Wednesday, May 20, 2009

1941 BMW R12 runs again after 50 years. setting up valves and magdynamo timing

Some fotos folks sent me on wartime BMW R12 bikes .

What is the location of this foto? my guess Southern Germany,or maybe Caucasus or Ural mountains...the mountains in the background are the only clue..

The BMW R12 engine when viewed from the front, rotates in the clockwise position, i had to find the correct R12 mag/dynamo that also rotated in the clockwise orientation for the mag to spark.The mag that came w it was a counterclockwise spin so it must have just been thrown in to sell the bike to the fellow i bought it from. No way in hell was it going to start spinning the wrong way.

After at least 50 years, the ww2 BMW R12 came back to life, serviced the cylinders w new valves, pistons, serviced the mag/dynamo. Carefully after much trial and error set the valves as in the foto above between #35 and the valve end ,.The original manuals call for one to fit a piece of paper between #35 and the valve end, Looking into the spark plug hole w a very bright flashlight , i could see the piston get w/in 10mm of TDC,as the piston ascended to the TDC position. remove spark plug wire terminal from mag, rotate mag sprocket until one is in the middle of the brass portion that is visible,thats where the point just starts to open, install mag, prime and kick started back to life. Theres plenty of great part sources in Germany and some very good online sites w proper instruction, The throttle control or "gasgriff" was worn out, the internal slider in the original Sum carb or Sum vergasser was sticking,so those parts will have to be ordered. should be a daily rider if there isnt any other big issues. W proper instruction its a relatively easy service.
There are R12 bikes w 2 carbs , this is a single carb bike. Be great to post a foto of a twin carb engine.

Here you can see where to check your valve gap. needed a 15mm, 13mm and 12mm wrenches. i found removing that inside center cylinder nut made it easier to make the adjustments.Position the valves at the relaxed position, then back off the 15mm "nut" closest to valve end. lock the 13mm lock nut against it, and valves are adjusted. By far Knucklehead valve adjustments are thee easiest.
It was kool to bring another relic back from the dead.

1 comment:

Anonymous said...

My husband has a 1941 BMW motorcycle. I'm not sure of the number (R12, R75, etc). I want to get him new tires for it as a surprise for his B-Day. Where do I go to get tires for it. I'm assuming it would be a specialty order right?