Carb Sync Tool Balances Your Carburetors

Carb Sync Tool Makes All the Difference as You Tweak Your Ride

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 with a carb sync tool 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.

You've read your manual, you've got the tools set up and you're ready to go, so let's get started. First make sure you've got plenty of ventilation. For most of this exercise the engine will be running, and while the smell of burning gasoline may be heady, the byproducts of combustion can put you to sleep for a very long time if you're not careful. I suppose I should also pass on the obligatory warning about spilling gasoline all over the place, especially when you're working with a hot engine, so consider it passed.

The first step is to remove anything that impedes access to the carburetors. Your manual will provide the details, but in most cases it's going to mean that at least the seat, the fuel tank and perhaps some of the bodywork will be coming off. Stow it all in a safe place before proceeding.

If required, connect your auxiliary fuel tank and hang it in a secure location high enough to ensure good fuel flow to the carburetors. If you plan to dangle the thing from the handlebars, make sure there's no chance it'll contact the exhaust pipes.

Sometimes things can get a little tricky here, particularly if you're working on something with EFI or a tank-mounted fuel pump. If that's the case you may be forced to keep the tank and its attendant fuel lines and wiring harness connected to the bike while you fettle the adjusting screws. In some instances the tank will be hinged to accommodate you; in others you'll just have to jury-rig something. As always, consult your manual; the answer is lurking in there somewhere.

The manual should also identify the correct vacuum ports, which will have either rubber caps or threaded plugs to seal them, and the "master" or reference carburetor you'll synchronize the others to. If the vacuum ports are threaded, you'll need adapter tubes to plug in your gauge (normally these are supplied with the gauges, but not always), so make sure you know what you'll need beforehand.

Following the manual or gauge manufacturer's instructions, connect the gauges in order-gauge number one to cylinder number one and so forth-and if need be, calibrate them according to the manufacturer's instructions. Then position them where they'll be easy to see yet out of the way, making sure none of the hoses foul any of the carb linkages.

Double-check your hoses and make certain there's nothing left open or unaccounted for, then start the engine (don't forget your fan) and let it come up to operating temperature before setting the idle to the manufacturer's recommendations. If the carbs are way out of whack, the bike may not idle well. If that's your situation, just get it as close as you can for the time being and proceed.

With the engine idling, observe your gauge. In most cases the carburetors should be set to identical marks, but occasionally you'll find some that are set unequally, so double-check the gauge readings against the manual before you start spinning adjustment screws.

If you're working on a twin, the procedure is dead simple. Normally there will be only one adjusting screw, and which way you'll turn it depends on the readings you're getting, but my bet is you'll be turning it clockwise to start.

As a rule carburetors are fairly sensitive to adjustments, so go slowly and work in small increments; 1/8 turn at a time should be about right until the gauge readings are identical (or at the factory-recommended settings).

When you're satisfied with the readings, gently raise the engine speed to a fast idle and then allow it to drop to the base setting to settle the slides. Recheck your work, performing any fine adjustments as required.

A word to the wise: You don't have to go crazy here-a fast idle is all that's required. If you're using a mercury gauge and you rev the snot out of the engine, the high vacuum created when the slides drop down can suck the mercury clean out of the gauge and straight into the cylinders. If that does happen it's not necessarily a disaster, but it will put paid to your synchronizing efforts until you can refill the gauge, and you'll have to change the oil and filter to get rid of the ingested mercury.

Triples and fours are a little more complicated, though not by much. Essentially the procedure is the same as that of the twins; there are just more cylinders (and adjusting screws) to deal with.

When you're happy with the adjustments, rev the engine to a moderately fast rpm-somewhere around 2500 to 3000 works nicely-and make sure the gauges remain equal. If they don't, but they remain aligned at idle, look for a vacuum leak, worn or sticking slide, or some sort of mechanical problem in the linkage.

The final step is to recheck your idle speed before disconnecting the gauges and putting the bike back together. Make sure all the vacuum caps and lines go back in the right place, and don't forget any clips, especially those that secure the fuel lines. In fact it's always prudent to start the engine one last time after the fuel and vacuum lines are reconnected but before any major body pieces are reinstalled, just to make sure everything is as it should be.

Tech Tip
The Lost Key Chronicles

A couple of months ago I was with a group of about 300 people taking part in a dual-sport ride. We were preparing to leave when the rider next to me reached for his ignition switch, said "Uh-oh," and began searching his pockets. Within 30 seconds he realized his keys were lost, and the next thing you know there were 300 people combing the parking lot looking for a tiny DR650 key. Believe it or not we found it-but if we hadn't he would have been truly screwed. The guy was miles from home with no spare. Yeah, he might have hot-wired the bike and made the ride, but that would have been a hassle, and most likely his day would have ended right then and there. The only bright side was that (a) he'd trailered to the event so at least he could get the bike back home, and (b) he lost the keys in a gin-mill parking lot, and the joint, which had excellent ambience (if you catch my drift), was open.

You might not be so lucky, so here are a few places to stash a spare key.

  • Tossing a spare in the saddlebag, tool kit or maybe a stash pouch is always popular, but make sure the spare is accessible without unlocking anything!

  • I've seen quite a few guys wire or tie-wrap a spare to some out-of-the-way place on the frame. Make sure the hiding spot is well protected and out of sight, and don't forget where you hid it.

  • Inside the headlight is a great spot-warm, dry and protected, and you can usually pop the headlight off without too much trouble.

  • Inside the taillight-ditto. When I was a dealer we always stashed a spare key in the taillight just in case. You'll need a screwdriver to remove the lens, but they're usually easy enough to come by, and in a worst-case scenario a rock makes a passable substitute.

  • Inside the handlebar may work but it can be tricky. In the first place, modern handlebars aren't always hollow, and in the second, you'll normally need tools to remove the throttle or a razor blade to remove the left-side grip and retrieve your hidden key. But either method is preferable to the alternative. If you do hide the key in the bars make sure it can't slip all the way to the center. Foam wrapped around the key will usually do the trick.

  • A hide-a-key box can be bolted, tie-wrapped or even duct-taped to the inside of the fender or frame or even to a convenient engine bolt. Tape the box shut and you should be all set.

Obviously you don't have to stash a spare on the bike; you can hide one in your favorite riding jacket or carry a flat-style spare in your wallet or anywhere else. The idea is to have a spare available in an emergency, and by extension it needs to be stored in a safe, secure, inconspicuous spot where it's protected from the elements and the unsavory types who might really like to ride your bike like they stole it.

Carb Sync Tool
Mercury Sticks
Floating Ball
This works so much better than a leaky coffee can with a petcock punched through the bottom.
A carb-adjusting tool and adapters may be required.
It doesn't get any more graphic than this: The left-side carburetor, which is the one in the right side of the photo, is the master carb. The balance screw is attached to its shaft; when it moves it forces the right-side, or slave, carburetor's shaft to turn. So long as both of those throttle plates open at the same time everything is good.
If you're working on a twin, normally there will be just one adjusting screw; go slowly with your adjustments.
As a rule, multiple carburetors and throttle bodies are fastened together with a stout bracket that serves two purposes: It makes the carburetors easier to handle, and it prevents them from flexing, which would affect the alignment and operation of the throttle shaft that joins the butterflies.
It's either go or go home; that's what makes using the electronic gauges so easy.
The balancing screw (s) will be easy to identify.