to permit a reasonable amount of time to transition from drain screw to syringe and back. Once on, the syringe does a fine job of filling the fork with oil as long at the schrader valve from the air hose is removed, which allows the air you're displacing to escape, but the transition from screw to syringe and back is impossible. Luckily, I had a bucket underneath to catch all of the spillage, weighed it, and added it back along with another ounce and a half using the VACUUM METHOD! Yes, Ronco shoppers (showing your age if you know what that reference means), the vacuum pump method worked like a charm.

I removed the schrader valve from the air port and hooked a vacuum pump (usually used to evacuate automobile air conditioning systems) to it using a rubber hose. I used another rubber hose with a taper fitting on the end to suck the fluid out of a measuring cup (I weighed the fluid, not eyeball measure). It sucked it all in, pulled out the hose, nothing came out while the pump was running. I quickly started the drain screw threads and then switched off the pump. Not a drop was spilled. I then used the same method to suck an ounce and a half into the other side. Again, not a drop spilled. No fluid went up through the vacuum hose into the pump. There is enough air in there, and the fluid is thin enough that it doesn't travel up that far. I'll post some pics of my equipment in a little while.

From now on, I'm going to do regular fork oil changes using this method on all of my bikes. The only requirement is that they need to have some sort of air assisted front forks.

I have no idea why it's flipping this photo 90 degrees. If anyone does, let me know how to correct it.

Good deal Lucille. I was afraid the vacuum pump would end up sucking the oil out of the forks.

^^^Yeah, my thought too. Those forks can't be any more of a PITA than the cap-on-rod-with-spring version on the later DOHC CB/CL450. I put the fork oil in mine before connecting all those parts

I’ve done that on my first bike years ago (a cb450), and many bikes since, it’s a PIA to compress that spring and get the cap on sometimes without cross threading or ripping out the top thread on the cap or the tube. If you want to measure the springs or check those internal parts, it’s obviously necessary, but this method will entice me to do much more frequent fork oil changes, even though I’m not getting every drop out. Although, I bet putting that suction on the drain tube would encourage a lot more to come out. I have a mighty-vac brake bleeder bottle that could be used to suck it out without it going into the vacuum compressor.