Home arrow VMCA Newsletters arrow Volume 2, Number 2 - April 1, 2005 arrow Carburetor Balancing BMW Twins 1951-1969
Carburetor Balancing BMW Twins 1951-1969 PDF Print E-mail
Written by BMWVMCA NEWS   
Tuesday, 29 July 2008

Carburetor Balancing BMW Twins 1951-1969 by Craig 'Vech' 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 adjustor 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 speeded 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 it is as fast as it wants to run using the mix screw, then switch back to 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, so you can say for example, the left (or right) side  is running just a little bit faster and 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 the 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 evenly.  In other words, look at the position of the screw driver slot in each of  the idle stop screws, and then back each one off the same degree of rotation.  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 carb tops. 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. 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  substantially.  I do this adjustment  with my chin on the gas tank to feel the vibrations while I adjust the  left cable. As soon as it smoothes  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 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. 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 the 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 where you put the pencil mark.  More than likely it won't.  The One Hundred dollar question is: How  much difference is there between the left side and the right side?  If you have 3/16 of an inch or less linearly between the  first mark and the second, that's less than about 3 degrees, and that's okay. If you end up with  ½” or ¾”, your  advance is either 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.

By Craig Vechorik
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Additional Carburetor Tip           

With the bike on the center stand and engine running at idle, spray a little WD-40 or starting fluid at the bases of the carburetor flanges. If this causes the engine to speed up, there is an air leak where there should be a tight seal. Remove carburetor and dress the flange using a sheet of ‘wet or dry’ sand paper on a ‘true’ surface. If unsure about this procedure, send your carbs to Bing International for service. Reinstall and retest for leaks.


Bing International L.L.C.
1704 South 525 Road,
Council Grove, KS 66846
Orders - Toll free 1-800-309-2464
Tech 620-767-7844 

Unleaded Fuels:           

Here’s what Mark Huggett has to say about this subject.             

“For your info guys, leaded fuel was only introduced in Europe in the Early 1960's. Before that it was all lead free anyway. Converting healthy heads on pre 1969 models is an utter waste of money. Only convert if the heads need new valve seats anyway.”  


Last Updated ( Tuesday, 29 July 2008 )
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