More on carb syncing

[mikenixon]

We've dealt with carb syncing in previous posts, but 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 merely 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 means 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 even on the Gold Wing, 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--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.

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. 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. In this case, carb tuning greatly affects transmission function. In fact, these ATVs have a penchant for gradually having their idle speed increase, causing the vehicle to creep when stopped.

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 maintenance task. Food for thought?

[05c50]

Good one Mike. I wasn't aware of the air bleed method on some bikes, interesting. When people have asked me about syncing carbs, I've always tried to explain it as getting all the cylinders to work at the same efficiency at the same time. Like you brought up, many things like compression, ignition timing, valve clearances, etc. can keep the cylinders from performing uniformly and I think that this is mostly misunderstood. A way to measure cylinder performance is somewhat of a challenge and measuring and comparing each cylinders vacuum is "good enough" for our purposes.
In the 70s, I raced a 4 cylinder cylinder car with multible carbs and we used flow meters (made for this purpose) mounted on top of carb air horns to measure air flow through the carb and past the throttle plates. I'm not sure how the adjustable air bleed would have worked with this setup. The adjustment procedure was rather crude...loosen a lock screw and slide the linkage rod until "perfection" was obtained. lol

Keep up the good work

.....Paul

[mikenixon]

Good one Mike. I wasn't aware of the air bleed method on some bikes, interesting. When people have asked me about syncing carbs, I've always tried to explain it as getting all the cylinders to work at the same efficiency at the same time. Like you brought up, many things like compression, ignition timing, valve clearances, etc. can keep the cylinders from performing uniformly and I think that this is mostly misunderstood. A way to measure cylinder performance is somewhat of a challenge and measuring and comparing each cylinders vacuum is "good enough" for our purposes.
In the 70s, I raced a 4 cylinder cylinder car with multible carbs and we used flow meters (made for this purpose) mounted on top of carb air horns to measure air flow through the carb and past the throttle plates. I'm not sure how the adjustable air bleed would have worked with this setup. The adjustment procedure was rather crude...loosen a lock screw and slide the linkage rod until "perfection" was obtained. lol

Keep up the good work

.....Paul

Cool!

[flyday58]

I've picked up a couple fuel-injected BMW boxers and a "flying brick" recently, and they too use the air bleed screw method for final carb syncing. Throttle cables are set to a specific freeplay measurement, then the BBS's (big brass screws) are used with the syncing device of one's choice for the final touches. These screws change the amount of air allowed into the fuel spray in the throttle bodies: more turns out = more air in the idle mixture.

The three butterflies on the brick throttle bodies are tied together with similar adjustment screws as the GL, but are normally never disturbed, unless a noob takes them apart. I don't know anyone like that...

[mikenixon]

Ah. Good to know. Just for you, I have something. You'll recall that another customer has the same model CB750C as you do. And that both of you were experiencing stalling at high altitude. He finally reported that it is solved in his case. The difference? We put a big bore kit in the top end, significantly increasing cylinder compression.

[flyday58]

Ah. Good to know. Just for you, I have something. You'll recall that another customer has the same model CB750C as you do. And that both of you were experiencing stalling at high altitude. He finally reported that it is solved in his case. The difference? We put a big bore kit in the top end, significantly increasing cylinder compression.

Interesting. I'll have to keep that one in my kit bag. The 750 will be moving to West Texas here shortly, lots lower altitude than my hacienda. New owner shouldn't run into any such problems!

[mikenixon]

You sold it then. Good!