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Discussion Starter #1
The clutch in my 99 ST-4 slips when the bike's cold. Once the engine warms up, it stops slipping. I did a search on 'slipping clutch' and found one other case where someone had experienced this phenomenon and written about it here. There wasn't much detail.

Does anyone have a theory as to why the clutch would slip when cold, but be fine when the engine's warm? Is this normal behavior for a dry clutch, to be 'slick' until the clutch material warms (or things expand)?

Thanks for any and all insights...
Alan
 

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Alan, it should not slip cold or hot. It may be worn out; figure 10-30K miles depensing on the gearing and type of use.

Check for play in the clutch lever. There should be several mm of travel before you feel any real resistance. If there is not, someone may have misadjusted the master cylinder plunger screw and its not fuully releasing. Although typically, slipping when hot (and the fluid expands) would be a symptom of that. Has the fluid ever been changed? If its black then it could use flushing out with new fluid; brake/clutch fluid absorbs moisture and becomes useless with time, so if it is more than a year or two since replacing it, now's a good time.

Assuming the clutch lever feels normal, you might consider getting inside the clutch cover for a closer look. If it hasn't been opened up for a while, there can be quite a collection of dust, rust and moisture in the clutch. Not a tough job, just take out the 3 (or 4) bolts that secure the tin cover and remove it; nothing should fall out or jump at you. A good blasting with some compressed air can blow out a ton of crap. Then you can see if theres any oily or greasy residue (there shouldn't be) or see if any parts are amiss. Pul in the clutch and watch the action; the plates should release smoothly and return fully. You can do this with it running, just don't get your necktie wrapped up in the clutch!

There should be six screws securing the pressure plate, and these should be snug (not tight, just 2-3 NM). If you feel adventurous, take the pressure plate off and unstack the clutch plates (just note the order/orientation for reassembly!) and clean everything with a mild solvent (kerosene/mineral spirits). You can deglaze the friction plates by sanding them with some 80 grit on a flat plate, assuming there's any friction material on them! Look for blueing of the steel plates, a sign of overheating and slippage.

Check that the throwout bearing in the pressure plate spins smoothly; if its notchy, its toast and will need replacing. The pushrod and its various seals and bearings are areas for investigation; if the pushrod won't slide when the engine is cold, it could partially release the clutch. Ditto the slave cylinder; if its gunked up or corroded then it will not work smoothly. There are various seals and orings that go bad over time and can lead to slippage, sticking etc. On a 99 that's not had good clutch maintenance, its likely time for cleaning, lubrication and replacement.

Lots to check, but actually pretty easy once you dig into it. If you don't have a shop manual, that would be money well spent!

Ciao!

pg
DD







The clutch in my 99 ST-4 slips when the bike's cold. Once the engine warms up, it stops slipping. I did a search on 'slipping clutch' and found one other case where someone had experienced this phenomenon and written about it here. There wasn't much detail.

Does anyone have a theory as to why the clutch would slip when cold, but be fine when the engine's warm? Is this normal behavior for a dry clutch, to be 'slick' until the clutch material warms (or things expand)?

Thanks for any and all insights...
Alan
 

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Discussion Starter #3
Thanks for the tips, Paul. The clutch was reportedly replaced not long ago, and there's a new slave cylinder (Evoluzione), too.

I'll pull the clutch cover and check things out. I've got a Workshop manual for a 2000 ST4, Snyder's Desmoquatro book, and a Haynes manual for the 94 to 01 superbikes.
 

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Did you figure out why it slips when it's cold? My 2016 Scrambler is doing the same thing
John, Your's is a wet clutch. Are you using automotive oil, like Mobil 1? If so, you are not supposed to be using that on a wet-clutch m/c. The friction modifiers will tend to make the clutch slip. But for the older, dry-clutch DUCs, it's OK to use M1 auto oil. For your Scrambler, you should be using motorcycle-specific oil (Mobil 1 makes that, also, but you have to really shop around to get it.
 
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