T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
POINTS IGNITION AND ... COMPUTER AIDED DIAGNOSTICS!
THOUGHT I HEARD SOMETHING
In my quest to make this engine run and idle as smooth as practically possible I've gone
the usual way of jetting / rejetting / reaming / adjusting / float bowling, turning mixture
screws and tuning spark plugs gaps. Although all this brought me a long way in
reaching the final goal, there was one thing still out of reach for old school
tools. I am talking about checking ignition spacing on a running engine.
Dynamic that is, instead of using an Ohmmeter ( the archaic type )
and a hex key to turn the engine by hand ( Static method )
Have no fear for the dark black hole.
IGNITION INTERVAL SPACING
Some BMW boxers engines can give you a hard time if you like to check and
adjust ignition timing with the help of a stroboscopic lamp. In most cases the
mark you see on the flywheel will be blurry instead of sharp. This is not a
failure of your timing light, it's a fault in the engine. If the thin end of the
camshaft ( where the ingnition cam / advance mechanism sits on ) is
slightly bend or the ignition cam itself is out of specs ( not exactly
180 degrees devided ) you will wind up with a double ( blurry )
image seen trough the peephole. Ultimately A worn Timing
chain / tensioner combination can also be a factor in this.
Super definition hearing, now let the bike do the singing ...
I've engaged " modern " computer technology to help out, as my own ears and brain
are just not fast enough to make any proper judgement at all. So ... a PC, laptop
or such, maybe a smartphone can do it ( I do not use 'm, so I can't tell ) and
a microphone is what you need, and off course software that allows you
to measure deep in time for example AUDACITY or NERO editor.
Start up the engine and record the sound at the exhausts, practically it doesn't matter at
what speed it's running as long as it's turning without any hesitation. Now load the
recording into Audacity ( as you see in the image ) and use the controls in the
view menu or the magnifier tool to fill the screen and have a good view.
Sound of the engine running at 870 rpm
By using the View menu - expand screen vertical / horizontal or by using the magnifier tool
you can have a closer look at what you are doing.
Now by using the selection tool ( the button above the magnifier tool ) you can make
a selection of the exhaust pulses top / top and read out how long that took in
time ( msec ). In this case 0,069 sec ( 69 msec )
Now select the next top / top wave and see if there is a significant difference relative
to the former readout, In this second case, the number is 0,069 sec. 60 devided
by 0.069 makes about 869 revolutions, and that's about right for a boxer's idle.
Indeed airhead boxers can idle at slower speeds like 600 or thereabouts and
although ultra low idle sound really great, oil pressure will suffer ... and on
a hot day your engine will not be exactly happy idling like that for long.
If you like to hear extremely low rpm idle, you can make that happen
in the Audacity software as well, for an example click here that's
how the engine sounds idling at a mere 234 rpm. As you hear
that sounds pretty similar to a puffing archaic boat's engine.
NOW, BACK TO HANDS ON
Ignition Advance Unit, Camshaft, Oilpump
Suppose you do find unevenness in spacing between the pulses ( not unlikely )
than first thing you want to find out is: Do we have a bended camshaft end?
To check that out you have to fabricate a support for a measuring dial or
a simple fixed point, to be able to eyeball the thin shaft on straightness.
The shaft itself can be corrected with a plastic hammer ( careful now )
When the shaft is true enough, check if the points ( after being set
to open on the F mark, open on the Next rotation F mark as well.
If not, disassemble the advance unit and rework the " early "
lobe of the ignition cam ( oil stone grinding by hand ) to
make the ignition triggers in time at each revolution.
Make sure the thin end is straight. No problems, All ok here?
Go for the ignition cam.
For some reason the ignition cam can be out of 180 degrees offset.
Wear or fabrication error? I can't tell, it can be corrected with
a truly flat fine grit oilstone or diamond sanding tool.
Take the unit apart and rework the lobe that's opening the points to early
compared to the other side. Just a few rolling rubs to begin, take your
time, it's gonna be ok! Work in small increments to get the hang
of it, reassemble and check regularly. Use some ultra fine
grit oiled emery paper to polish up and finish the job.
Congratulations, you've done fine tuning!
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