I think I may have found the culprit behind my '94 ST1100 clutch slipping. A stuck thermostat. When the oil is too hot, the clutch slips. Thoughts?
Slipping Away
- Thread starter Dark Night
- Start date
ToddC
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Wrong oil.....
Andrew Shadow
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More likely that the excess heat is vaporizing the moisture in the clutch line causing it to expand and release the clutch a little.
When was it last bled and flushed?
When was it last bled and flushed?
I ruled that out by changing to the recommended, Honda OEM oil - to no avail.
Not knowing the type of hydraulic fluid in 0lace from the previous owner, I flushed they system, rebuilt both the master and the "release", and used DOT4 fluid. I also replaced the clutch plates and the springs. Everything worked fine - until the engine got too hot. Then the clutch slipped just as it had done at the outset. What's too hot? I don't know in degrees, but once the temp gauge gets to halfway, the clutch slips. I grounded the sending unit to the fan to make sure the fan was functioning. It kicks-in when the temp gauge gets a little above half. I then opened the thermostat housing, removed the thermostat, and found that it was frozen in a slightly open position. I'm awaiting a new one and hope that it solves the problem. Fingers crossed.
But I'm curious. Have others encountered this issue?
I've never heard that overheating causes clutch slipping (but then, I'm not an expert in this area). If the oil is overheated, I would expect it to thin out, lose viscosity, and allow the plates to still function as designed. From what I know about clutches and thermostats, there is no reason at all why a stuck stat should affect the clutch. What kind of riding were you doing when this slipping occurred? Were you shifting a lot, slowing and accelerating on curvy roads or doing a constant speed on a highway?
My 1300 suffered from a stuck (closed) stat on a hot day in PA. I was able to let it cool, drive a couple of miles at low speed (construction traffic was stop and go) until I got 4+ bars, stop and let it cool. After a bit, the traffic eased and I was able to get the bike up to 50 mph, and that kept the temp at 3+ bars until I got home. Nary a whisper of the clutch slipping.
What brand of oil and what weight did you use after you changed the clutch plates? Did you use OEM parts?
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How about the clutch fluid getting too hot, expanding and cannot escape - the pressure build up disengages the clutch. I have experienced this on my brand new 1300.
Check the brass bush in the clutch lever. Make sure that the plunger is seated in the hole in the side of the bush.
Also check the tiny compensation port in the reservoir under the chromed cover. If that is clogged, expanding fluid cannot escape.
When you operate the lever, check that the seal can be seen moving completely past the inlet port - viewed from through the fluid in the reservoir. You may need a bright light.
Under the clutch slave cylinder, front of the engine, on the joint between the crankcase and the slave - in the 6 o'clock position there is a small hole. Poke it with a blunt pointy thing, and catch whatever comes out. There should be nothing - but that hole is a weep hole for any brake fluid that gets past the slave piston seal, or for any oil that gets past the clutch bearing seal. Either way, you want to know what it is and how much. Once it is clear, pump the clutch lever a few times, see if it pushes anything else out.
I guess that the hole is also a vent. I never thought about it before, but the space between the slave piston seal and the clutch bearing is air. Its Ok, air will compress, but nevertheless, the hole should be clear.
Check the brass bush in the clutch lever. Make sure that the plunger is seated in the hole in the side of the bush.
Also check the tiny compensation port in the reservoir under the chromed cover. If that is clogged, expanding fluid cannot escape.
When you operate the lever, check that the seal can be seen moving completely past the inlet port - viewed from through the fluid in the reservoir. You may need a bright light.
Under the clutch slave cylinder, front of the engine, on the joint between the crankcase and the slave - in the 6 o'clock position there is a small hole. Poke it with a blunt pointy thing, and catch whatever comes out. There should be nothing - but that hole is a weep hole for any brake fluid that gets past the slave piston seal, or for any oil that gets past the clutch bearing seal. Either way, you want to know what it is and how much. Once it is clear, pump the clutch lever a few times, see if it pushes anything else out.
I guess that the hole is also a vent. I never thought about it before, but the space between the slave piston seal and the clutch bearing is air. Its Ok, air will compress, but nevertheless, the hole should be clear.
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Interesting problem .
Is there any shifting of the release point on the clutch lever between cold and hot ? Is there any different feel in the lever cold vs hot ? In other words does the clutch start to engage sooner (on the lever) when the bike is cold ? If there is , it would seem like there is some kind of pressure build up in hydraulic the system .
- Master cylinder not getting released enough to let fluid back to the reservoir . Cogged return ? The Brass bushing problem ( does the ST11 have one?) ? . But it seems strange this would only effect the ST when warm.
- Sticky / problem with the slave cylinder not resetting all the way back . Clutch might work when oil/engine is cold ,but start slipping when oil is warm and thin. Again strange that this would only happen the bike when is warm
It dosent sound like your engine is too hot. Open thermostat and working fan . Brake fluid dosent expand that much anyway and if at the point of boiling ,you would expect the opposite -clutch engaged . The only part that would get heated is the slave cylinder
Your using the right oil, so that should not be the problem.
Weak / wrong springs ? - again might work when engine /oil is cold They are new ? But..
Just my thoughts . My ST seems to engage so far out on the the lever now, that I think I'm in neutral half time I start from a stop. But it dosen't slip- yet
Good luck !
Is there any shifting of the release point on the clutch lever between cold and hot ? Is there any different feel in the lever cold vs hot ? In other words does the clutch start to engage sooner (on the lever) when the bike is cold ? If there is , it would seem like there is some kind of pressure build up in hydraulic the system .
- Master cylinder not getting released enough to let fluid back to the reservoir . Cogged return ? The Brass bushing problem ( does the ST11 have one?) ? . But it seems strange this would only effect the ST when warm.
- Sticky / problem with the slave cylinder not resetting all the way back . Clutch might work when oil/engine is cold ,but start slipping when oil is warm and thin. Again strange that this would only happen the bike when is warm
It dosent sound like your engine is too hot. Open thermostat and working fan . Brake fluid dosent expand that much anyway and if at the point of boiling ,you would expect the opposite -clutch engaged . The only part that would get heated is the slave cylinder
Your using the right oil, so that should not be the problem.
Weak / wrong springs ? - again might work when engine /oil is cold They are new ? But..
Just my thoughts . My ST seems to engage so far out on the the lever now, that I think I'm in neutral half time I start from a stop. But it dosen't slip- yet
Good luck !