® Synchronization study

We've dealt with carb syncing in previous articles, basically the mechanicals. But now let's deal some with the theory. I'm coming back to this subject and expanding on it because, well, it fascinates me. (I have a simple mind--I'm easily entertained--LOL!).

In the midst of our discussion involving the importance, skill and process of synchronization, while making the point of how neatly air flow measurement affords us the opportunity for throttle synchronization, I referred to fuel injection throttle body syncing,. Maybe you missed it, but I mentioned that on Kawasaki sport bike (ZX-14R, ZX-10R) fuel injection there are no screws for adjusting throttle plate orientation one to the other. The sport bike's inline throttles are all bolted to a single non-adjustable shaft. Synchronization is performed using air bleed screws.

These screws actually allow minutely increased or decreased amounts of air into the intake manifold. Think about this for a moment. Several very interesting things are apparent. First, it syncing in this manner is a much easier proposition than doing so by moving the positions of the throttle plates relative to each other. Second, the fact that all it takes is a skosh more or less air quantity in each manifold to make the vacuum levels equal demonstrates how tenuous, how delicate, how precise syncing is (and note the interesting fact that the adjustment of the air bleed is too minute to affect air/fuel mixture, another wonderful mystery!). Third, it emphasizes rather dramatically (in my simple mind) just how awfully basic synchronization really is. All we're really doing is equalizing airflow, or more precisely, the pressure drop in each manifold that results from airflow. That's it. When we synchronize the throttles, we're not actually syncing throttles, because on the ZX-10R that isn't even possible. And this means that even on carbureted bikes, we're not really syncing the throttles. And, did you notice we skipped the most obvious lesson of the ZX-10R? Why would throttle bodies that are manufactured as one permanently integral unit ever need syncing in the first place? Meditate on that one. Take your thoughts all the way back to carburetors, where we have to move the relative throttle positions to get sync. When we adjust them, what are we really doing? And why is it often necessary to sometimes have the throttle plates not perfectly aligned in order to achieve perfect vacuum sync (you know this by first doing a perfect bench sync and then finding the engine doesn't regard it so)? And once synced, why would syncing ever be needed again, if all it was was mechanical alignment?

Because of the engine's imperfections. When I bench-sync carburetors I am working to the premise that this will suit the engine. But it doesn't always. In those cases where despite a careful bench sync the running sync shows the carbs are off a little, what does this mean? I tell my customers it means something important. It should be noted, paid attention to. Because it means something in that engine, whether cylinder compression, spark plug condition, individual cylinder temperature, ignition timing or strength, carbon buildup, manifold sealing, valve clearances, head gasket condition, valve or ring sealing--something. Something is different. Something is not uniform across all the engine's cylinders. And I tell my customers, go ahead and check running sync because it will confirm or deny the engine's state of tune and give the opportunity to find out what is making it not uniform, not harmonious, throughout the cylinders. Bench syncing therefore offers a diagnostic opportunity when compared with a running sync. It's important to take advantage of that opportunity.

Recently some folks on forums have been very vocal in opposition to bench syncing. We have already noted that it is mandatory, not an option, as part of a carburetor rebuild. What these folks are contending is that people like myself should not advocate to customers that their rebuilt carburetors are sufficiently synced on my workbench and not needful of a running sync. They are only partly right. As is so often the case on forums, the ease of speaking extemporaneously and without qualified experience has got the better of these people. Ninety-eight percent of my customers are not going to be able to synchronize their CBX carburetors. That's just a fact. It's a rare skill even among professional mechanics. For most of my customers, the careful bench sync does the trick. They don't need anything more, and I have had customers tell me this when they do endeavor to sync their carbs. They are very good as-delivered. Should they be synced on the running engine? Of course. That little bit closer sync makes a difference. But not so much that the average CBX owner is badly served by just the bench sync.

Another thought. On carbureted twin-cylinder Kawasaki ATVs, there are found both a screw for mechanical throttle plate syncing, and air bleeds for the same purpose, further syncing. The factory manual ignores the mechanical sync screw, making one assume it is for carburetor installation only, to get the linkage roughly set up. Then the actual syncing is done via air bleed screws on each carburetor, just as in the ZX14. Interestingly, this arrangement is repeated in reference to the idle speed. There is a mechanical, factory-preset (locknutted) idle speed screw, but again, the maintenance manual ignores it and has you adjust the idle speed using the same air bleed screws as for syncing, and at the same time, the same operation, as the sync. Why? Why both mechanical and air bleed sync and both mechanical and air bleed idle speed? This unusual setup is doubtless due to the ATV having a CVT transmission, which system is very sensitive to idle speed. These ATVs have a penchant for over time having their idle speed increase, causing the vehicle to begin to creep when stopped. In this case, carb tuning greatly affects transmission function.

The point of all this is, synced airflow is not always equal to and is more important than-- actually trumps, mechanically-measured sync. Synced airflow is our goal, no matter the outcome of synced mechanicals. And isn't this such a fine, hair-splitting thing? Marvelously so. Appreciating this is what separates the men from the boys when it comes to performing this important maintenance task. Food for thought?

Last updated November 2021
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