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Here are some BMW tuning considerations. Questions keep coming
up about the two carbs and how to synchronize them. Do you have a "hard to start" BMW motorcycle?
These are only some of the factors in tuning. This is a mental exercise
only and is not intended to be important to anybody at all. The net effect
will be to confuse some riders. It's amazing that they run so well with
all of these factors affecting the tuning. It is only natural to
want to keep your BMW tuned.
1. The BMW twin is basically two engines with a common crankshaft.
In all tuning aspects, they must be treated as being two separate engines.
2. The BMW exhaust systems are of a different length, due to the
staggered cylinders. So they tune differently. A cross wind can
change the back pressure between the two sides.
3. The BMW twin intake is pretty symmetrical for each side, but not
exactly.
4. The right side, of 1970 until about 1980, has some crap (crankcase fumes)
dumped into it from the crankcase breather. This changes the tuning
slightly, it's especially noticeable at lower rpm.
5. The Bing carbs are pretty consistent, but I have found differences between
them, don't assume.
6. The BMW valves may seal unevenly, or may not. Same for the rings.
Remember the time that you rode for 2 hours in a cross wind? That is when you
changed, forever, the two sides. The cooling was totally different for
them.
7. If the compression is different between the two sides, then one
can't expect to balance them. Compression is measured at low rpm. At
higher rpm they can act differently, and we can't even measure it.
8. Do you think that the camshaft was made with equal lobes? Maybe so,
but do you think that they wore equally? Not likely.
9. Often the ignition timing is slightly different. If fact, why
even think that these two cylinders even need the same timing? Proof that they
don't is that when one encounters a ping (pink in the UK) it is usually on one
cylinder.
10. The BMW timing advance curve is mechanically derived, not "need"
derived. The ideal timing is to be retarded enough to be just under the
ping timing at all rpm. To determine that, one must have the ability to
independently change the timing of each cylinder under load. Then find the
optimum timing increments of 50 or 100 rpm. Then what you see will be the
real curve needed. There is no way to get that mechanically or
electrically. The curves for each cylinder will be very different from
each other and from the one provided by the advance mechanism. That's why
one can find that the pinging will be slightly different in each cylinder as it
goes through the "ping" rpm range. One may even find that it
only pings in one cylinder.
11. Have you ever used vacuum gauges to tune the carbs and found them
to be way off?. It is possible, even easy, to get the vacuum equal with
the settings at idle and have them run quite differently, even badly. A
deaf person would NOT be able to tune them with the gauges only. A person
with experience and sensitivity can tune them faster and better than anyone with
only gauges. It is natural to want a "magic" fix, but all of the gauges
and level tubes filled with this and that, are a very poor way to do it.
Sorry, but you need to "know something" to do the job well.
12. To adjust the cables to pick up at the same time is one way.
The error in that thinking is that the two engines (cylinders) won't rev up at
the same horse power vs. rpm rate. Then, at road speed they may be
putting out different amounts of hp. We call that "vibration." Some vibration is normal
and is due to the offset cylinders.
13. Another way is to take the engine up to 3500, or so, rpm and short
out one spark plug at a time and adjust the cables for equal rpm. This
won't produce equal HP (low vibration) at that rpm, but it will get you into the
ballpark. The reason is that the two cylinders usually produce a different
amount of horse power at any one rpm. The two sides aren't able to
contribute equally to the work. Note, I really don't approve
of that method of balancing the carbs.
14. The choice is easy for the touring rider. One will find that
a narrow range of rpm will be normally used on the road. At this rpm,
adjust the throttle cables for minimum vibration. Just know and accept
that it won't be optimized at any other rpm. More info on this procedure
at the end of my controls page.
14. The sport rider must accept that it will never be "perfect" at any
rpm.
14. Now let's throw in the fact that your engine will want different
timing and mixture at different ambient temps, pressure and humidity.
The bottom line on carb tuning for the BMW twin
A good BMW twin tuner with a lot of experience will almost never find a
perfectly tuned engine. Each of my mechanics went through a "crazy period"
when they were learning to tune carbs. While riding their own bikes, every
few minutes they would reach down and do a slight tweak on a carb.
Eventually, they learned to accept that it would never be perfect. The very
best that you will ever achieve is a workable compromise. Be happy.
Trivia
The Bing carbs are,
or were, made by a company named Fritz Hintermayer Gmbh in Germany. There
was no company called Bing. They were very good to me.
To adjust the Bing carbs on the /2
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