Synching the throttle bodies on a K bike is one of the most overlooked, yet most important, parts of keeping your K bike running as smoothly as possible.
WHY? The purpose of synching the throttlebodies is to balance the volume of air entering each cylinder. The usual result is a smoother, better running engine. It's nitpicking but using this procedure you're not actually balancing the volume of air but balancing the vacuum in each throttlebody.
WARNING: On the throttle assembly you will notice some silver screws with blue paint on them. DO NOT adjust those or you will be in a world of hurt unless you really know what you're doing. They are factory set and should not be adjusted.
TOOLS REQUIRED:
Regular screwdriver
Comparative vacuum measuring device (a.k.a. carburetor synchronizer)
There's a few options for a carb synch tool:
I use a TwinMax Carburetor Balancer.
www.casporttouring.com usually has the best price. This instrument compares two vacuum levels. If I recall correctly the TwinMax hoses aren't a perfect fit on K bikes so I went to the auto parts store and bought two feet of vacuum hose. It's cheap.
There are other options out there that will allow you to simultaneously measure 4 vacuum levels like Carb Stix, CarbTune, Carbmate, etc... Theoretically synching the throttlebodies all at once will yield ideal results but a TwinMax does a good enough job in my opinion.
You can also look around on the Internet for methods to build your own if you want to save a few bucks.
PROCEDURE: Synching the TBs is actually quite easy and quick. Usually the major portion of work is removing/reinstalling fairing parts to get to them. If you have an LT or RT model then I did a write-up on IBMWR a while back which will show you how to remove the radio and the fairing pieces. Here's a link to it:
K75RT K100RT K100LT K1100LT fairing removal.
Next, start the engine and get it up to operating temperature. I usually start the bike and then let it idle until the fan comes on.
You will notice that each of the throttlebodies (one per cylinder) has a brass screw at it's base. You will also notice little rubber caps at the base of each throttlebody. On two valve K bikes, there is a vacuum hose on the last cylinder that goes to the fuel pressure regulator.
Turn the TwinMax on. Use the left knob to set it to it's highest sensitivity and then use the right knob to zero the meter.
On the number 1 throttlebody (front of the engine) gently turn the brass screw in until it bottoms out. Then back it out one and a half turns.
Remove the little rubber caps (replace them if they look old/dry/cracked) from the number 1 and number 2 TBs and connect the TwinMax hoses to them. Adjust the brass screw on the number 2 TB until the needle movement on the TwinMax meter is bracketing zero. Number 2 is now balanced with number 1.
Remove the hose from the number 2 TB and replace the rubber cap. Repeat the above procedure for the number 3 TB to balance it with number 1. Then do the same with the number 4 TB.
If you have a 2 valve K bike with a vacuum hose on the last cylinder then in theory it's best to put a "T" vacuum junction on that TB and connect the TwinMax to that but if you're not running dynos and trying to squeeze the last tenth horsepower out of the engine then you should be OK just removing the fuel pressure vacuum hose and connecting the TwinMax hose. That's how I do it.
Remember to turn the TwinMax off when you're finished.
Put everything back together and go for a ride.
Note that if you find that one or more of the cylinders doesn't respond to turning the brass screws then you probably have an air leak between the throttle body and the cylinder head. This is easy to confirm by spraying some starting fluid or propane/butane around the boots between the TBs and the head. If that causes the idle to increase then the 20 year old rubber between the TBs and the cylinder head has most likely cracked and needs to be replaced.
Also, if you're replacing one of those boots then it makes sense to replace all of them while you have things apart. You also probably want to replace them with new ones from BMW, not used ones, since I don't see much sense in replacing 20 year old rubber with 20 year rubber.
Here's the part numbers:
K75 or 2V K100: 11611460408 Buy 3 for a K75, 4 for a K100.
K1, K100RS4V or K1100: 11611461621 for the boots themselves and 11611465169 for the O-ring installed at the base of each boot (buy 4 of each)
Here's a link on how to replace them on a 4V K:
4 Valve K Bike Intake Manifold RehabAnd, while you're at it, you might want to consider
Getting The Fuel Injectors Reconditioned.
For K75s and 2V K100s be sure to clean around the bases of the injectors (vacuum or compressed air) before pulling them as crud can build up around their bases and fall into the head if you're not careful.
