R27 running problem

The bike runs just fine untill it gets hot. Then it dies and will not start untill it cools off. Not so bad if you are just Sunday ridding. Not so great if you have to be somewhere. Any advice would be great.

If there were a twin /2, I'd say the problem was a coil going south. Usually on the twins of the era with the original magneto coil, the bike would start OK and run but it would be difficult to start later after stopping. The R27 is a battery ignition system, not magneto like the twins. It might be still a coil that gets hot and internally separates the windings.

I'd find a way to measure the coil resistance cold and then when hot and failing. See if the resistance goes way down when hot.

Try the condenser, cheapist first fix. Then valve adjustment, both less$ than a coil.

Even w/o a condenser there should be a ragged spark.
Sometimes the coil will be hot to the touch and sometimes not.
There could be an open created when the coil gets warm, check this with an ohm meter with the coil warm..

I lost a coil on an old Guzzi and had to zip-tie a large automotive unit to the frame to make it home 800+ miles.

Sure sounds like a coil. I had a similar experience years back. Even a few minutes of cooling would allow me to re-start. New coil solved the problem.

This is really a vintage question, so I'm moving it over there.

How can you test the armature(rotor) on one of these things? I was getting weak spark and took it all apart and cleaned it replaced the the coil(wich was not in spec) and the condensor. Now I have no spark. I replaced the oraginal parts and still no spark. My only thought was I disturbed the armature in moving it. I dont wanna spend the $400 plus just to test it. There has to be a way to test it.

DSBMW1 said:

How can you test the armature(rotor) on one of these things? I was getting weak spark and took it all apart and cleaned it replaced the the coil(wich was not in spec) and the condensor. Now I have no spark. I replaced the oraginal parts and still no spark. My only thought was I disturbed the armature in moving it. I dont wanna spend the $400 plus just to test it. There has to be a way to test it.

Click to expand...

Rotor and no spark should not be related, except perhaps by your disturbation....Rotor failure will cause no generator output, but the bike should be able to run with the power in the battery for some time. Unless your battery is flat dead, then there will be no spark. Period. The ignition depends on voltage from a battery to be able to get it started, and then the generator should put out enough to keep it going and to charge the battery.


There is a close inter-relation between the points and the timing. Check that your points are opening to the correct gap, then check your timing (statically if need be), then go back and re-check the points gap.

If you can, take the rotor out and take it to an alternator shop.... they will know what to look for and can test it in 30 seconds.

You can do a simple go/no-go test on a rotor while it is in place on the bike, but you must make sure that the brushes are not part of the "circuit". Place a piece of paper (perhaps folded over) between the brushes and the shiny place (ring) on the rotor that it sits. This will isolate them electrically. Do this for both brushes. Then take an ohm-meter, and check that there is the proper resistance by touching one lead to one, and the other lead to the other ring. I don't know what your resistance should be, but if I remember, the resistance should be in the 4-6 ohms i.e. pretty low. A failed coil will either show a short (no resistance) or an open.