New fork seals/oil, not quite right

[zooter]

The bike shop supposedly blended oils to give me 15 weight, all the old mushiness has gone and it goes nice on corners but cruising straight it tends to pogo on undulations. Wassup?

[geoffm]

The bike shop supposedly blended oils to give me 15 weight, all the old mushiness has gone and it goes nice on corners but cruising straight it tends to pogo on undulations. Wassup?

Could be the oil viscosity is now to high, giving you to much compression damping, so that the forks aren't moving quickly enough on hitting small bumps. A lighter weight oil with a smaller air gap may help. This is assuming it is skipping over the bumps.
If it is wallowing and pogoing - ie it keeps on bouncing after the bump, then the oil may be a bit light. Have you set the static sag (amount that the fork moves when you sit on it at rest). Does it bottom out? If so, you may need stiffer springs and/ or a smaller airgap above the oil.
Decreasing the air gap gives a higher effective spring rate due to the compression of the air above the oil. It is a rising rate due to the compression of the air, so has more effect at higher displacements. Don't go overboard or you will hydraulic lock the forks (REALLY stiff!) or pop the fork seals.
Preload is used to set the static sag, but once the fork starts moving, then the fork is controlled by the springrate and damping (compression going up, rebound going down). With conventional forks, you have a compromise between rebound and compression damping, as both use the same oil and holes in the damper rod.

The other issue is that a lot of problems are caused by the other end, which may be showing up now the front is sorted. What are the back shock(s) like? If they are rooted and bouncy, then this can be the cause of the problems.
Geoff

[zooter]

Cheers Geoff. Could be the back end, 40,000 ks on the bike but feels ok there to me, not that I know zip. Any rule of thumb for gauging the shock performance or is all seat of the pants? ( I'm after a replacement from a gsxr750 '88 to '95 or so BTW )
The front is not too hard/`skipping over bumps. It is multiple rebounding after the bump. It is way stiffer than it used to be when it would dive really badly on braking but not wallow, go figure (please).
Actually typing all this out has kind of crystallised a theory. Assuming the new front end is now approprately stiff but the rear too soft then the rear could get loaded up by a slight pitching of the bike then the back hits same bump and loads up some more rebounding and pitching bike back as the front is also rebounding. What a mouthful. Would definitely cause the pogo effect I'm getting. It would also be speed dependent which I haven't tested out.
Any more theories floating around?

[geoffm]

Cheers Geoff. Could be the back end, 40,000 ks on the bike but feels ok there to me, not that I know zip. Any rule of thumb for gauging the shock performance or is all seat of the pants? ( I'm after a replacement from a gsxr750 '88 to '95 or so BTW )
The front is not too hard/`skipping over bumps. It is multiple rebounding after the bump. It is way stiffer than it used to be when it would dive really badly on braking but not wallow, go figure (please).
Actually typing all this out has kind of crystallised a theory. Assuming the new front end is now approprately stiff but the rear too soft then the rear could get loaded up by a slight pitching of the bike then the back hits same bump and loads up some more rebounding and pitching bike back as the front is also rebounding. What a mouthful. Would definitely cause the pogo effect I'm getting. It would also be speed dependent which I haven't tested out.
Any more theories floating around?

Sounds quite possible, especially if it continues bouncing after a bump. This is often a sign the damping is gone - and the shock is overdue for the bin. The standard shocks of the era (is your GSX the one with the 'lecy screen?) were pretty crap and went to lunch pretty quickly. The later GSXR ones (I had a 1990 GSXR750L) were very good. You could try seeing if the shock can be rebuilt. Most factory shocks aren't, but it is worth asking about... if it is rooted, there is not much you can do, without spending money.
Is there anyone around with a shock dyno? It is the only real way to check the shock for fading, etc.
Geoff

[zooter]

Thanks Geoff. My bike is a naked shafty. The gsxr shock is the one I'm after.

[FROSTY:Me]

15w oil in the gsx sounds right to me. maybee they as geof said put slightly more oil in as well.
how many Kms has ya G done??

[zooter]

40,000 ks, just run in. I'm going to try unwinding the rear damping adjustment tomorrow. It's set to stiff at the moment so it should make it worse if I unwind it nay?

[geoffm]

40,000 ks, just run in. I'm going to try unwinding the rear damping adjustment tomorrow. It's set to stiff at the moment so it should make it worse if I unwind it nay?

It depends if the rear shock is to hard, so that the back end bounces and upsets the front. If the damping is gone, it will wallow.
Came across this http://www.georgestock.co.nz/div_koni_workshop.html while looking up BMW part numbers. I see they have a shock dyno and my be able to test your shock to see if it is rooted or not.
Geoff

[zooter]

so I wound out the damping adjustment and now much improved, 99% happy with it now but haven't had it for a high speed blat as yet.