Forum bmwbike.com

my 1952 R51/3 starts very well and runs good but as I have told before my top speed is blocked to 110 km/h. I do not want to trave at full throttle but for me this is a sign of a problem. The engine sounds good until 4000 rpm and it seems to me that it changes after that. I check the ignition by timing light and its ok. (magneto body and magnet is alligned, compression is around 75-80 psi, OT mark is in correct position on flywheel, spark plugs NGK and gap 0.6mm, valves are set correctly).... the only question in my head is: I did not replaced the valve springs. It seems like the engine does not want to rev any more since my uphill and downhill topspeed is SAME!!!. I tried with and without airfilter and nothing changed. So airfilter is ok. Also it may be that not enough fuel is running through. What do you think.

You don't mention whether you saw the F on the flywheel appear beside the mark in the window around 2500 rpm. Maybe the ignition is not advancing enough.

Or perhaps the carbs are not correct for the bike, or the main jets are too lean for the carbs. An R51/3 should have carb numbers 1/22/41 abd 1/22/42, with main jet size 100.

Or perhaps there is insufficient fuel flow, perhaps due to a plugged up petcock filter. There may be a mesh sleeve or sock type of filter over the the petcock's tube in the tank, and this filter can become blocked by paint chips etc.

Also how much above sea level (altitude) are you? Higher altitude = less power.

Does the bike missfire or just run have no more go?

I see the F mark clearly on the flywheel. I will check the jet size and clean the petcock filter.

Thank you for the reply,

I had a similar problem with my R50 - 1961.
The problem turned out to be that the auto advance was going too far, allowing the ignition to be over advanced when I reached 50mph. On the R50 there is a spring that limits the distance the weights can move. I dont know if the 51/3 has the same feature. I used a timing light to see how far the auto ignition was going. It was reaching over 50 degrees of advance. I adjusted the spring.
Good luck
Dave E