Fork Oil Weight & Air Gap. 1977 CB400F

[Oldbuthopeful]

Sorry to be the one to bring this debate back down to earth!!!.....................    I was about to appeal for help with a similar problem when I came  across all these words of wisdom and thought my prayers had been answered without having to put the question.

Not so.  All this talk of weights and air gaps leaves me floundering.  All I  need to know is  -  how many fl.ozs. of ATF should I top up my 350F
with after having drained each leg via the drain plug?    My Clymer manual says 4.2ozs per leg, but when I poured in the measured amount, I found I had almost no travel on the forks.  So I deduce that 4.2ozs is too much.  Could this in fact be the amount needed when filling the  legs from empty after a stripdown?

When I had my 550, I seem to remember Mr Haynes giving figures for both 'filling from empty' and 'topping up'.
Has anyone by any chance got comparable figures for the 350Four please?

[K2-K6]

As the damping action is at the base of the fork, the oil level for damping only has to cover the valved section to avoid cavitation and causing a loss of damping.

Looking at the damper rod design of this type of fork it appears that the upper level for damping purposes is not highly critical......but too high a level will most certainly give you a hydrailic lock, in that the top of the oil reaches the fork leg cap and so prevents any further compression of the suspension taking place.

If the level is unknown, then it would seem logical to wrok it backwards to get a decent start point.......so if you take the total suspension travel (4.5inches?) and then double it to give you a minimum air gap with the forks fully extended.......using a syringe and piece of plastic pipe, mark off 9inches on the tube, insert it into the fork leg from the top until the mark is level with the stanchion, then draw off any excess to make both legs equal.

You should be able to verify with the fork caps still off by pushing each leg in turn up and down to make sure the damping is effective over the full range before riding it.

[Oldbuthopeful]

          Thanks K2-K6 for the lucid explanation. I'll give it a go in a day or two and report on progress.

[K2-K6]

Forgot to write an important bit to make this method effective......that you should leave the fork springs in the legs while you do it as they take up an appreciable amount of space in the same tube as the oil.

So if you do it with the springs removed....when you put them back in, it would increase the oil level by far too much.

It's interesting if you try to write an instruction just how much you have to remember to include all the little bits you take for granted when it's in front of you.

Hope you get to make a decent reference point for future use.....also to get them how you want as well.