® The Master Carb Myth

From page 3-9 of the 81/82 Honda CBX factory manual, dated May 1981. This is the sole indicator in the manual that there is such a thing as a master or base carburetor. Nowhere else is it reinforced. Not in the adjustment tutorial, not in assemebly/disassembly, nowhere. The idea of a master carb is just that, an idea. It is not borne out in the carburetors themselves.

Despite what Honda says in many of their manuals, there is no such thing as a "master" or "base" carburetor. I don't say this to be argumentative, and I don't say this without a lot of thought. But all this talk about a master carb is nonsense.

Carburetor throttle synchronization is extremely important. In many cases throttle sync is the most effective end result of my rebuilds for my customers. Most people have never ridden well-synced carbs and are thrilled when they finally do. And for the DIY, the task of synchronizing carburetors is difficult enough for many folks, why introduce notions that will have no bearing on the job and only confuse? Moreover, all the observation I have made in working on powersports vehicles for over 45 years tells me there can be no benefit in having a fixed carburetor, that is, one whose throttle cannot be adjusted. Consequently, I can only disagree with the "master carb" idea. Nice idea, but only an idea, not a reality. Here are three things you may not have considered.

Number one. First, you will say, "but the manual says so!" Okay, fair enough. A legitimate argument. No one reveres the factory book as much as I do. But did you know the manual makes mistakes in many areas? Item: The Honda CBX manual shows 1979 model carburetors in all versions, even the Pro-Link manuals, despite there being significant differences between the four model years. Item: A lot of Honda manuals show prototype carbs, and even non-US model carbs. Item: The CBX book instructs, in all manual versions, a way of installing the carb manifold clamps that is wrong for the 80 and later models (due to the addition of vacuum valve parts in later models that interfere with the clamps if installed incorrectly). Item: The Pro-Link CBX manual also has tool numbers listed incorrectly, an incorrect method for servicing the Pro-Link shock, and several other glitches. Am I slamming the official book? Not at all. I am merely pointing out that, as superior as it is, it is not without fault. But this is not about the manual, this is about the supposed master carb. And here's the $64,000 question. If Honda didn't say so in the CBX or DOHC manual, how would you know there was a master carb? Could you tell it by looking at the carbs themselves? The answer is no. There is absolutely nothing different about any one of the carburetors compared to its mates. The linkages are the same. The throttles are the same. Nothing is different. The bottom line is the idea of a master carb is supported only by a sentance in the manual, and in the minds of the idea's adherents, and nowhere else.

This is three of the six throttle shafts of a Honda CBX carburetor. In fact, shafts one through three. See how the method of connection is the same at each adjuster? Note the ball joint on the far right of the picture. The presence of the ball joint, which merely allows the two banks of three carbs each to meet at an angle, does not presume a fixed or master carburetor.

Number two. Second, if you said, "but the six-cylinder CBX carbs have only five sync adjusters and similiarly the four-cylinder DOHC four carbs only three, so that means one carb is missing an adjuster, ergo it has to be the master carb." Really? Is that what it takes to have a master carb? Did you know four-cylinder Gold Wing carbs have only three adjusters? V4 carbs also? Is there any mention in their manuals about a master carb? Nope. And again, point out for me the master carb. Tell me how that carb is different. "Well, there is a master carb because of emissions regulations", you may say. Sorry, no. The 1975 Wing and the 1982 V4 are at different ends of the emissions standards timeline, yet both carb sets have only three sync adjusters, and neither bike's manual mentions a "master carb". In the case of the Wing, emissions laws weren't even applied to motorcycles until the 1978 model year, so that leaves the 1975 through 1977 Wing out. And as for the Honda V4s, the sync screws on those carbs are quite obviously set up one screw for the front pair, a second screw for the rear pair, and the third screw is dedicated to balancing the front and rear pairs to each other. Exactly like the Gold Wing. And what about the CB/CM400/450 and the CX/GL500 and 650, those two-carb sets having only one sync adjuster? Where is the "master carb" on those carb sets? There isn't one, again. So, having one less adjuster than carburetor is no sign that there is a master carburetor. In fact, if you take either the Gold Wing carb set or the V4 set, bith of which are arranged in a square pattern, and break the square so that the carbs are arranged in a straight row, you have the exact same sync screw arrangement as a DOHC four carb set. And visa-versa. If you took the DOHC and bent it around in a square, guess what? Yup, the sync screws would be identical to the Wing and V4. In all these carbs, each sync screw adjusts two carbs, or two banks of two carbs, at the same time. One screw balances the two carbs on either side of it. The second sync screw does the same for the other two carbs. And the third sync screw balances the two carb pairs to each other, pair to pair.

This is how Honda carburetor sync adjusters are arranged. This is an early example, the later differ only in that they have no locknuts.

Number three. What we've covered so far is proof enough. Byt there is more. Because the third point is, since one screw adjusts the balance of either two carbs or two banks of carbs, as a consequence when multicylinder carburetors are each connected to vacuum gauges and one of the carbs sync adjusters is rotated, the result is the vacuum reading for those two carburetors change. The carbs on BOTH sides of the adjuster change readings. While the one goes up in vacuum, the other goes down. This is further proof that there can't be a fixed, non-adjustable, master carburetor. If there were a master carb, this would not happen. All the carbs are in fact adjustable. As a matter of fact, best practice technique for carburetor throttle synchronization is to focus on just two carbs at a time, which the linking of muliple carbs into sync-able pairs makes a natural thing.

This shakes your world, no doubt. No master carb! What you have assumed all these years is incorrect! (Hang around, you'll get a few more shocks!) But does knowing the truth about the so-called "master carb" help you? Yes and no. Yes because it should make the synchronization process make more sense, and be much easier and faster to do. It certainly helps me, and I've been doing this for almost five decades. But at the same time no, because whether you believe in the master carb concept or not, not a whole lot changes. Everyone still ends up at the same place, carefully synchronized carburetors. And that's all that matters, silly ideas notwithstanding.

 

An early Honda V4 carburetor set. One of these three screws adjusts the front bank of two carbs to the rear bank of two. There is no "master" carburetor.

 

A Honda GL1000 carb set. Again, just as with the V4, one of these three sync screws is a bank adjuster. It syncs the left two carbs with the right two. No master carb here either. If these carbs were rearranged from their square configuration and made inline, you would have the exact same setup as in a later Honda inline four, No difference.

A CBX1000 carb set. Note there are just five sync adjusters for the six carburetors. This in no way illustrates or supports the idea of a master carburetor. Screws 1/2, 2/3, 4/5 and 5/6 balance their carb twins, and screw 3/4, by virtue of it's being in the center, balances the left three carbs with the right three. No master carburetor.

A Kawasaki GPZ1100 carb set. Three sync adjusters, again one fewer than there are carburetors. But here note something really special. The factory says the center adjuster adjusts the two banks of two to each other! Just as I have been saying! No master carb here. From literature dispensed at factory Kawasaki tech training school.


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