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:

1) 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:

 

Warning: 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.

1) Start the bike and get it up to normal operating temperature.

(Only 2V K bikes have a fuel pressure regulator vacuum hose.)

1a) 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 before proceeding.

2) 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 of the caps between your thumb and forefinger to check for drying/cracking.  Replace as needed. The BMW part number is 13547694924 but any little cap that seals well will do.

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.

Here's an example of where to add the T connector to the rearmost hose when using a Morgan Carbtune on a K75. A K100 will need one more "plain" hose on the left.

3) 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.

3b) 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.

4a) 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 process.

K75 Example:

4b) 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) Once you've got the TBs balanced it doesn't hurt to rev the throttle one last time and then wait a bit for things to calm back down to verify the balancing. Then turn off the bike and remove the hoses.

 5) Replace the vacuum caps and go for a ride to enjoy how smoothly your K bike is running now.

Other notes:

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.

Intake manifolds:

K75/K100: Part 11611460408 - about $54 each

K1/K100RS4V/K1100: Parts 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.