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Zenier Diode and Crossover Tubes

Ben, Ben, Ben, Ben:

First, the crossover tube that people are talking about IS the vacuum line
that is attached to the throttle body test port.  There is no other crossover
tube.   The Cruiser models only have one throttle body, so no synching is done
on them at all, but there is no crossover affair that I've ever seen on any
other of these bikes.

The new hexhead bikes have a crossover on the exhaust side, but I don't think
there's anything on the intake.

When you pull the vacuum lines on the bike to hook up the gages for a TB
synch, you automatically have disconnected the connection between the two
sides.   This small diameter "crossover" doesn't do much.   You can cap the
vacuum ports or hook them together and you won't notice much difference in the
performance or feel of the bike.

Again, if you have the 02 sensor hooked into the system, you don't want to
modify the TPS position for a richer mixture.  This will only make the problem
with the FI cutting in and changing the fuel curve worse.    You can only do
this if you have a non-cat exhaust or at least have the 02 sensor unhooked.

We learn from the Techlusion guru that the motronic brain assigns different
fuel curves depending on whether it senses an 02 sensor hooked to it or not.

If you leave the 02 sensor on with the standard Cat Code Plug in, you can do
this 0=0 procedure which will calibrate the TPS so that zero throttle opening
equals approximately zero voltage.   The effect of this is to make the TPS
slightly leaner, not richer.   It keeps the 02 sensor from causing the FI to
kick into the default lean fuel curve, causing the surge.   It works pretty
well.  Part of the magic of it is that it's precise, so you don't overdo
things and cause engine damage.

Any adjustment to the TPS voids the warranty.   The screws are painted over
because they are set at the factory with a flow meter. They're not designed to
be messed with at all, but the 02 sensor and the rest of the logic for
limiting pollution on the bike really mess with the original intent of the
engineers.   The result is a pretty severe compromise in rideability...worse
on some bikes than on others.   Again, probably because of variability between
02 sensors more than anything else.

Again, messing with the TPS voltage to get a richer mixture with the 02 sensor
on will just make things worse.

As to alternative air filters, you should know that the air filter box is the
limiting factor on these bikes, not the filter itself.   You can put a K&N on
your bike and it will do nothing.  You have to modify the air box to get an
effect.    Then change the exhaust and remove the 02 sensor...then maybe a
variable voltage device on the TPS circuit would be worth having.   In the
end, you'll get a bit more ultimate power, at a sacrifice to low-end
torque....sort of the opposite effect that the GS intake tubes have.  Put me
in the camp that likes strong mid range torque and isn't completely crazy
about HP numbers.   Also put me in the camp that wants a well made air filter
with no leaks in the gaskets.   These are 100,000 - 200,000 mile bikes.   A
good filter is a good thing.


If you can get the system back to the original German setup...you'll have a
sweet running bike.   Some people have done this by removing the CCP and
unhooking the 02 sensor.  In place of the 02 sensor you put a pot...available
fairly cheap from BMW.   This needs to be in place, because if it's not there,
the system will run too rich and waste fuel.  The system looks for the pot.
If it doesn't see the pot, it runs on a different fuel curve.   When you
change from the 02 sensor to the pot, you need to disconnect the power to the
FI for a minute or so to erase the setting.  You can adjust the pot as a crude
idle speed emissions control...just set it so it idles well and doesn't smoke.
This is the way the original oilheads came to the German market from the

There are a bunch of ways to make these bikes run well.    Part of the fun, is
noodling around with them and seeing which way works best for your taste.
Because there is such a load of data collected on this subject, you owe it to
yourself to read up.

- -TB