Carburetor Balancing BMW Twins 1951-1969

by Craig Vechorik

Step 1: The bike must be correctly timed before you set the carbs.

Warm up the bike
Fire up the bike: get it warm, not real hot, but warm. Obviously, your timing has to be correct first, with no differential timing, because the last thing you adjust and balance when tuning up the motorcycle is the carbs.

Check to make sure you don't have a vacuum leak between the carb and the head. While the engine is running at idle, spray carb cleaner right where the carb and the head meet. Do this to both carbs. If you hear any speed change at all, you have a vacuum leak, fix that first.

But let's assume you have no vacuum leaks and it's sitting there idling and it's warmed up. Before you start adjusting, make sure you have plenty of slack in the outer sheath of each throttle cable. Just to be sure you do, loosen the lock nut on each cable adjuster on the top of each carb slide chamber and screw the adjuster in to give yourself a little more cable slack. Let's start by setting the idle mix on the left cylinder first. Screw in your idle stop screw (the one with the spring on it) just a little to lift the slide slightly, and speed the left side up (not real fast because you want it to run on the idle circuit and not the main jet) and get your 9mm wrench on the lock nut of the mix screw and break it loose. By the way, if you are starting from scratch, the idle mix screw should be set between 1.5 and 2 turns out from bottom, more or less. Then take your spark plug wire off the right side so it's only running on the left cylinder. While it's running on that left cylinder because you have sped it up, play with the mix screw, turning it in and out to find the maximum RPM you can get the engine to run at that given throttle stop setting. When it sounds like that is as fast as it wants to run using the mix screw, then switch back the idle stop screw and back it out, and slowing down the RPM slightly. And then go back again to the idle mix screw and adjust for the max RPM at the new throttle stop setting until finally, that cylinder will fire and run at approximately one hit per second. Then lock your jam nut on your mix screw and repeat the process on the other side, the same way.

Then when you get both of them about right, stand over the bike at idle, with both cylinders running and momentarily remove the left plug wire and listen to how fast it runs, put it back on quickly and immediately remove the right plug wire, listen to the speed, put it back on. To fine tune the idle speed you must do it by ear. If the left (or right) side is running just a little bit faster, back off that idle stop just a little bit until you can lift one wire, listen, put it back on, lift the other wire, listen, and can hear that each cylinder is running at the same speed.

You will find when both of plug wires are connected, the engine will probably run just slightly too fast for an idle. What you must then do, is to back each idle stop screw off in a graduated manner. In other words, look at the position of the screw driver slot in each of the idle stop screws, and back each one off the same degree of rotation. And that will slow the engine down to where it is supposed to idle.

Next check your carburetor cables. You want each of the sheaths of your cables to have approximately 1/8 to 3/16 of an inch slack where the cable goes into each of the carbs. What you are going to do is to synchronize the left one to the right one. Leave the right side alone and make the left carb match it. How this is accomplished by loosening the lock nut on the cable adjuster on top of the left carb and then rolling the throttle on r-e-a-l slow so as the engine just comes off idle, the bike will start to rock and vibrate. It will vibrate because one carb is opening before the other one. As it is vibrating, hold that throttle setting with your right hand, and use your left hand on the left carb adjuster screw to screw it in or out. You will find a position where the smoothness begins to improve dramatically. I do this adjustment operation with my chin on the gas tank to feel the vibrations while I adjust the left cable. As soon as it smooths out, shut the throttle down to relax the cable and sheath to remove any twist you might have imparted to the sheath when adjusting. Then repeat the process again, s-l-o-w-l-y opening the throttle again to see if the smoothness is maintained the second time. If not, again adjust the left carb cable adjuster. Do this as many times as necessary. When you get it to where its glass smooth, tighten the jam nut on the adjuster and try again s-l-o-w-l-y to see if the engine takes the throttle nice and smooth. Sometimes, tightening the jam nut will change things again. If it does, readjust again. That is all there is to it.

Remember that it will perform correctly only if the timing is correct and you have no differential timing. If the engine has differential timing between the left and right cylinders, it will never be glass smooth.

How do you determine differential timing? Disconnect the coil from the points, put an ohm meter on the points, one lead to the little nuts and bolts of the points the other lead to ground, pull both your spark plugs out and put your meter where you can watch the fly wheel and the meter at the same time. Take a lead pencil; turn the engine in the direction it runs with an Allen wrench, r-e-a-l slow. It should ideally break points open, and the meter will react, at the line above the "S" mark. But, for this test, no matter where the meter reacts in relation to the flywheel, when you find that exact spot where meter starts to jump, showing that the points are just starting to crack open, put a pencil mark on the fly wheel. Then turn the engine one complete revolution. See if the meter jumps and breaks the points at exactly the same spot you put the pencil mark. More than likely it won't. The One Hundred dollar question: How much difference is there between the left side and the right side? If you have 3/16 inch linearly between the first mark and the second, that's about 3 degrees, and that's okay. 3/16 of an inch or less. If you end up with 1/2 inch or 3/4 inch, your advance is worn out or there is a wobble in the shaft when it rotates. There is nothing you can do but throw that advance away and buy a new one, and/or check the shaft of the rotating magnet for trueness, and repeat the test.

Back To Tech Articles