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Old 18-May-2009, 23:46
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Question More clutch wierdness - help.

Here's the long story:

Did a trackday at Brands on 7th May. Halfway through the afternoon I noticed the clutch lever was a lacking in travel and a bit "floppy".
Inspection revealed the tell-tale signs of hydraulic fluid on the sidestand - "Aha" I thought, "clutch slave cylinder seal is weeping".
Blagged a bit of fluid off a kind chap and finished the day with the clutch still working but not perfect - it wasn't slipping so it was fine.

Couple of days later, invested £2.81 on a new seal and plopped the piston out, cleaned bore, new seal, bled system and felt satisfied that the old girl had done over 130 miles on track for one minor and easily remidied fault.

Couple of days later, went up to Box Hill, after a few miles the clutch was slipping like a *******.
Came home and on inspection noticed that the clutch pressure plate was not sitting tight on the clutch pack - it was like the clutch was partially engaged.
Re-bled the hydraulics and in doing so the pressure plate sat back tight on the clutch pack. I have a bleed nipple at the lever end so I bled it from both ends.
Went out for another ride and again, slipping after a few miles.

This weekend I stripped out all the plates etc and they are showing signs of serious slippage i.e. shiney bronze on the friction plates.
As a bit of a bodge, I swapped some of the knackered friction plates with some not so worn ones from a 748 (yes, I know they are thicker than SPS ones but they were part worn and about 2.5mm thick so should not effect the stack height).

Just been out for a ride - no slipping to report after some very serious "testing" - however, the clutch bite point changed after a few miles so that it would disengage with only a slight lever pull. It made pulling away from junctions a bit of a pain.

Something is expanding in the system so that when it gets hot it's like the clutch pushrod is getting longer and pushing on the pressure plate!
I marked the pressure plate centre bush (the bit that the rod engages in) with a chalk mark and started the engine - the bush does not rotate with the spinning clutch. This means the rod isn't rotating and so all the rod bearings must be OK. This means that the rod isn't heating up the slave cylinder piston and doing wierd stuff to the fluid.

Any thoughts from clever people out there?????

Cheers,

R
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Old 18-May-2009, 23:49
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Have you replaced the O-rings on the slave end of the clutch push rod, they play havoc with the action when they harden off


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Old 19-May-2009, 00:06
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Good point Kev, I'll get some tomorrow and see if that helps.

R
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Old 19-May-2009, 00:09
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I've had a clutch push rod seize solid on my 750SSie due to those two o-rings getting so hard, No amount of pulling the clutch lever could get it to move, had to remove the clutch and bash it out with a copper mallet.


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Old 19-May-2009, 01:57
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My guess is that one or both master cylinder reservoir ports are partially or fully plugged and the fluid can't return to the reservoir when the fluid heats up.

So when the fluid expands it pressurizes the system and either leaks past the slave cylinder seal or pushes the pushrod causing slippage. First it pushed past the seal, but because it couldn't get by the NEW seal it moved the slave cylinder and therefore the pushrod. Just a millimeter or two is all it takes.

This would also account for the moving engagement point at the lever.

An alternative explanation is that someone adjusted the MC piston stroke incorrectly and the return port is being covered by the MC piston.

Properly adjusted, there should be a 5-10% dead-band of the full lever movement before there is pull resistance felt at the lever. This freeplay is necessary to prevent covering the port as the piston seal expands normally over time, and to avoid placing the piston seal rest location (where corrosion occurs) right at the return port orifice.

Once adjusted, the manufacturer places a potting compound over the screw adjuster to prevent accidental or incorrect adjustment. Check to see if the potting compound has been removed.

Last edited by Shazaam! : 19-May-2009 at 02:06.
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Old 19-May-2009, 15:00
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Shazaam,

Sounds like a feasible theory - I will add it to my list of stuff to check.

Since it has only just started exhibiting the fault and nobody has adjusted any screws/potting compound - how exactly do I check if the ports are blocked?

R
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Old 19-May-2009, 15:55
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Look for a squirt of fluid in the reservoir during the first 5% of the lever pull (Feed Port)
Push the slave cylinder back in its bore and look for a squirt in the reservoir (Return Port)
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Old 19-May-2009, 16:04
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OK will do. What happens if I don't get the correct squirts? Is it possible to clean the ports?
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Old 19-May-2009, 17:58
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I don't recommend trying to disassemble and clean master cylinders or to use rebuild kits. Leave it to the manufacturer to rebuild them. The skill level demands are too varied ... and the safety issues are not inconsequential.
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Old 19-May-2009, 21:42
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Mood: Ugly girls (749) are more dirty.....
Took a look in the reservoir for evidence of squirts - NONE!
Mind you, it's a standard 916SPS with a remote reservoir so would you still see the squirts?
I remember my old Suzuki had a non-remote (brake) reservoir and you could see the squirts as you pulled the brake on and off.

Anyway - dunno what to do now!
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