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Library Home Page > 1st Generation (1983 to 1993) > Fuel System
Carburetor Synchronization Information
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Carburetor Synchronization Information

Thanks to Rey for this info from Motorcycle Consumer News.

All Together Now

CARB SYNCHRONIZATION IS a fairly simple operation you can perform in your own garage at home with a minimum of special tools. Any time you tune up a bike with more than one carburetor, you should also synchronize the carbs. Synchronization needs to be performed at regular intervals, too, as part of every tune-up. Whenever your valve clearances are adjusted, you are likely to effect the vacuum balance of your cylinders. And on many machines, wear on the linkage that connects the carbs together, particularly on carbs that work back-to-back on opposing cylinders, will also dramatically change the synch over time. An engine that needs synchronization will sound rough and stumbling at idle. It may also be much noisier than it would be otherwise, making you suspect cam chain tensioner problems, or other mechanical causes. Throttle response will suffer, as well.
Consider for a moment what synchronization actually is; the harnessing of individual cylinders into a team that pulls precisely together. Just like oarsmen in a racing skiff, when none are overexerting and making the effort of another less effective, and none under-exerting, forcing another to overheat; a synchronized team effort is most efficient. We are fortunate that our modern multi-cylinder motorcycles are typically equipped with a carb for each cylinder, despite the additional complexity. Cylinders that share carburetors rarely share them equally. Manifold variations and uneven cylinder firing tend to cause unequal fuel distribution, depending on the length and volume of the manifold runners, the timing of the intake pulses and the rpm of the motor.  Ask a veteran engine builder about the classic example of the four-barrel carburetor feeding eight cylinders on a big American V-8, and he'll tell you that the air-fuel mixture will be different on the cylinders furthest from the carb, at the corners of the engine, compared to those in the middle, closest to the venturis, depending on the manifold design.
But, even then the effect may not be not constant. In fact, the corner cylinders might tend to run rich at high rpm, but lean at low rpm, relative to the middle cylinders. Hence, the big business in aftermarket V-8 intake manifolds, all attempting to find the best compromise.  Our motorcycles, on the other hand, by keeping the distance to the carburetor and manifold shapes constant, eliminate most of these problems. The latest high-performance engine designs have done away with the centrally located camchain, and now drive the cam from the side of the block, in part as a way to minimize intake manifold variations from one cylinder to another.


To synch your own carbs, you'll definitely need a synch tool of some type and typically, a remote source of gasoline to fuel the engine while the gas tank is removed, a specialized carb-adjusting screwdriver, a drill, bit and self-tapping screw to remove the tamper-proof caps that may be covering your idle mixture screws, and a fan to cool your engine while you work. 
Carb synch tools come in many types.  The most common is simply a four-tube mercury manometer that utilizes engine vacuum to lift individual columns of quicksilver from a common pool. To reduce the jumping of the columns as the engines' vacuum changes through the various strokes, the tubing to the individual manifold connections will incorporate restrictors, little drilled pills jammed in the tubing. These must be hung upright at all times, or you risk losing the mercury, not a good thing to have loose in your garage-it's very toxic.
The popular mercury manometers are available in deluxe and economy models and sell for prices ranging between $44-$80 from Motion Pro and others at your dealer.  More sophisticated are the multi-dialtype vacuum gauge sets. These are usually fitted with adjustable restrictors to control the vacuum fluctuations. Naturally, being more complex, they are also more expensive, several times the price of the cheaper mercury manometer (approximately $ 175 for a four-gauge rack from K&L Supply through your local dealer). A drawback is that because the mechanism that moves the needle is mechanical,'it is more delicate and vulnerable to damage. Also, the accuracy of the multiple gauges must be very good to give perfect results.
The most accurate are the electronic types. Similar to the factory tools used to synchronize electronic fuel injection, they utilize pressure sensors with an electronic readout. They have no moving parts and so are not affected by fncaon, or the accuracy of multiple springs, tubing, or gauges, and they are adjustable for high sensitivity. Kowa Seiki, a top Japanese manufacturer of shop tools, sells a four-cylinder model for approximately $500 (from K&L).
The Twinmax balancer, from England, is another, more affordable electronic balancer that you can buy, available through Adventure Motorcycle Gear (800-217-3526).   Because it works by comparing the vacuum present on either side of a single pressure sensor, only two cylinders can be connected to the device at one time, making it ideal for twin-cylinder engines. But, it can also be used on multi-cylinder motors by designating one cylinder the "master" and adjusting the others to match-slightly more bother, but very accurate. It is sensitive enough to synch fuel injection as well, which a mercury manometer is not. The price is $79.95.
Synchronization is the fmal step in a tune-up. You'll perform it after your valves have been adjusted and whatever .other maintenance has been performed.  Two distinct steps should be performed.  The first is to synch the throttles to open simultaneously from the same vacuum level. The second is to optimize the individual idle mixtures, starting from the factory-specified adjustments.  The combination of these two makes an engine run very smoothly, with excellent power and response.


by Dave Searle

Reply to keithert, your Note of 7/29/99 06:44 AM -0700:

Thanks for presenting this information. It is accurate except for the part about removing the gas tank. On a Venture that is not required.
Very easy to do on a Venture. Adapters not required for the vacuum line hookups either.



Last update: 11:56 AM Sunday, September 26, 2004

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