Forum bmwbike.com

Dear All,

My 1952 51/3 bike is running well but I think that it is far away from its normal performance. the top speed is 135 kmph but it is hard to travel at 90 kmph. I do not think of traveling at top speed but I want to be sure that the engine is in goog condition. The engine is new, at 7800 km, coil is new, spartk plugs new, air filter new (K&N), Firing is ok (checked with light gun). The only thing that I am not sure is the position of the needle. Is it possible that the engine is not getting enough fuel? And I want to measure the compression. What is the compression value?

I think your bike's compression should be a little under 100 psi. Measure on hot engine, plugs removed, wide open throttle, using several hard kicks in succession.

Thank you for the reply, can you help me about the ideal or normal cruising speed of a R51/3?

This page says the R51/3 has 24 HP and a top speed of 135 kph:
http://bmw.mc.no/bmw/r51-3.html

This page says the later R50/2 has 26 HP and a top speed of 140 kph:
http://bmw.mc.no/bmw/r50-2.html

The R50/2 manual says top speed is 87 mph crouched or 81 sitting. Top speed for an R51/3 is probably 2-3 mph less. Cruising speed would be 65-70 mph in my opinion.

I agree with the other lads. Can one safely assume you're not riding with sidecar ratios in the final drive? That would knock your cruising speed down noticeably even if your engine was perfect. Just a thought.

Someone just emailed me that he rode for 10 miles at 80 mph on his R50/2. He has a full Glaser fairing, which may have helped. This is in the area of the bike's top speed; I would not recommend cruising at that speed as a habit.

I have measured my compression and the value is 70 psi which is not good I think. My bike has just 8000 km on it. The pistons and the cylinder are renewed, I myself checked the valves. I do not know but I think this explains my why my top speed is around 60 mph. Maybe my piston is not correct type. Do you have a picture of an original R51/3 piston?

The data sheet I have for the R51/3 shows a compression ratio of 6.3.

The online calculator at smokemup.com shows that for a 6.3 compression ratio and an atmospheric pressure of 29.92 in, you should have a compression test reading of about 77PSI, so 70 is a little low. It should be tested as Allan described in his earlier post.

The same datasheet show the top speed to be 135 km/hr.

You can see Ersatzteillistes with pictures at www.beemergarage.com

Best regards,

Bruce

Thank you for help. I also visited smokemup and calculated the value but Mr.Allen said that it must be below 100 psi. If this is ture than I must check the cylinders. If 77 psi is true than nothing to worry about . this value must be the start point of checking a healty engine, is this info so unknown?

I think that people believe that all the postwar bikes are the same. They are similar, but there are differences.