Forum bmwbike.com

My 1951 R51/3 has a very weak spark and is difficult to start. I have a new magneto coil and the rotor re-magnatised. I've tried different condensors and it makes no difference, HT leads and plugs seem OK and the points aren't pitted. I've checked the timing and that seems fine too. The spark is so weak that the plug can be held in my hand, the motor turned over and only a small charge is felt. Any ideas? The nearest auto electrician who deals with motorcycles is 400 kilometres away. Thanks for your help - Bob

Who re-magnetised your rotor? I took my /2 rotor to a shop that repairs aircraft magnetos, and they said it was a permanent magnet and could not be remagnetized. I then called a person who repairs vintage BMW magnetos, and he said the /2 of rotor was a permanent magnet, and only the very early BMW rotors could be re-magnetized - I believe he said pre-war BMWs. Your rotor should not be much different than a /2, and I wonder what could have been done to your rotor.

Where did you get your coil (from a reputable source)?

Are the spark plug wires and caps the non-resistor type?

Has the abrisz alignment been properly set to make the strongest spark?

Thanks for your reply Alan. The guy reckoned he re-magnatised it and it appears to have a stronger pull to it - Or is it my imagination and I got ripped off??!!!??
I had the wrong spark plug heads on and changed them to the non resistor type.
The spark was still weak, although I think a slight improvement but still not good starting.
Then a mate brought me around an article on magnetos by Craig Vechorik off the net.
In this it was stated that once the abrisz alignment was set with the "S" "V" notch and line on the rotor tighten it and leave it.
Altering the advance and retard by moving the plate on the back of the points.
This I did and straight away a nice fat blue spark.
I'd been doing the ignition timing as was stated in the Clymer manual on page 18 which says that if the timing is out loosen the 10mm retaining nuts on either side of the magneto and move it clockwise or anti clockwise until the "S" is in the right place.
This contradicts what Craig Vechorik says. So I think the manual is wrong judging by the difference of the spark.
Interesting eh? Thanks again Alan for your advice.