The Carburetor Shop LLC

204 East 15th Street

Eldon, Missouri 65026


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 Selection and synchonization of triple carbs on a six cylinder inline engine




Since we receive more inquiries concerning 235 Chevrolets, we are using this engine as our example; but the selection process and the synchonization process are independent of make.


For best results, the very best thing is to do one’s homework BEFORE one purchases anything. A triple setup MAY have clearance issues. There are several major criteria in the selection of one’s puchase, but the first criteria is how the unit will be used. Consider that 50~60 years ago, triple carb manifolds were sold for RACING! Today, while a very small percentage will submit their vehicles to the race track, most are interested in triple carbs for either LOOKS ONLY, or looks with a modest increase in performance. These two uses have vastly different requirements in carburetors.


Typically, the manifold sold in the period would be drilled for three carbs the same size as the original (a 216 Chevrolet would have 3 S. A. E. size 2 carbs, whereas a 235 Chevrolet would be drilled for 3 S.A.E. size 3 carbs). By using this arrangement, the user could order two addition original carburetors from the dealer to go with original carb on the engine. At the same time, the user would install a full-race cam, a magneto ignition, headers, high-compression pistons; all to go with a numerical higher rear-end ratio BECAUSE THE ENGINE WOULD HAVE AT BEST A VERY HIGH IDLE, AND VERY LITTLE LOW END TORQUE. This is a good selection for a trailered race car, but a poor selection for the “cruiser” on the street.


The second criteria is to decide if solid linkage, or progressive linkage will be used. We highly recommend solid linkage unless this is strictly a “show” item, and the outer 2 carburetors will be “dummys” (in place for “looks”, but not used). The reasoning for this recommendation is really quite simple. If one uses progressive linkage, then the center or primary carburetor must be of sufficient size to idle well, and run the engine well at normal cruise RPM. Since the manifold is designed for three carburetors, the runners to the outer cylinders will not deliver the same A/F ratio as to the center. To prevent a lean A/F ratio on the outer cylinders, the primary carb will have to be artificially rich, thus reducing fuel mileage. For performance work, it is very difficult to get the end carbs to come in without a lag of a third to half a second. If the reduced fuel economy and the lag are acceptable to the enthusiast, then progressive linkage may be used. Carburetor selection for progressive linkage is easy. Simply use 3 of the original carbs for the engine. I am not saying that progressive linkage won’t work, as many use it. But it cannot work as well as solid linkage. Did you ever see an XK-E or Healey 3000 with progressive linkage? Or the triple YH setup used on the early ‘Vettes?


Since the manifolds of today are basically copies of the manifolds of yesteryear, generally the street enthusiast will need to modify the manifold for smaller carburetors. Example: for a stock or modestly built Chevrolet 235, an enthusiast might choose 3 original Carter W-1 carburetors as used on the Chevrolet 216. These will be marginally on the large size, but can be dialed in without too much effort. A better choice (but possibly more difficult to find) would be the Carter WA-1 carburetors used on the small (202 CID) Husdon in the early 1950s. Two were used on the Hudson; three work well on the 235 Chevrolet. Still better, but much more expensive, would be the carbs used on the Willys 134 CID 4 cylinder. Carter W-0 and Carter YF carburetors were used. The key in selection is to choose carburetors that are individually too small for the engine. The manifold would be drilled for size 3 carbs (2 15/16 inch mounting pattern), but the 216 carbs and the Hudson carbs are size 2 (2 11/16 inch mounting pattern). There are three methods of doing the conversion: (A) use size 3 to size 2 adapters (extra cost, and looks “hokey”); (B) slot the holes in the carburetor (no additional cost, but looks even more hokey); or (C) have a good welder fill the original holes in the manifold, and then drill and tap for the correct size (extra cost, but looks much more like a factory installation, and weren’t looks the major criteria?).


If you plan to use on a trailered racecar, use the larger size.


The third criteria for selecting carburetors is that the EASIEST AND LEAST EXPENSIVE good results will be obtained by selecting either (A) carburetors which were originally designed for used in a multicarb environment (example: Carter WA-1’s used on the Twin-H Hudson) or (B) carburetors with a mechanical (as opposed to vacuum) power system. Carburetors with a vacuum power system may be used, but the tuning MAY be more difficult. If you have a spring winding kit and feel comfortable hand winding small precision springs, you may ignore this paragraph. The Carter W-1 units have a mechanical power system, and the Hudson WA-1’s were designed to run in a multiple carb environment.


The next criteria is the one that gets the most arguments – the brand of carburetor to use. After almost 60 years of working on carburetors, we recommend Carters for most applications (readily available, reasonably priced, and parts readily available). For the high end user to whom cost is not an object, but perfection is the goal, we suggest either Strombergs are Zeniths. Other brands may be used, but there are no others that we recommend. In any event, one should choose a brand with which the user or the mechanic of the user is extremely familiar. In any event, ALWAYS use matched carburetors by tag numbers. Trying to use non-matched carburetors is simply asking for trouble.


If you start with used cores, acquire good rebuilding kits (we would suggest the ones we make); and rebuild the carburetors to absolutely stock calibration. By building to the stock calibration, you build a repeatable “baseline”. Calibration adjustments, if necessary, may be made later.


One issue that MIGHT be of concern is manifold type. The types are the common plenum type, and the individual runner (IR) type. The IR type will require SIGNIFICANTLY LARGER carburetors than the plenum type for the same engine.


Once the carbs are rebuilt (or new carbs), one is ready for the installation and synchonization procedure, which is independant of brand, type, and size. Solid linkage will be required (unless you have some funky home-made manifold that has both carbs feeding into a centrally located plenum). Progressive linkage is not an option.


Synchonization (solid linkage)

(A) Make certain that you have three IDENTICAL carburetors (check that the tag numbers are exactly the same, except for the production date), and that the carburetors being used are the approximate proper size for your application. If this condition is not true, stop reading and start looking!

(B) Screw the idle mixture control screws in on all three carbs until they lightly "bottom". Check the carburetor manufacturer's spec for idle adjustment, and set all three screws in the middle of the range. Example: if the spec is 1 to 2 turns, then use 1 1/2 turns.

(C) Set the throttle positioner screws higher than normal (so the engine will start and run at a high idle).

(D) Install all three carburetors but NO linkage.

(E) Acquire a manometer. Uni-syn is a brand name that is readily available. Try the local motorcycle shop.

(F) Start the engine, and run at a high idle until the engine is at normal operating temperature, and that the chokes (if used) are completely off on all three carburetors.

(G) Reduce the setting on the throttle positioner screws approximately an 1/8 of a turn at a time on each carb until the idle approximates desired RPM.

(D) Set the Uni-syn on the center carburetor. Adjust the control knob such that the plastic bobber is directly in the center of the column.

(E) Remove and replace the Uni-syn and verify that the engine does NOT change RPM with the addition/subtraction of the Uni-syn.

(F) Move the Uni-syn to either of the end carburetors (it makes no difference which). Adjust the throttle positioner screw of the second carburetor such that the plastic bobber is directly in the center of the column. Do the same for the other end carburetor. When the carbs are synchronized, one should be able to move the Uni-syn from carb to carb with no change in RPM and no deflection in the position of the plastic bobber.

(G) If the idle is too high, again adjust the throttle positioner screws on both carbs to the desired RPM, and repeat the synchronization process.

(H) Install the linkage to all three carburetors. Make certain there is NO movement of the throttles.

(I) Install the linkage from the footfeed. Again make certain there is NO movement of the throttles.

(J) With the throttle linkage installed, verify the synchronization. If adjustment is needed, remove the linkage, and start over.

(K) Once synchronization is complete with throttle linkage in place switch off the engine and have an associate move the footfeed inside the passenger compartment to WOT, while you visually ensure that the carburetor throttles also move to WOT.

(L) Its play time!