Carburetor Balancing BMW Twins 1951-1969
by Craig Vechorik
Step 1: The bike must be correctly timed before you set the carbs.
Warm up the bike
Fire up the bike: get it warm, not real hot, but warm. Obviously, your
timing has to be correct first, with no differential timing, because
the last thing you adjust and balance when tuning up the motorcycle is
Check to make sure you don't have a vacuum leak between the carb and
the head. While the engine is running at idle, spray carb cleaner right
where the carb and the head meet. Do this to both carbs. If you hear
any speed change at all, you have a vacuum leak, fix that first.
But let's assume you have no vacuum leaks and it's sitting there idling
and it's warmed up. Before you start adjusting, make sure you have
plenty of slack in the outer sheath of each throttle cable. Just to be
sure you do, loosen the lock nut on each cable adjuster on the top of
each carb slide chamber and screw the adjuster in to give yourself a
little more cable slack. Let's start by setting the idle mix on the
left cylinder first. Screw in your idle stop screw (the one with the
spring on it) just a little to lift the slide slightly, and speed the
left side up (not real fast because you want it to run on the idle
circuit and not the main jet) and get your 9mm wrench on the lock nut
of the mix screw and break it loose. By the way, if you are starting
from scratch, the idle mix screw should be set between 1.5 and 2 turns
out from bottom, more or less. Then take your spark plug wire off the
right side so it's only running on the left cylinder. While it's
running on that left cylinder because you have sped it up, play with
the mix screw, turning it in and out to find the maximum RPM you can
get the engine to run at that given throttle stop setting. When it
sounds like that is as fast as it wants to run using the mix screw,
then switch back the idle stop screw and back it out, and slowing down
the RPM slightly. And then go back again to the idle mix screw and
adjust for the max RPM at the new throttle stop setting until finally,
that cylinder will fire and run at approximately one hit per second.
Then lock your jam nut on your mix screw and repeat the process on the
other side, the same way.
Then when you get both of them about right, stand over the bike at
idle, with both cylinders running and momentarily remove the left plug
wire and listen to how fast it runs, put it back on quickly and
immediately remove the right plug wire, listen to the speed, put it
back on. To fine tune the idle speed you must do it by ear. If the left
(or right) side is running just a little bit faster, back off that
idle stop just a little bit until you can lift one wire, listen,
put it back on, lift the other wire, listen, and can hear that each
cylinder is running at the same speed.
You will find when both of plug wires are connected, the engine will
probably run just slightly too fast for an idle. What you must then do,
is to back each idle stop screw off in a graduated manner. In other
words, look at the position of the screw driver slot in each of the
idle stop screws, and back each one off the same degree of rotation.
And that will slow the engine down to where it is supposed to idle.
Next check your carburetor cables. You want each of the sheaths of your
cables to have approximately 1/8 to 3/16 of an inch slack where the
cable goes into each of the carbs. What you are going to do is to
synchronize the left one to the right one. Leave the right side alone
and make the left carb match it. How this is accomplished by loosening
the lock nut on the cable adjuster on top of the left carb and then
rolling the throttle on r-e-a-l slow so as the engine just comes off
idle, the bike will start to rock and vibrate. It will vibrate because
one carb is opening before the other one. As it is vibrating, hold that
throttle setting with your right hand, and use your left hand on the
left carb adjuster screw to screw it in or out. You will find a
position where the smoothness begins to improve dramatically. I do this
adjustment operation with my chin on the gas tank to feel the
vibrations while I adjust the left cable. As soon as it smooths out,
shut the throttle down to relax the cable and sheath to remove any
twist you might have imparted to the sheath when adjusting. Then repeat
the process again, s-l-o-w-l-y opening the throttle again to see if the
smoothness is maintained the second time. If not, again adjust the left
carb cable adjuster. Do this as many times as necessary. When you get
it to where its glass smooth, tighten the jam nut on the adjuster and
try again s-l-o-w-l-y to see if the engine takes the throttle nice and
smooth. Sometimes, tightening the jam nut will change things again. If
it does, readjust again. That is all there is to it.
Remember that it will perform correctly only if the timing is correct
and you have no differential timing. If the engine has differential
timing between the left and right cylinders, it will never be glass
How do you determine differential timing? Disconnect the coil from the
points, put an ohm meter on the points, one lead to the little nuts and
bolts of the points the other lead to ground, pull both your spark
plugs out and put your meter where you can watch the fly wheel and the
meter at the same time. Take a lead pencil; turn the engine in the
direction it runs with an Allen wrench, r-e-a-l slow. It should ideally
break points open, and the meter will react, at the line above the "S"
mark. But, for this test, no matter where the meter reacts in relation
to the flywheel, when you find that exact spot where meter starts to
jump, showing that the points are just starting to crack open, put a
pencil mark on the fly wheel. Then turn the engine one complete
revolution. See if the meter jumps and breaks the points at exactly the
same spot you put the pencil mark. More than likely it won't. The One
Hundred dollar question: How much difference is there between the left
side and the right side? If you have 3/16 inch linearly between the
first mark and the second, that's about 3 degrees, and that's okay.
3/16 of an inch or less. If you end up with 1/2 inch or 3/4 inch, your
advance is worn out or there is a wobble in the shaft when it rotates.
There is nothing you can do but throw that advance away and buy a new
one, and/or check the shaft of the rotating magnet for trueness, and
repeat the test.
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