How To Synchronize Your Carbs - It's All About Balance

The 411
For reasons of performance and ease of manufacturing, motorcycle engines are often equipped with one carburetor or throttle body per cylinder. Consider for a moment the kind of manifold you'd need to feed a BMW boxer or inline-four motor through a single carburetor and you'll understand exactly why that's so.

The catch is that when you feed each cylinder through its own carburetor (from here on we'll use the terms carburetor and throttle body interchangeably), what you've got are essentially several independent engines joined by some shared hardware.

That being the case, it's important to keep them all working in concert, and one of the ways you do that is by synchronizing the carburetors so that each cylinder is always at the same rpm as its neighbors for a given throttle opening. This keeps the crank turning smoothly and results in an engine that idles well, runs sweetly and makes good power.

Problems occur when the throttles get out of step with each other. When that happens each cylinder receives a slightly different charge, so the engine is constantly trying to speed up or slow down depending on which cylinder is carrying the load. The continual speed changes, small as they may be, cause the engine to vibrate, make the idle fluctuate and, in the worst case, cause off-idle hesitations and surging at steady speeds.

What causes the throttles to go out of synch? Typically, multiple carburetors are connected to each other via linkages. When the linkages start to wear and small amounts of play develop, the carburetors get out of phase with each other. In most cases it's simple wear and tear that creates the problem, and because of that, synchronization problems tend to come on slowly. Most bikes go thousands of miles between adjustments, so if your bike is fine when you put it away but refuses to idle properly the next time you ride it, it's a pretty safe bet that an out-of-synch carburetor isn't causing your problem.

As an aside, I should mention that before you synchronize anything you'll need to make certain the rest of the engine is up to snuff. If you've got a mill that's down on compression with three tight valves and two fouled spark plugs, you can synchronize the carbs until you're blue in the face and the bike still won't run worth a lick.

What You'll Need
Synchronizing carburetors sounds a lot more difficult than it is. As always, the most important tool (besides your willingness to get dirty) is the service manual. The manual will detail not only the basic procedure but where all the adjusters and vacuum ports are located and, of course, any nuances peculiar to your bike.

In some instances there may also be a fair amount of motorcycle to disassemble before you can even see the carbs, and in that case the price of the manual will usually be cheaper than the side panel you broke 'cause you didn't realize there was a hidden screw holding it on.

Along with the shop manual and a basic set of hand tools, you'll need the following, which with the exception of the cooling fan can be found at any good motorcycle shop.

* Some sort of cooling fan. No motorcycle, whether air- or liquid-cooled, should be left to sit and idle without the benefit of cooling air blowing across it to prevent overheating. Overheated engines don't respond well to subtle adjustments, and if it gets really hot there's a chance you may damage something. An old window or box fan will work just fine, and new ones can usually be bought for under 20 bucks at the nearest Home Depot.

* An auxiliary fuel tank will come in handy if you need to remove the bike's tank to access the carbs. It won't be required in all circumstances, but if the tank has to come off, the backup container is a lot easier to use than an old coffee can with a couple of holes punched through it. Motion Pro makes a nice one that's reasonably priced.

* A dedicated carb-adjustment tool. Again, this isn't a strict requirement, so check the manual before you spend your dough, but in some cases one of these tools, which usually contains a screwdriver tip and lock-nut socket combined into one unit, will make life a whole lot easier.

* Vacuum gauges, technically called manometers, are the one must-have item-without them you're dead in the water. Vacuum gauges come in a variety of styles and shapes. Right off the bat I'm going to recommend you pass on the analog-style multiple-gauge setups as well as the type that uses a floating steel ball. Both of those need to be calibrated at each use and require a damping adjustment if you want to get an accurate reading. Mostly they are more trouble than they're worth, especially for the casual user.

The mercury type of gauge commonly called a carb stick is very easy to use and requires no calibration. However, there is a slight chance that the mercury will get sucked into the engine if you rev the thing up and then chop the throttle suddenly, and of course if you're the careless type you can also break the tubes or tip the gauge the wrong way and spill the mercury. Which risks turning your garage into a home version of a Superfund cleanup site.

My favorite synchronization tool is the digital type, like the CarbMate (see this issue's product review, page 90), which uses LEDs to indicate differences between the carbs. The CarbMate and other similar tools are very easy to calibrate and use, particularly if you're new to this sort of thing, and while they tend to be more expensive than mercury gauges, I think it's money well spent.