Lost Drive Power

KirkJ

KirkJ

Joined
May 4, 2025
Messages
3
Country flag
While riding yesterday my 750 lost drive power and coasted to a stop (engine still running, bike in gear). I checked the clutch operating lever and it is in correct position. Pulled off the chaincase outer cover. When I pull the clutch lever I see deflection of the clutch spring plate. With bike in gear I can easily spin the rear tire which in turn spins the front chain however I do not hear or feel any movement in the engine. When I engage clutch the front chain does not move. The kickstarter feels as if the clutch is engaged and does not turn over the engine. Do you think this is a clutch adjustment issue or something more serious?
 
To confirm, you see the "front chain", aka Primary chain, turning when rotating rear wheel in gear with clucth not held in? That would mean the sprocket on the crankshaft is spinning on the shaft. That is a taper fit on the crankshaft with a woodruff key...

Or did I misunderstand and you get no primary chain motion rotating rear wheel in gear, whxi indicated clutch plates are slipping, not holding? If thats the case, try fully slackening clutch cable at the handle bar, undo the clutch adjuster screw and its locking nut on the clutch assembly, making it fully loose, then slowly turn it in until you can just see/feel the clutch diaphragm spring start to comppress. Now undo that samexadjuster 1/2 to 3/4 of one turn, tighten the locking nut without moving the adustment screw. Now take up cable slack at jandlebar, leaving a few mm's freeplay at cable outer sleeve end cap. Confirm cable is correctly engaged at the gearbox operating arm, which can drop down out of position during this setup.
 
baz said:
Sounds like a sheared woodruff key
But we need more details
Click to expand...
My understanding is woodruff is not meant to take loads of torque, it is the taper being clamped hard down from the big nut on outerside of rotor that takes all the load. On Velocettes, the actual crankshaft is held in precise alignment by nothing other than a taper fit, no cross bolting or pins like other makes. A taper is a very strong way to joint such things indeed!
 
Tornado said:
My understanding is woodruff is not meant to take loads of torque, it is the taper being clamped hard down from the big nut on outerside of rotor that takes all the load. On Velocettes, the actual crankshaft is held in precise alignment by nothing other than a taper fit, no cross bolting or pins like other makes. A taper is a very strong way to joint such things indeed!
Click to expand...
Yep in engineering the taper should take all the load
But in the real world those tapers may not be perfectly ground
And people fitting new crankshaft sprockets may not have taken the time to lap the two surfaces together for a perfect fit
That's the reason that triumph clutch centres have an extra keyway broached into them when tuned for performance
 
KirkJ said:
While riding yesterday my 750 lost drive power and coasted to a stop (engine still running, bike in gear). I checked the clutch operating lever and it is in correct position. Pulled off the chaincase outer cover. When I pull the clutch lever I see deflection of the clutch spring plate. With bike in gear I can easily spin the rear tire which in turn spins the front chain however I do not hear or feel any movement in the engine. When I engage clutch the front chain does not move. The kickstarter feels as if the clutch is engaged and does not turn over the engine. Do you think this is a clutch adjustment issue or something more serious?
Click to expand...
@KirkJ, Welcome, you've come to the right place.

As mentioned above.

And to further clarify, clutch is considered "engaged" when the lever is out and providing engine power to the gearbox. It is considered "disengaged" with the lever in, like when changing gears or stopped at a light.
Not splitting hairs, but being accurate so we can help.

If the primary (you said "front") chain moves, engine off, then the sprocket is spinning on the shaft or worse.
Usually, the engine sprocket/rotor/spacer is misassembled and rattles loose.
Crankshaft damage is likely.

Any unholy noises heard before the stoppage?
 
When mind blew it didn't just dropped all the balls and cage to the bottom of the GB, so no lock up at all just lost drive when I put in gear, so not all lock up, my mate's 850 did the same but 100 miles from home manage to put it 4th gear and got home just using 4th gear and a lot of clutch slipping at the bottom off Nambor hill climb before the bypass
robs ss said:
If it was that Ash, the gearbox would have likely locked up
Click to expand...
 
Tornado said:
To confirm, you see the "front chain", aka Primary chain, turning when rotating rear wheel in gear with clucth not held in? That would mean the sprocket on the crankshaft is spinning on the shaft. That is a taper fit on the crankshaft with a woodruff key...

Or did I misunderstand and you get no primary chain motion rotating rear wheel in gear, whxi indicated clutch plates are slipping, not holding? If thats the case, try fully slackening clutch cable at the handle bar, undo the clutch adjuster screw and its locking nut on the clutch assembly, making it fully loose, then slowly turn it in until you can just see/feel the clutch diaphragm spring start to comppress. Now undo that samexadjuster 1/2 to 3/4 of one turn, tighten the locking nut without moving the adustment screw. Now take up cable slack at jandlebar, leaving a few mm's freeplay at cable outer sleeve end cap. Confirm cable is correctly engaged at the gearbox operating arm, which can drop down out of position during this setup.
Click to expand...
Thanks for your response Tornado. The woodruff key was sheared off.
 
concours said:
@KirkJ, Welcome, you've come to the right place.

As mentioned above.

And to further clarify, clutch is considered "engaged" when the lever is out and providing engine power to the gearbox. It is considered "disengaged" with the lever in, like when changing gears or stopped at a light.
Not splitting hairs, but being accurate so we can help.

If the primary (you said "front") chain moves, engine off, then the sprocket is spinning on the shaft or worse.
Usually, the engine sprocket/rotor/spacer is misassembled and rattles loose.
Crankshaft damage is likely.

Any unholy noises heard before the stoppage?
Click to expand...
Yes sorry for the error regarding the clutch. The key was sheared and a replacement is on the way. I did not hear any noise at all prior the drop in speed. The engine was running and sounded normal as I coasted to a stop. I pulled the plugs and was able to turn over the engine. No strange noises so I hope it was just an alignment issue which caused the key to fail.
 
baz said:
So was the alternator nut loose?
What's the history of the engine?
Don't just fit a new woodruff key as you will need to address any scoring etc
And you absolutely must lap the two surfaces back in again until you have perfect contact
Click to expand...
..exactly! The tapered fit should carry the load, not the key.
 
As Baz is suggesting the key is just for location purposes, it is too weak to transmit the engines output, the taper on the crank and the chain sprocket carries the torque. So the key breaking tells you the taper surfaces are not good or the crank nut was not able to apply enough force to seat the taper. The crank nut could have not be tightened enough or a burr or such could have got in the way.
 
@KirkJ
As mentioned above, the scoring/galling on the two tapered surfaces must be closely inspected. A talented mechanic with a polishing stone may be able to save the crankshaft.
The sprocket should be replaced.

Pictures please.
 
robs ss said:
Doesn't really matter, the spinning damage will have occurred to the taper fit.
Click to expand...
It matters a LOT.
The shearing of the key deforms much around it.

The remnants of the mangled key will destroy adjacent parts.

Whole different deal. Missing key would be less damaging.
 

Similar threads

T
norvil belt drive.....positioning the extra steel plate
Replies
1
Views
302
Schwany
Schwany
Tornado
Compression/Power loss Left Side
2
Replies
30
Views
1,631
kommando
kommando
NickZ
tricky drive chain adjustment
Replies
3
Views
334
ebyker
E
click
850 MK3 speedo drive/gearbox
Replies
5
Views
385
marshg246
marshg246
Saber
Lost Clutch Issue
2 3
Replies
47
Views
2,826
Saber
Saber
Back
Top