I think you need to determine for sure what
is wrong with this machine before you fix it.
First off you need to do a compression test
and a leakdown test, both cold and after it
has been acting up.
The leakdown test will tell you if there is
a compression loss, and if it is from rings,
intake, or exhaust valves.
If you are not blowing air out the crankcase
vent, then there is nothing wrong with the
compression rings.
Is this motor hard to start when it is hot?
If so then suspect the mag coil. I've seen
more carb and ring problems go away when
a weak ignition is repaired.
I will also relate a very recent series of
events that happened to me. This bike:
http://www.metalworking.com/RCM-gallery/files/Rozen,Jim/Bmw.jpg
had the ignition advance unit blow up while I was
out riding one day. The timing was all messed
up but stupidly I tried to limp it home. There
was major damage to the pistons so I wound up
having to do a top end job on it. Bore, hone,
new pistons and rings.
AFter re-building it it had great power but one
jug used a lot of oil, and smoked.
Compression was fine, but the plug was wet with
oil on that side.
In frustration I finally tore that side down
again to inspect the scene of the crime. The
only thing I could find that was possibly amiss
was with the third, oil control ring.
This was of the newer, three piece design, with
two very thin scrapers and a wavy expander ring
that fits between the side rings. The expander
ring had the corrugations so that one end was
going "up" whereas the other end was cut so that
it was going "down" with the result that the
ends of the expander could slip by each other
and catch.
Because I always save teardown parts, I inspected
the expander ring that I took out - it had both
ends directly opposed (both pointing "up") and
there was no chance the ring ends could catch on
themselves.
In desperation I simply re-installed the older
expander ring.
The bike has not burned any oil since that time.
A very small thing, and I am not positive that
there was not some equally small change during
the teardown that mattered - but I think from
now on I will look at the expander rings very
carefully when installing them....
Jim