Author Topic: Jammed motor  (Read 689 times)

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Offline Jonem

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Jammed motor
« on: 22 Oct 2019 at 05:37 »
I have a 1959 MkvVthat has sat for a few months. I rode it yesterday around the block and this morning I noticed that there was a stiffness to the motor when I kicked it over.
After a couple of kicks it then locked up!. The gearbox turns ok when the clutch is in, so I suspect the motor is jammed or seized. Is there anything that I should look for that is a peculiarity of this motor?
Any ideas, thanks.

Jonem

Offline eddie

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Re: Jammed motor
« Reply #1 on: 22 Oct 2019 at 07:33 »
Jonem,
            Firstly, we need to establish what bike you have (there wasn't a 1959 MkV). The most obvious options would be a 1949 Mk3 or a 1954 MkV - there are slight internal differences within the engine.
   Going back to your problem - engine seemed stiff, then locked up - on old bikes with magneto ignition this can be down to the shellac melting in the magneto armature and then setting as it cools. On Mark series engines, this can be easily checked by removing the magneto and see if the engine then free's up. If it does, a mag rebuild will be required. Obviously, the ignition will need re-timing when the magneto is replaced.
   If the mag is not the culprit, I'm afraid a total engine strip will probably be needed in order to check out the crankshaft/ main bearings. Worst scenario is that the crankshaft has twisted and will need re-aligning. The second possibility is that the front main bearing bush has turned and it's locating peg has distorted the main bearing flange to the point it has locked up the motor. This is the point where the 2 models differ - the Mk3 had a single bush inserted from inside the crankcase, whereas the MkV had a 2 piece bearing - the 2 halves being pressed in from either side. The later 2 piece design proved more troublesome than the earlier type (they have a habit of working loose in the cases) and a lot of owners have reverted to the earlier bearing. When replacing the main bearing bushes, don't make them too good a fit on the crankshaft - Mr Douglas recommended a running clearance of 3 thou - anything less is likely to result in another 'nip up' of the front main bearing (and another strip down) - when these bearing nip up's occur, there is no obvious sign of anything being wrong until the crank is removed from the motor.

  Hope this is of some help,
                                           Regards,
                                                           Eddie.

Offline Jonem

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Re: Jammed motor
« Reply #2 on: 22 Oct 2019 at 09:45 »
Thanks Eddie,
  It is a 1950 model, it was a typo. I have play in the magneto gear and the camshaft gears so it is definitely the motor. I thought perhaps a piston had seized, but it happened when it wasn't running, just being licked over. I think that your bushing idea is probably correct. Cheers Jonem

 

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