rear end locked up

I was riding my 1974 850 Sunday, when the motor suddenly died, rear end locked, pulled in clutch, slowed to about 25mph, let out clutch and immediately re-started. Rode home about 10 miles and seemed ok. On examination oil on kickstart shaft. At time of lock-up was going about 60 up steep hill and probably should have been in 3rd not 4th, about 3500 rpm. Any suggestions or help is appreciated. Thanks :?

michael levato said:

I was riding my 1974 850 Sunday, when the motor suddenly died, rear end locked, pulled in clutch, slowed to about 25mph, let out clutch and immediately re-started. Rode home about 10 miles and seemed ok. On examination oil on kickstart shaft. At time of lock-up was going about 60 up steep hill and probably should have been in 3rd not 4th, about 3500 rpm. Any suggestions or help is appreciated. Thanks :?

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Could be lots of minor things, so let's start with worst case scenario first ( :twisted: ): If a motor dies and the rear end locks, and you pull in the clutch it frees, it could be a seizure.

I would drain your oil and look for bits of aluminum from you pistons. Also pull a spark plug, rotate the motor with the kickstart, and get in there with a probe light to get a look at the cylinder walls. If they are scuffed (vertical lines up and down), you know you had some rubbing. Hope it's not this, but if so, you are lucky it didn't spit you off onto your head.

Holmeslice said:

michael levato said:

I was riding my 1974 850 Sunday, when the motor suddenly died, rear end locked, pulled in clutch, slowed to about 25mph, let out clutch and immediately re-started. Rode home about 10 miles and seemed ok. On examination oil on kickstart shaft. At time of lock-up was going about 60 up steep hill and probably should have been in 3rd not 4th, about 3500 rpm. Any suggestions or help is appreciated. Thanks :?

Click to expand...

Could be lots of minor things, so let's start with worst case scenario first ( :twisted: ): If a motor dies and the rear end locks, and when you pull in the clutch it frees, that sounds like a seizure. I would drain your oil and look for chunks. Hope it's not this, but if so, you are lucky it didn't spit you off.

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He did ride it home. But you're right, check the oil.

Oil on the kickstart shaft is normal, probably has nothing to do with the bike stopping.

swooshdave said:

He did ride it home. But you're right, check the oil.

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Yep, bumped it while rolling at 25mph. Could be enough to free it up. Let's hope it's something less sinister though.

Michael, very bad news even if lucky it ran good afterwards.
Kicker oil likely the tire lock up shock back into drive train twist up.

Consider the force applied to something major to lock up on such momentum.
Not fun for me to contemplate what flash backs this elicits.
I'd be pensive a piston was injured and fatigue failure lurking.
I'd be pensive a rod cap bolt or two was over stressed.
Possibly crank shaft bent and/or stuff its ends attached to, leash jerked too.

If it seems to start and run ok then about nothing you can do externally will help make a decision to just ride it on out or open everything up.
I vote takes ya chances for a while staying fairly close to home or ride with others to comfort you until retrieved.

hobot

Oh yeah the flash back just keep flooding in. Can't forget the jerking the drive train got too, think bent/twisted main shaft and distorted wobble sleeve bushes, etc.

hobot

I took a short 15 mile ride yesterday, shifted fine, no loss of power, no smoking, in short ran fine. Changed gearbox oil and some very miniscule metal shavings. Then changed motor oil and filter. In sump plug bottom were a few small metal fragments, greenish in color. Not sure how long they've been there as this was only my second oil change on recently purchased bike and the first time I had neglected to dismantle the sump plug.
Your suggestions are appreciated and I too am glad I didn't go on my head. (have done that twice before and that's enough) :?

Ok then, stiff upper lip in pensive expanding test rides till ya forget about the event for ever more we hope. Copper makes greenish compounds, can't think but maybe cam thrust washers or bushes that could show up in sump. If a Combat it'd be its known to shed washer tabs, in your case, who knows till opened someday for just fun and new rings and shells. Yes please, no more hammer heads for any of us, any more.

I had a two stroke Bultaco Alpina that used to do that all the time when at WOT for extended periods. In that case it was the piston seizing up in the bore. Let it cool a bit, it ran fine. Did that for years and since it was seldom at WOT for extended periods, I never worried about it.

However, the main point is that it IS a serious issue that needs to be checked out. Frankly, I can't think of any other scenario than the piston seizing in the bore.

Yes temporary piston seizures are well known to make that a first thought.
Some can go a long ways afterwards, others not. I'd suspect a horizontal imbalance if one piston dragging-hanging more thean the other for a sense of buzz getting past isolastics. Then again maybe not. But you are on a Commando dear sir, they automatically attract new people any where they stop, by pilot plan or chance stuck somewhere. Go forward and face fate to see whats next and report back the lingering effect in brain.

hobot

From personal experience, cylinder seizures have generally been preceeded by a rapidly increasing loss of power rather than a sudden lock-up (the last time due to super-expanding Chinky pistons in an A7), but the only sudden lock-up was due to a big-end grabbing...again, on a BSA :roll:

I also has a big-end grab on a T150...it was just a momentary loss of power then it carried on just fine for another hundred miles or so, until the rod punched a window in the crankcase. I suspect that the shells had grabbed on the crank, and the rod was then running on the shells, but difficult to prove, as the evidence was left on the M61 near Bolton!

I was wondering if this particular episode may have been the layshaft collapse issue, but I'm guessing that pulling in the clutch wouldn't have helped if this had been the case?
A couple of years ago I had a chain snap on my T140 whilst accelerating hard in 2nd, resulting in a sudden lock-up.
This cost me a mainshaft and a second gear...the Tooth Fairy had paid a visit :cry:

Sorry to say this, but if this had happened to me I'd be pulling both the engine and gearbox apart before things develop into a major let-go. There's just too much uncertainty given the lack of a root cause for the lock-up, IMHO

"Recently purchased bike" means you should look into your gearbox at the layshaft bearing anyway, so I'd start there. At "worst," you get rid of a bearing that wasn't causing problems yet but may have at any time (okay, at worst, you find the PO already did... still, you can't take the chance). At best, you've found your problem.

Some dunce here already suggested the depth of the bad news pensive riding or deep wrenching to follow. I guess this episode answers the Buddic Question of a Perfect Commando, its the fully fettered fresh one, not yet started on its next rebirth cycle, each time trying to get it all right in the end.

hobot

While accelerating I kept close eye on kickstarter and it never moved, which apparently will do if layshaft bearing is gone. I don't really relish tearing down a good running engine and smooth shifting gearbox, especially considering no real British Bike shop or mechanic within 100 plus miles (NNY 20 miles from Canada

" was riding my 1974 850 Sunday, when the motor suddenly died, rear end locked, "

"I don't really relish tearing down a good running engine and smooth shifting gearbox"


Are the above two phrases in reference to the same bike?

michael levato said:

While accelerating I kept close eye on kickstarter and it never moved, which apparently will do if layshaft bearing is gone.

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Sometimes, yes.

Other times, the bearing goes without warning, probably taking your gearbox with it, and perhaps a limb or two or your life.

Winter's coming. Wrenching season.

Have cylinders been bored oversize and new pistons fitted recently? Inadequate piston clearance?

"considering no real British Bike shop or mechanic within 100 plus miles (NNY 20 miles from Canada"....

I'm guessing most of us are our own mechanic to some degree. 20 miles from Canada suggests you might see some snow in a few months. No better time than now to line up some tools and manuals in preparation of getting your hands dirty. Hell, you already know where to look for some answers.