Vintage BMW Motorcycle Owners

Hello,

I recently acquired a restored 1938 R71 and it gives me a real headache to start when hot. When cold, it starts with the first kick, always. (spät, trickle the carbs, no throttle). It idles fine, goes well in all load conditions.
When after a ride you stop the motor, and immediately start again, it will fire. When you wait 10mins, it's over, no chance to fire again (no matter if you flood the crabs or not, spät or fruh, throttle or not...)
When cooled down, let's say 1 hr, it will fire again from the first kick.

Plugs are brownish, 0,6mm
Valves checked and set at 0,1mm
Carbs are new PZ28D (Chinese G28 copies, unfortunately no Graetzins). There is a thick heat gasket between the carb and cilinder.
Coil is new (at least looks so, I have no detailed history)

Coil problem (didn't had the change to try another one)?
Carbs getting to hot and additional heat shield needed?

Thx for you ideas

I suspect your coil is shot, but do a very easy and fun experiment: run the bike (in nice weather, no rain, no wet roads) without the front cover in place. This keeps the possibly failing coil cool, keeps it from shorting out as when hot. If the hot starting problem is solved...just get a new coil!
Otherwise, maybe somebody else tell you what's wrong?

Hi: I would try a new condenser. Your symptoms are exactly what happens when a condenser gets weak. Hidden behind the distributor body, often overlooked. Is it original? About 85 years old. I bet that will fix it. Todd

Sounds like the classic condenser "gone bad". Any condenser will work; they all have the same value. Most new ones are pretty bad as there's less demand for them.

Did some further testing today. Installed a brand new coil (PVL #12 13 8 030 025). Did a 20 min ride. After shut down, could restart the bike with first kick. Five min later, start at first kick. Another 10 min later, not possible to start anymore… Checked the plugs, as good as no spark.
Took off the front motor cover, coil was really hot, you could hardly touch it. Cooled it down with compressed air. Restarted at first kick!

Could this be the condensor causing this? Other reason why the coil gets too hot?

Your coil had a short. Condenser was not causing that problem. Sounds like you found the problem. Todd

TaosTwinCam wrote: ↑

Mon Apr 08, 2024 12:00 pm

Your coil had a short. Condenser was not causing that problem. Sounds like you found the problem. Todd

Hi Todd, I don’t believe it’s the coil. I now already tried 3 new coils, two different brands, always the same problem. They get too hot, and when cooled down, work again. I now have to find out why they get too hot.

Poking around on Google suggests this is not unique to just old motorcycles, but happens to old tractors and cars too, especially the 6V variety. I'm assuming your R71 is still running 6V? I think your part# is for a 6V coil, which would need a ballast resister if you've upgraded to a 12V system.

I found several posts indicating the condenser can be the problem. One suggests:

"I think your wiring, or ignition switch is bad. When the defective areas get hot, they will break down and cause enough resistance to reduce the spark. Check how much voltage is actually reaching the coil when everything is hot. If there is a reduction with the points open, that is your problem. Also check it with the points closed to see how much voltage drop you have. Also, not as likely, but it could be in the distributor. Are you positive that you have a good condenser, and the points plate grounding wire is in good order?"

I also found a number of folks saying new condensers have a bad reputation, so it's better to troubleshoot with an old one that is known to be working.

This is the "poor boy" method of checking a condenser (electrolytic capacitor) with an ohm meter.
1) remove any charge on the condenser by shorting the input lead to the case.
2) set the ohm meter range to its highest value.
3) hold the positive meter probe to the case and the negative probe to the input lead. If the meter doesn't read infinity, the condenser is shorted out. Throw it away.
4) hold the negative meter probe to the case and the positive probe to the input lead. The meter should give a high resistance reading which will increase to infinity as the condenser takes a charge. Note: to check this again you will need to discharge the condenser.

If the condenser passes these 2 tests it is most likely ok.

Craig

Craig

Yes that is a good static test if that fails it is bad
Note:
Condenser must be disconnected
Easier just unground the condenser so you don't have to remove the lead from the points or coil wire, and don't have bare hands on test leads.

Rare case, only a dynamic test is best with a know good condenser