Forum bmwbike.com

My R69US coughs!
It starts right up after I changed a point set and condenser to new ones.
And it idles great. But if I open the throttle and hold it still (anywhere from 1/8 to all the way),
it starts coughing from pipes. It sounds almost like backfires, but I think it is just exhausting
unburned gas (same thing?). However, when I gradually or suddenly open it more, it sounds fine.
Only when throttle is held in one position, it coughs.
I tested advance unit by swapping it with a healthy bike.
I cleaned both carburetors well. Now I can't figure this out.
Could anyone please give me an advice? Thanks.

Aki,

My first guess would have been the centrifugal advance but obviously it is not since you checked it with a good one. Since this problem appears to have started when you changed your points and condenser then perhaps you should try putting back the old set or a new one just to see! I would then check the static magneto position with the S timing mark on the flywheel and finally reajust the timing if the latter was offset.

Perhaps you should re-check the points and the timing. You may have installed the points with improper gap or dwell settings, and the timing could also be affected.

Hi,
You have described perfectly one of the many strange symptoms of a bad condensor. Note that the problem happened right after you changed the points and condensor? This has happened to me before also. Try swapping it with the old one.
Mike

Thanks all for replying to me. But I cannot stop the backfiring (or misfiring? How can I tell?).
I have tried an old condenser, which didn't help.
The only way that I can make it somewhat less noisy is to change the needle position
from 2 to 3. But it runs too heavy. Sparkplugs turn black but not in the middle (white)
which seems to indicate the existence of some lean spot.
I was wondering if carburetor's slide could cause this backfiring.
The slides have some visible worn place.
Could this be it? If not, I suspect a coil, but is it reasonable guess?
How do I test an ignition coil with tester anyway?

Thanks again,
Aki

It sounds like an ignition problem to me. Backfiring could be caused by timing to far retarded. Did you check the static timing?

Aki,

Try to remember what you did when you changed the points and condenser! As you mentioned, it started doing that the minute you changed the latter. You may have set the point gap too wide and if you reset the dwell then check it dynamicaly.

Pierre-

You're right. The gap was too wide because the new point had a cam follower (?),
which was too big, creating an unwanted gap. I filed it down until I can have a correct
gap. But I also found that the cam on the advance unit was unevenly worn.
I only adjusted timing for one cylinder assuming that they're the same.
There is a pretty big gap between the timing for each cylinder. I finally manage it
to find a "middle" timing point, so the bike runs ok.
I guess I have to buy a new advance unit!

Thanks.
Aki