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Found this online, hope it helps

Here is the procedure, step by step!

First you need to remove the air cleaners, and make sure there are no vacuum leaks (spray starting fluid around the carburetor bases and intake manifolds while the engine is idling. Make sure the engine's ignition timing, valve adjustments, and other items are all correct also.

You want to do one thing at a time, right? Make sure you end up adjusting the carburetors to a TUNED engine, not one that has problems.

There are two easy ways to synchronize dual carburetors, and the method is the same, but the tool used is different. One tool is called a Uni-syn (or similar), which uses a ball or "piston" in a glass/plastic tube. The piston moves higher in the glass if MORE air is moving through the carburetor (when the Uni-syn is placed on top of the carburetor throat, with NO air bypassing the Uni-syn). Remember, no vacuum leaks, and the Uni-syn must seal tight on the top of the carburetor or velocity stack). If you have Kadron carburetors, the air cleaner stud gets right in the way of using the Uni-syn, so you have to use a section of tube (I use a cut off Pringles' can) to space the Uni-syn above the top of the air cleaner stud.

The other method I use is to get a length of 1/2" hose (or so), and place it sideways over the throat of the carburetor, and carefully listen to the other end. WARNING! *MAKE SURE THIS ENGINE WON'T BACKFIRE OR YOU WILL BE DEAF IN NO TIME*

Warm up the engine (normal operating temp), and then disconnect the carburetor linkage from one or both carburetors (I do both). Now, when you put the Uni-syn on the top of the carburetor (one throat), you need to rotate the disc (that is on the threaded piece) up or down to center the piston in the glass. It doesn't matter how much air is moving through the carburetor now, as long as the engine's idle speed doesn't change when you put the Uni-syn on the carburetor. If the idle speed changes, either the Uni-syn's opening is practically closed, or that cylinder has a vacuum leak which you missed! Now, go ahead and center the piston (measuring cylinder #2), then measure #4. (You don't have to check #1 and #3 since they are on a common shaft, unless you took them ALL THE WAY APART). The piston will be higher or lower. If it's higher, the second carburetor you are measuring is allowing MORE air into the engine than the first one is, and if it's lower, the opposite is true. With the "hose method", you just need to listen for the pitch of sound you hear (you want them to sound the same)!

Next you adjust the little set-screw which is the throttle stop, and which is the idle speed adjustment on these carburetors. Turn the screw in (clockwise) to flow more air, and out to flow less air. You want to match the intake airflow from side to side on the engine.

Now, here's my little trick. If you want a slower idle speed, screw the "more airflow" carburetor adjusting screw out, decreasing the flow, and slowing the idle while evening out the two carburetors. If you want a faster idle while evening out the two carburetors, screw the idle screw in on the "slower" carburetor (speeding it up). If the idle is already OK, move one out a little, and the other in a little! Just do this a few times, and re-measure (and calibrate your Unisyn) each time until the airflow is equal and the idle speed is OK. After you synchronize a few sets of carburetors, you will be able to do it really quickly and look like a Pro! In fact, the most time consuming part of adjusting a set of dual carburetors is removing and reinstalling the linkage and air cleaners!

After this is done, you need to connect your linkage up (a little tricky!) so it DOES NOT CHANGE THE THROTTLE POSITION ON THE CARBURETORS (which you just spent time measuring and adjusting). Usually, the linkage will be off on one or both sides. This is what the threaded rods are used for. Loosen the locknuts on the linkage (leave the ball-joints connected at either end), and thread the rod one way or the other. Sometimes the rods and ball joints are left-hand threaded on one end, so that when you turn it, the entire linkage rod gets longer when it's rotated one way, and shorter the other. Make sure you don't thread one side too far out (it will fall out), or too far in. Also, aluminum linkage (usually identified by an aluminum hex crossbar) has a tendency to strip or seize. If you can move the adjustable rods, get some grease or never seize on it ASAP!

Now comes a part of synchronization that is usually overlooked. You want both carburetors to have identical throttle positions at all points (NOT just idle). You also need both carburetor adjustments to be unaffected when sitting at idle position (same carburetor position with linkage attached as when it was disconnected).

Next you need to adjust your idle mixture. The adjustment screws are located at the base of the carburetor, and usually on the outside (Weber or Dellorto). Back each of them out 3-4 turns. With the engine fully warmed up and idling, slowly turn them in (do one at a time), and CAREFULLY listen to the idle quality and speed. As you turn the screw in, you will hear the cylinder misfire (it's the one you are turning in, and it's running out of fuel). Once it misses, turn the screw back out until the cylinder fires properly again. Now, turn it out 1/2-1 turn more. You are done with that cylinder. Now, do the next cylinder, then the next, until you have done all the throats.

Of course, I am assuming the jetting is already in the ballpark for this application. If you Can not get the cylinder to lean misfire (miss), or can't STOP it from missing, there is an ignition or jetting problem that needs to be addressed before you synchronize your carburetors. If this is the case, start following the carburetor jetting procedure (another article) before you attack the idle adjustments (don't waste your time on fine tuning until your main jetting is close). After the jetting is correct, double check your synchronization, then finish with the mixture screws.

The procedure I have outlined here all dual carburetors, even dual single bbls (except you only have one mixture screw per side, since one barrel feeds TWO cylinders). Other important things to check are that the linkage pieces "match" from side to side, and from carburetor to carburetor. The linkage has to keep the carburetors at the same adjustment at ALL throttle positions, not just idle! Part of this is that the carburetors have to open at the same rate, and if one side opens faster than the other, the car may run fine at idle, but hesitate and miss when on the road. Make sure the linkage is symmetrical. Also, you need the vertical throttle rods (If it's crossbar linkage) to be matched in their pitch from vertical. This ensures that the two carburetors open at the same rate, since the crossbar is rotating the same on both sides.

Sometimes you have to add/remove washers from various ball-joints on the linkage to adjust the threaded rods so they are at the same angle on both sides of the engine, but the engine will run SO much better, and it's amazing how many "experts" miss this important detail.

Another dual carburetor tip: It would be good if your carburetors have a throttle STOP (full throttle) also, so the butterfly's or carburetor arm's do not get bent if they are opened too far. Make sure you have full throttle at the carburetors when you have the gas pedal to the floor, without bending or stressing anything. You do not want to bend anything; those carburetor parts are expensive!

One final tip: I tend to go on the small size for carburetors, since the engine makes more USABLE power throughout the rpm band. The engine may make less peak power, but you will out accelerate the same engine with larger carburetors (you have more power across the rpm band)! I can only say to trust my experience.

I hope this article has cleared up the "voodoo" that surrounds proper dual carburetor adjustment. Many people do not want you to know how simple it is, and others are afraid of purchasing dual carburetors for their car for fear that they need constant adjustment. This is simply false: the carburetors do not lose their adjustment. For this to happen the screws would have to MOVE or get clogged with dirt, varnish, etc. What does happen though, is that the engine changes, and carburetor adjusting is needed to get back to a perfect setting! Adjusting dual carburetors is just common sense once you know the principle behind it.
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