900sd Darmah Marzocchi Forks Advice

I have a 1980 Darmah which was rebuilt around 7 years ago and hasn't been used since. The forks were very 'stiff' with little suspension or damping in evidence at all.

I've checked that the wheel is fitted correctly and the legs are not twisted. When I drained the oil, there was around 180cc in each leg, but very discoloured and quite viscous - possibly 20 grade or more. The spring lengths were similar, but I can't find what the spec is, so have no idea if they are right or not. The spacers below the top nut which are supposed to be 35mm long were actually 55mm but had been marked by someone at 35mm, as though they had meant to cut them down.

I cut them to the correct length and refilled the legs with 10 grade fork oil and they are much better, but still not great. The travel seems about right (can't be certain as the brakes are too weak currently to bottom them out- another job) but they don't really iron out the bumps.

I'm thinking to try lighter oil and possibly removing the spacers altogether - the forks would still be slightly loaded without them, but any suggestions or advice would be appreciated.

 

Thanks Paddy, really helpful! Do you know the correct spring length?

 

Where am I measuring this from and to? The length from the top of the fork to the bottom is 800mm.

Mine has 26mm compression with just the bike's weight, so I'm guessing the springs have had it. What do you recommend to replace them with? Are the YSS progressive springs any good?

 

As an update to this, having stripped the forks and rebuilt them, I found that the expander ring which sits behind the damper piston ring was missing from one leg altogether. Having cleaned everything and replaced this missing part, the forks now work better, but they are still far from great.

I haven't managed to source any suitable replacement springs. I tried some YSS progressive items, but they wouldn't fit in the spring guides and trying to fit them without is a non-starter as they just twisted up inside the legs and made a dreadful noise scraping the sides.

I'm pretty sure that losing one of the two oil seals would help matters, as would polishing the inside of the stanchions where the sliders move, but I still need to find some springs which will fit. If anyone has any suggestions, I'd really appreciate it.

Also, the damper pistions were installed as they are shown on the parts diagram - with the crown pointing up, but I've see a few instances where the piston is facing down in the leg instead. I'm not sure what, if any difference this would make, but does anyone know which way it should be?

 

Another update - just in case anyone ever needs to know:

I'm now pretty sure that there are two different configurations for these Marzocchi 38mm forks. Some, possibly the earlier ones, have the damper piston crown pointing down and use a different recoil spring from that shown in all of the diagrams (I've seen) which buts against the piston crown. Others, mine included, use a conical recoil spring which fits partially inside the piston which is fitted with the crown facing up, as shown in the diagrams.

If anyone has information to contradict this, please share by all means.

Also, on the subject of springs, I finally tried a set from Back to Classics. They took an age to arrive, but they were the correct diameter - 23mm - to fit in the guides and at 460mm, 5mm longer than the ones I had. I fitted them with the 35mm spacer, as specified and they have transformed the suspension. Never underestimate how tired springs can ruin the handling of a bike.

So after 6 months and a chunk of change, the bike now goes (like a rocket), stops (eventually) and handles really well. Just the slipping clutch to sort out now.....

 

I've already rebuilt the callipers, had the discs skimmed and fitted Brembo organic pads, so the brakes are as good as they will get. Not brilliant, but good enough. I think the clutch will either be tired springs- like the forks- or someone has put synthetic oil in at some point. We'll see....