A strange clutch problem

[Ainslie]

Hi All,
The clutch on my '95 ST1100 outfit works perfectly on start-up. However, the clutch does not engage if I use the clutch lever when the engine is warm. Put simply; all is normal until I snick the bike into 1st gear with a warm engine. Nothing happens when I release the clutch to move off! The clutch remains disengaged with the clutch lever fully out.

This problem appeared suddenly, out of the blue, two days ago. Everything else on the bike is operating normally. I bled and changed the hydraulic fluid for clutch operation three weeks ago. The clutch operated faultlessly until two days ago.

In a way, I'm relieved that it is a clutch problem only. I thought I was dealing with a broken drive shaft when it first happened.

Any advice or comment will be greatly appreciated. Yep, I have already ordered a workshop manual. Nonetheless, advice coming from the lived experience of a problem is often more valuable. Safe riding, all!

 

[naedlaen]

Plugged line (as fluid returns) in the master cylinder? Flushing it might have allowed some crud to loosen and float up to plug the return orifice in the master. It would be the simplest to check.

Nice hack!

Neal

 

[Ainslie]

Thank you for the prompt and insightful comment Naedlaen!

Come to think of it, I did need to take a BIG handful of the front brake just before the problem emerged. I just wonder about the relationship of this weird problem with a warm engine. Maybe the hydraulic oil causes the crud to swell as it heats up. Will remove the master cylinder and blow out with air. Its then over to removing the slave cylinder if that doesn't work!

My brother and I co-own the outfit. We both love long-distance bike touring but now have trouble with heavy solo's. I'm 74 now and will soon need sell my cherry BMW K1100 RS solo.

Here in Australia, it is a long way between things. There are lots of deserts and long hot days in the saddle. Thousand-kilometre days are not unusual in sparsely populated central Australia. Our ST1100 outfit fits the bill perfectly. It will easily run at 130 kph all day with reasonable fuel consumption. We can stop anywhere to camp as the sidecar is a double and has a boot. There is plenty of room for food, tools, camping gear (and cold beer).
Safe riding

 

[naedlaen]

Without starting the bike, put it in gear without pulling the clutch and try to rotate the rear wheel. Rotates like in neutral, your clutch is disengaged. If it does not rotate, clutch is engaged. If it is engaged, pull the clutch to disengage it and then release and try the wheel again. If it now turns, the hydraulic pressure is not released. Crack the banjo at either end (protecting the paint) and check the rear tire for rotation.

The hard braking may have caused crud to break loose, but the two systems are separate, so that event is coincidental to the clutch issue.

Neal

 

[TerryS]

In the floor of the clutch master is a small metal shield that sits over the compensation port. When you pull the clutch you should see some fluid movement from that port; the shield is removable, and is there to stop you squirting brake fluid out of the master when you have the lid off the master. If that port gets blocked the line pressure can't release from the hydraulic system and eventually your clutch slave is holding the clutch plates apart enough that you get no drive.

 

[jfheath]

The other alternative is that the push rod is not sitting inside the hole in the side of the brass bush. Instead it is sitting on the side of the bush - which will push on the push rod, (always partially disengaging the clutch). It also closes off the compensation port. As the engine warms up, so does the fluid. It expands and has nowhere to go to - except to the clutch slave cyclinder. So in effect the heat from the engine is expanding the fluid and disengaging the clutch. So you are almost right with your heat expansion theory - but it is the fluid expanding with nowhere to escape to that is probably the issue)

If there is crud blocking the compensation port (as @TerryS described), exactly the same thing happens. Each operation of the lever allows a little more fluid into the clutch line, but it can never escape.

These are two easy checks. With the silver cover off the port inside the reservoir, you can see the piston go past the inlet port when the lever is released. With it fully retracted (watch it go past), you can poke that hole without fear of damaging the seal.

The brass bush - you would have to remove the clutch lever. One lock nut underneat, one screw. Before you remove the lever, make sure you can see where the plunger meets the bush - then you will know if that was the problem. Before you put it back mark the location of the hole on the circular top of the bush - so that you can line it up with the push rod.

edit - It is unlikely to be the slave cyclinder. The clutch is disengaged, therefore there is something pushing it in. So the slave is holding its pressure.

 

[Ainslie]

Thank you so much for the excellent advice. I feel that jfhealth and TerryS have provided a way forward for me.