Synching the throttle bodies
equalizes the vacuum across the all of the cylinder intakes.
The result is a smooth running engine that will probably get
better mileage to boot. It's a very simple process and
should be done, at a minimum, after every spark plug change
or valve adjustment.
Tools: Aside from the tools
required to expose the throttle bodies on K bikes with side
fairings, the only tools you need are a regular screwdriver
and a comparative vacuum tool Some of the more popular
vacuum tools are:
Morgan Carbtune: This
replaces the old mercury-based vacuum devices, allows you to
balance all the of the throttle bodies simultaneously,
requires no battery and is my preferred tool for the job.
2) TwinMax Carb Synchronizer:
This tool allows you to compare the vacuum on two throttle
bodies as once. Instead of synching all cylinders at once,
it synchs two at a time.
3) Do-It-Yourself: If you
search The Internet there are plenty of examples of people
inexpensively building their own carb/throttle body
synchronizers. Here's one example:
When working on K bike throttle bodies, DO NOT turn or
adjust the silver screws/nuts with blue paint on them. These
are factory set and should not be adjusted.
Start the bike and get it up to normal operating
(Only 2V K bikes have a
fuel pressure regulator vacuum hose.)
If your bike has a lot of miles on it then it might be a
good idea to remove all of the vacuum bypass screws and
give the tips of them a quick cleanings with fine steel wool
Remove all of the throttle body vacuum caps and connect the
vacuum hoses from your vacuum tool. (Or just the first two
if using a TwinMax.) Roll each one between your thumb and
forefinger to check for drying/cracking. Replace as
needed. (BMW part 13547694924)
2a) 2V K bike only: Though not
100% necessary, on 2 valve K bikes you'll get slightly
better results if you put a "T" connector into the measuring
tool's last vacuum line and attach the fuel pressure
regulator vacuum hose to that. It's a good idea to inspect
that vacuum hose for drying/cracking from age and replace as
needed. You can buy a new vacuum hose from BMW (part
11727545323 - about $15) but generic vacuum hose from an
auto parts store is less expensive, works equally well and
your bike will never know the difference.
Turn the first vacuum screw all
of the way in and then back it out one and a quarter to one
and a half turns.
TwinMax only: Before attaching the vacuum hoses, turn
the TwinMax on, set it to it's highest sensitivity and then
use the other knob to zero the needle.
Carbtune or other multivacuum device: Use the vacuum
bypass screws to adjust the vacuum on the remaining throttle
bodies so that is equal to the vacuum on the first throttle
body. Rev the bike a little every now and then during this
TwinMax: Synch the second throttle body to the first,
replace it's cap, then synch the third throttle body to the
first and then (if you have one) synch the fourth throttle
body to the first. When synching each pair, before moving on
to the next, rev the throttle a couple of times and then
reverify that that pair is in synch.
5) Replace the vacuum caps and go
for a ride to enjoy how smoothly your bike is running now.
If you find that one or more
throttle bodies is difficult or impossible to synch then
this is usually an indicator that there are cracks and air
leaks in the intake manifold boot(s) between the throttle
body(ies) and the cylinder head. If one or more is leaking then it makes
sense to replace all of them.
K75/K100: Part 11611460408 -
about $54 each
11611461621 (intake manifold) and 11611465169 (O-ring below manifold) -
about $25 and $2 each respectively
By varying the first throttle
body and then resynching the others to it you can also fine
tune the idle when synching the throttle bodies.
© 2013 Drake Smith - Please do
not use or reproduce this elsewhere. Feel free to link
to it though.