1976 cb750K rotor test

[riettic]

I thought that the rotor would have slip rings on the cb750 k6 so I could check the ohm reading, but it doesn't.  Anyone know how I can test this rotor when it has been removed from the bike?  

many thanks,
Claudio

[Re-run]

Do you have the manual? That will tell you how to test various systems.

[riettic]

I have the shop manual, but it has no indication on how to test the flywheel/rotor.  It seems to be completely demagnetized - it's not attracting any metals to it.  Anyone know if they make OEM rotors for these bikes?  I know they make them for 1978 onwards, but haven't been able to locate '70 - '77.

cheers

[LukeM]

Hmm..

If memory serves (and Wikipedia agrees), only one of the two components needs a magnetic field, and it's usually the stator.  Spinning the rotor induces changes in the magnetic field, which induces voltages in the stator coils due to back EMF.  So I would expect there to be magnetic attraction inside the stator but not in the rotor.

Sorry if I'm mistaken.  I've been wrong before.  Someone with the spare parts can check this.

About all you could check with the rotor is to make sure there are no breaks in the rotor winding.  I think this is done by putting an ohm meter across the coils and it should read close to a dead short.  Sometimes the contact surfaces for the brushes or the brushes themselves come loose or don't make contact, which would result in no output.  Some degreaser and then some contact cleaner would help here.

Hope this helps.
Luke M

[sgtslag]

Luke is correct:  it uses an electro-magnet, not a permanent magnet.  Without current flowing through the coil, there will be no magnetism present.  As for testing, the coil should read up to 4 Ohms, or less.  You could try applying a 9V battery to it, with some paper clips just below the coil:  the current from the battery should be enough to create a magnetic field strong enough to at least move the paper clips a little bit.  If you connect a full 12V to the coil, it will likely get warm, quickly, as well as generating a very strong magnetic field.  Just be careful.

With regards to replacing it, you may be able to have it re-wound.  Otherwise, check with local motorcycle junkyards for parts.  E-Bay is always available, too.  There are other forums, and a Google search on "rewinding CB750 rotor" (try variations of wording, more/less restrictive) may turn up some resources.  Cheers!

[riettic]

guys, thanks a lot for the responses.  I'll post my findings when I get her running in good order.  

Claudio