Vintage BMW Motorcycle Owners

After an absence from the 1957 R50 for a few months, I have changed out a leakey transmission seal and am back at trying to diagnose why the bike seems to hit a mid level RPM (maybe around 2500 rpm) and not want to increase much from there. (It used to run much better).

Yesterday I did a static timing set up, and today checked the timing with the strobe light. Idle strobes on the "S" mark and at acceleration it does not go beyond the "F" mark.

Start up is easy, and idle is pretty good sounding. I can get the left side to idle cleanly on its own down to about 3 or 4 rpm without any "missing"; however I can't seem to get the Right idle to go as low rpm as the Left side without stalling out. Right side idle also does "miss" every couple of seconds.

I am starting to think this may be a carburetor issue, as things on the electrical & timing side seem to be good. I only have one see-thru gas filter on the R side and it seems to have be only about 1/4 full of gas during operation. That seems low to me but when I disconnect the float top, gas flows freely out of it...

Alternately, I think there might be a possibility that after a recent piston ring end-gapping, that I may have ground the gaps too wide (at .5mm) based on some information which I may have interpreted erroneously. Other members expressed that the specified piston ring end-gap is less than .5mm for an R50. I did do a compression test and it seems to be normal on both sides (about 120psi after 5 kicks).

So... any ideas?
Gas flow? Blocked carb jets? Piston ring end gap too wide?

I would love to end the summer with this bike riding well!

Thanks!

Air/fuel mixture?
3 or 4 rpm sounds weirdly low.. what do you mean?

ignition seems to be ok. Did you also check and adjust the valves ... all good on that front?

With regards to the carbs ... idling and not accelerating beyond 2500rpm are two very different circuits within the carb system and yes, I agree with S.R. ... what do you mean with 4rpm ... sounds veeeeery low.

My first advice: Don't adjust idle speed all to low. It sounds cool, I know, but it is veery bad for your engine and the oil flow. Put idle speed up to some 800 rpm ... means, you cannot hear the single beats anymore. In the long run, your engine will be thankful to you.

Now, with regards to your acceleration issue, here's what I would do.
Take off the petcock and clean the filter screen
Put it back on with a new gasket
Take out all extra gas filters (between petcock and carburator) ... they restrain gas flow.
Then check gas flow. I think it should be around 200ml/min. Don't forget, when you accelerate the bike, it takes way more gas than at cruising speed ... 200ml/min accounts for that.

Let us know what you find out
Klaus

westeagle wrote: ↑

Sun Sep 13, 2020 5:05 pm

Other members expressed that the specified piston ring end-gap is less than .5mm for an R50.

I recently confirmed the gap spec is .25 - .40 mm for my ‘55 R50 before gapping new rings last week.

I checked gas flow and timed it coming out of the float top into a 200ml receptacle. Both sides hit the 200ml by about 40 seconds in, so I can cross gas flow off.

I dismantled, carb cleaned and cleaned out passages of both carbs: No difference in performance afterwards.

Yes I did check valve clearances previously. The were both spot-on.

I guess next step is to install the new rings I purchased last month, thinking that it would eventually come to this. I'll be gapping them at .25mm this time...

What do you mean by "gapping" the rings to 0.25mm?

Silly suggestion probably but I have been caught out when I have left the choke active. Idles fine loses power when throttle opened more than 1/2 way.

Thanks Waldoverkill
I will check again but I'm pretty sure choke isn't in...

Kurt,

We were referring to piston ring end gap.

Yes, I was aware you were talking about end gap. But how do you "gap" it? Typically that's done by boring a cylinder to match a specific oversize and/or piston size. I've heard about filing the ends of rings, but this can result in a poor fit due to the way the geometry is changed.