Part and parcel of any carburetor work is of course synchronizing the throttles. Syncing is not completely understood by most folks. Let's explore this subject.
This is the factory Suzuki sync tool, no longer available new but one of the best ones out there.
First, there is just one thing that syncing benefits, and that is idle and low speed. Syncing is in fact for the purpose of smoothing the idle, primarily, and secondarily for improving off-idle throttle response. There is no other benefit. However, the idle and throttle response one is important enough to make syncing mandatory whenever called for, as well as a needed periodic maintenance item.
Syncing is periodically necessary not because the sync screws move but because carb linkage and engine internals wear slightly. Folks who put paint or (gasp!) thread locker on their sync adjusters are not correctly understanding this.
The traditional syncing routine involves attaching vacuum gauges (any of a myriad of types) and adjusting the sync screws until all gauges read the same. What is actually being measured is the airflow through each carburetor (more accurately, its manifold) and this more or less approximates the combustion strength of each cylinder so that the cylinders are in harmony. Syncing the throttles is not only the last part of a carb rebuild, it is also the final part of a tune-up. The reason it is the final part of a tune-up is because everything that went on before, valve adjustment, ignition adjustment and service, etc., affects and lays the groundwork for the carb sync. So that the sync actually builds on the prior work, and depends on it. If one were to readjust the valves, say, after syncing the carbs, then the sync should be rechecked.
Bench vs. Running Sync
When a carburetor rebuilder syncs the throttles at the end of a rebuild, he of course does this on the bench, a "bench" sync. Some have supposed this to be inferior in some way to a running sync and technically it is. But such folks don't usually stop to think under which conditions it is inferior and in considering that they might begin to value the bench sync more. Because, the bench sync is inferior to the running sync only to the degree that the engine has some tuning deficiencies. In other words, the only time a bench sync does not exactly match a running sync is when the engine has, say, some cylinders exhibiting less compression than others, or the ignition timing is not perfect, or the valve adjustment could be more consistent. See the point? If the engine is in the condition it should be, then a bench sync is as good as a running sync. Always. I used to tell my customers they should double-check the sync when their carbs arrive, and again, technically they should, But what I found early on was they universally reported that they put the gauges on and the carbs did not need syncing. They were perfect. So I stopped telling folks to check sync.
Symptoms of Bad Sync
Poor synchronization can create a number of problems. The classic "hanging idle," wherein the engine idle fails to come back down upon deceleration, is probably the best known of them. But there are other symptoms as well, and there are other causes of a hanging idle also, such as a vacuum leak, for example. Other symptoms caused by poor synchronization include engine knocking, and "hunting idle," wherein the idle goes up and down.
This article is too short to unpack the concept of synchronization technique, but a few tips are in order. First, use a large box fan. Career techs don't need to, but everyone else definitely needs a fan to keep the engine cool. Not only does this prevent engine overheating, but it also actually improves the process because an overheated engine will make the gauges do strange things with the result you cannot sync. Let the engine cool off and resume syncing. A second tip is to never rotate the sync screw more than 1/8 turn at any time. Just that little bit will move the throttle plate 0.010" and change your vacuum gauge reading almost a full CM HG. Third, watching the gauge while you turn the screw is a bad idea. It may be the intuitive thing to do, but you will find it very frustrating as first, snugging the locknut changes the reading, and second, the obligatory rap of the throtttle afterward to settle the springy linkage will also reveal a change.
All sorts of syncing gauges are available -- everything from mechanical round gauges to tubes filled with water or glycol or mercury, or steel balls or steel rods. There are even electronic LED and LCD systems. It hardly matters which you choose, but if you select the round gauge type, be sure the gauges have calibration screws on their faces. The round gauge type wear internally and so must be readjusted to each other periodically. Despite that requirement, the round gauges are very good to have.
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