How to balance V11 throttle bodies

Posted March 31, 2004

Balancing throttle bodies on the V11 Moto Guzzis (this article uses a Le Mans as an example) is a fairly simple procedure. This procedure was related to me by a mechanic at Moto International.

You will need the following tools to perform this service:

• Small flathead screwdriver
• 2.5mm hex key
• Manometer (TwinMax, carb stick, water manometer, etc.)
• Large fan
• Voltmeter or multimeter

Description	Pictures
First, get the bike up to operating temperature. Normal procedure is to get out and ride -- letting the bike idle in the garage will not do it (and risks the bike overheating in any case). Ride for a minimum of 10 minutes, 20 minutes is ideal. Note that you cannot accurately set the throttle bodies unless the engine is in otherwise good tune: spark plugs gapped and clean, valves adjusted, mixtures about right, etc.
Once the bike is up to temperature, prepare a fan aimed at the engine, to keep it from overheating as you fiddle.
Attach your manometer -- TwinMax, mercury carb sticks, or water manometer, whatever. Attach it to the vacuum nipples right at the intake flange.
Adjust each of the air bypass screws so that they're 1 full turn out: Gently screw in the bypass screws until they are screwed all the way in, then back each one out the amount you desire. Be careful not to screw them in very hard, or you could damage the air passage. The key here is to set them to the same value. If you like, you can use 3/4 of a turn, or 1 and 1/4 turn, if the bike won't idle nicely with one turn. The air bypass screws require a small flathead screwdriver.
Start the engine, and turn on the fan if it's not already running.
Observe your device, and the tachometer. If your idle speed is not as specified in the manual, you'll have to do this several times until the engine is both balanced and idling at the right speed. Using a 2.5 mm allen key, adjust the idle stop screws until the idle speed is correct, and the manometer shows about even. You may have to adjust each screw several times as the idle speed and balance change. Note that you'll never get the cylinders exactly evened out with a water manometer -- it's too sensitive. Just try to get the water levels within a few cm of each other or you'll go crazy chasing a moving target. The right-hand idle stop screw is a pain to access. Each throttle body has the idle stop screw on the left-hand side, which makes it easy to work with on the left, and an utter pain on the right-hand throttle body. You'll want a flashlight to see it, if you're not doing this outside. DO NOT use the air bypass screws to adjust balance and idle speed, no matter how much easier it looks. Air bypass screws should only be adjusted away from the "one turn out" setting to change idle mixture, using a CO meter.
Now that the idle settings are balanced, use the throttle to rev the motor up to around 3000 RPM -- the actual speed doesn't matter, you just need to be using the right grip to set engine speed. Note the balance you see on the manometer. If it's not close to balanced, you need to adjust the balance knob on the left side of the bike. Release the throttle grip, make the adjustment, and rev the motor up to the same speed. Note the difference in balance -- if it went the wrong way, adjust the knob in the opposite direction. Keep revving, noting, adjusting until you get the engine close to balanced. It'll never be perfect, but get it close.
If you have to run the motor for more than 5-7 minutes with the fan on, shut it down as soon as you can. Leave the fan running, and go grab a drink. It's very easy to overheat an air-cooled engine while balancing the throttle bodies. Let the engine rest for at least 20 minutes before you continue working. If the ambient temperature is cold (below 40° F or 5° C), you can probably run the engine longer, and let it cool off for less time between runs. If it's warm out, you have less time until overheating, and will have to let it rest longer.
Finally, check and possibly adjust the Throttle Position Sensor (TPS). Apply the black lead of your voltmeter to some bare metal on the bike's frame, and the red lead to the indicated pin [to be filled in]. It should read right around 500 mV at a good idle. [Adjusting procedure to be written]

Discussion

The goal of balancing the throttle bodies is to set up the engine so that it's generating the same amount of power from each cylinder. The easiest way to measure whether that's happening is to measure the amount of vacuum being created by each cylinder. And the easiest way to compare the vacuum of one cylinder to that of another cylinder is to hook them together with some kind of pointer in the middle that indicates which one is pulling harder.

So, your manometer (with the exception of separate vacuum gauges) works by comparing the vacuum from cylinder 1 to that of cylinder 2.

The throttle plates in a carburetor or throttle body restrict the amount of air that's allowed to flow into an engine. This produces a lower pressure as the engine tries to suck more air past the throttle plate than it's capable of passing. The opening around the throttle plate determines the difference between atmospheric pressure and intake pressure for each cylinder.

So really, what you're doing when you adjust the different idle stop screws or the big balance knob is setting the throttle plates to be at exactly the same angle between the throttle bodies. This produces the smoothest running engine, and the best power. The cylinders are then working together at the same rate, instead of one trying to run faster or slower than the other.

Controversy

There is some controversy in the V11 community (as interpreted from the v11lemans.com forums) on two of the points raised in this article. The first is the use of the right-hand idle screw, and the other is how to adjust the air bypass screws.

Right-hand idle stop screw

The argument goes that, because the throttle plates are linked together, adjusting either of the idle stop screws changes the idle speed. Thus, there's not really any way to affect the balance between cylinders at idle.

I refute this argument with an experiment you can do. Go out to your bike, and follow the instructions above, but back the right-hand idle screw all the way out, and don't adjust it. If you can get your idle balance set properly without it, you have a rare bike, and I invite you to never touch that annoying right-hand screw again. Most people will find that the left-hand cylinder will consistently pull less vacuum.

The reason for this is that the linkage between the throttles has some play. Because idle adjustments are made in fractions of a millimeter, and the play in the linkage is larger than the adjustments being made, moving the left-hand screw a little bit won't change the angle of the right-hand throttle plate. This results in an imbalance at idle, although it may not be a large imbalance.

If you're not concerned about small imbalances, it's quite acceptable to simply adjust the idle speed with the left idle stop screw and not worry about throttle body balance at idle.

Air bypass screws

The air bypass screws are used to set idle mixture. They adjust the amount of "extra" air allowed into the combustion chamber at idle. By unscrewing the screw, you're opening the passage, allowing more air to pass. This leans the mixture; tightening the screw enrichens this mixture.

The average home mechanic doesn't really have the equipment necessary to adjust the mixture correctly. It takes a CO meter, which is a multi-hundred-dollar piece of equipment on the used market. So, the argument goes, why adjust them at all? They were almost certainly set at some earlier date by a competent mechanic with the proper equipment.

The argument presented by the MI mechanic mentioned at the head of this article is that one turn out is almost always the right setting. Unless your bike runs poorly at idle, it's probably not necessary to adjust the bypass screws. However, adjusting them to be the same makes balance adjustment easier. Setting them to one turn each puts them in a reasonable range which, within about 3000 feet of sea level, should put the idle mixture about right.

So, you don't have to adjust the idle mixture screws. You won't hurt anything if you do, though.

Created by Ian Johnston. Questions? Please mail me at reaper at obairlann dot net.