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Discussion Starter · #1 ·
I write this from the side of the road, my clutch slipped at above 5000 rpm last , once, then after a 20-30 stop it did not reappear. Now on way home it started slipping at 5000 rpm, then 4000 and I'm on the side of the road cause I can't get it to move. I can release clutch in any gear from the stopped position and the bike won't move.

It like the clutch is no longer engaging. No leaks, clutch handle pressure feels ok, oil level ok, don't know what else to check for.

What could it be?


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Old Wizard
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If the return port in the master cylinder is plugged, as the clutch fluid heats up the fluid expands and instead of returning to the reservoir, it pushes on the slave to separate the plates and cause slippage. After it cools the clutch will function normally until it heats up again.
 

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Life is too short to worry !
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If it is excessive pressure in the line then you could release the bleed nipple on the slave and let some fluid out enough to allow you to get home with a poorly functioning clutch but at least you could get home to renew the fluid.
 

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I write this from the side of the road, my clutch slipped at above 5000 rpm last , once, then after a 20-30 stop it did not reappear. Now on way home it started slipping at 5000 rpm, then 4000 and I'm on the side of the road cause I can't get it to move. I can release clutch in any gear from the stopped position and the bike won't move.

It like the clutch is no longer engaging. No leaks, clutch handle pressure feels ok, oil level ok, don't know what else to check for.

What could it be?


Sent from my iPhone using Motorcycle

What year/model ST? Was the clutch assembly recently serviced/bled? How many miles on the clutch?
 

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If the return port in the master cylinder is plugged, as the clutch fluid heats up the fluid expands and instead of returning to the reservoir, it pushes on the slave to separate the plates and cause slippage. After it cools the clutch will function normally until it heats up again.
The return port is probably blocked by the piston not fully returning when the lever is released. Back off the adjuster in the lever to give a little clearance.
 

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Discussion Starter · #7 ·
The clutch did work normally after it cooled, I sat there for about 30 min till temp got to about 160' then within a few min it started again.

So from a beginners perspective, how would I unclogg the line and/or tighten basket bolt as suggested?


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i had the identical problem with an SS.

it took a master cylinder replacement to solve. i changed slave first, cheapest, to change, changed master and viola' it was fixed.

bob
 

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Life is too short to worry !
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The clutch did work normally after it cooled, I sat there for about 30 min till temp got to about 160' then within a few min it started again.

So from a beginners perspective, how would I unclogg the line and/or tighten basket bolt as suggested?


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If it worked after cooling down then you dont need to worry about the bolt as it will be some sort of problem with the hydraulics.
 

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Old Wizard
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Here’s a simplified illustration of a master cylinder.

The red arrow on the left points to the vent port a.k.a. the compensating port. This port allows fluid to expand back to the reservoir when the fluid heats up and increases in volume. This port has a tiny orifice and can easily get plugged from debris that has entered the system.

The right red arrow points to the inlet port a.k.a replenishing port. This port refills the system as the fluid cools and decreases in volume. This port is larger and presents less of a plugging potential.

Replacing the master cylinder will solve the problem but there may be alternatives.

A common reason for the vent port being covered is the installation of improperly-sized aftermarket levers that interfere with the complete retraction of the piston. Using the proper levers will solve this.

If the lever has been mis-adjusted by a mechanic or previous owner you can have it readjusted. 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 free-play is necessary to prevent covering the vent port as the piston seal expands normally over time, and to avoid placing the piston seal rest location (where corrosion occurs) right at the vent port orifice.

When the lever is pulled the piston moves forward with the lip of the seal covering the vent port. Before this port is covered, piston motion forces excess fluid back into the reservoir. If you look into a reservoir just as the piston begins to move you will see slight fluid squirt.

If there's debris plugging the port (no squirt) you can disassemble the MC and clear it with compressed air
 

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Discussion Starter · #11 ·
Thx all for input, the reservoir was filled over max. I also saw some oil at drops at mastercylinder. Siphoned out some and will see how it goes tomorrow


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I'm having a similar problem. I let the bike rest for a few minutes, then I could limp home. Shazaam, you rock. pictures, clear explanation. very cool.

I had a similar problem with a Honda rear brake seizing. I wound up redrilling the vent port with a teeny/tiny drill bit I borrowed from our machine shop. (it was ages ago when money was quite tight and rebuild kits were more common).

Just wanted to say thanks for a great forum. when so little goes wrong with my ST4S, I don't have much to post about. suppose that's a good thing.
 
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