Carburetor balancing/synchronizing

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models

Created: 2006 June 13

Updated: 2006 June 13

Contributed by Patrick Hayes on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io).

I presume you have the more common Dellorto square slide carbs. There are some early loops with round slide Amal carbs and these comments will be all wrong for them.

I presume you have plenty of slack in the choke cables so that they close fully and are NOT hanging off the end of the taut cables.

I presume you have vacuum tap ports on your intake manifolds. Again, early loops did not have these ports. They can be added easily. Later loops all had them.

I sure hope you have some form of vacuum measuring tool. Forget about doing it by ear, or feel, or counting rotations before stall, or sticking toothpicks in the carb throats. Those techniques are all horribly crude as related to vacuum measurement.

I hope you do not try to do this with analog gauges. It is impossible. It is far easier to read and far more intuitive if you use vertical column manometer vacuum measuring devices.

I like CarbStix the best, but they do have the danger of spilling toxic mercury. Mine are 25 years old and intact (tap wood). There are modern copies which use steel rods instead of mercury. That would be my choice if I were starting anew. There is an electronic TwinMax. Not as intuitive. But very small and portable as long as you learn how to read it. For about $2 you can make a tool out of fish tank air hose and ATF oil.

Get the bike running and warm and hook up the vacuum.

Begin by setting the idle speed. This is the big, exposed, knurled screw. DO NOT TURN THE SCREW!!! When the slides rest on the pointed tips of these screws there is a lot of contact load. You can damage the screw or the slide. Even for slight adjustments, always crack the throttle ever-so-slightly as you turn this screw so that it is not loaded while turning.

You probably will not have equal idle vacuum readings. They certainly won't be steady as they pulse up and down with each cylinder suction. You can visually and mentally average out where there center positions are. Whichever column is averaging lower, that side is open too far in relation to the other and that cylinder is working harder than the other. Depending on your idle speed, you can choose to slow down the harder working side (and its mercury column will now rise slightly on average) or you can open and accelerate the non working side. It doesn't matter if you choose to open one or close the other, you need to get them pulsing in average parallel. Remember to crack the throttle as you adjust this screw. Counterclockwise drops the slide, slows the cylinder, and raises the mercury column. Clockwise is opposite.

Once you have the idle speeds equal (showing equal pulsing vacuum) you can adjust the mixture. This is the smaller, recessed screw. Turn it inwards clockwise until you hear the motor stumble. Turn it outward counterclockwise until the stumble eases. Back out at least an additional ¹⁄₄ turn and maybe even ¹⁄₂ turn from this non-stumble point. You want the idles a little bit rich.

Go back and look at your idle speed balance as it may have been altered. Repeat the above two paragraphs.

Now to acceleration balance. Many will write to you about mid-range and high range balance. Hooey!! At big throttle openings and higher RPMs, your engine cannot possibly detect minor vacuum imbalances. The place where it is CRITICALLY important is at a stop light or traffic light when you are feathering the clutch and adding throttle to accelerate away. Cylinder imbalance is one of the biggest aggravators of the famous loop frame grabby clutch. It doesn't grab at 75 MPH, it grabs at 3 MPH when you are pulling away from a light.

If you did the above idle work accurately, the two carbs are drawing equally at idle and the two cylinders are producing equal power pulses. Now, when you open the throttle it becomes critically important that BOTH sides open and draw simultaneously and both cylinders increase their power output equally. Crack the throttle open. Not a WFO twist, just a 10% crack. Look at the vacuum columns. Just as you crack the throttle, one side suddenly drops or loses vacuum before the other. That side has a tighter cable and is starting to work harder first. Again, you can either loosen that cable or tighten the other.

The effect you want is two equally pulsing vacuum columns at idle and then they both drop simultaneously and equally as you begin to open the throttle. Everything else is superfluous.

You will never get perfection. You are talking lots of sliding parts in the carburetor and the cables and the throttle handle. Its all sloppy stuff and it will look a little different each time you blip the throttle. You will be amazed at how nicely the motor runs when it is in sync and how easily or quickly it goes out of sync. You will soon learn what it looks like to have these columns perfect and also what it looks like to have these columns close enough for everyday smooth operation without being fanatic about it.