VW Carb Tech 103: How to Synchronize Dual Carburetors
This article contains the step by step, detailed procedure for synchronizing dual carburetors on an aircooled VW engine. But before you actually tackle the syncronization, 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 do this to ensure that you are adjusting the carburetors to a TUNED engine, not one that has problems.
Carbs 103: How to Synchronize Dual Carburetors
There are two easy ways to synchronize dual carburetors, and the method is the same, but the tools used is different. One tool is called a Uni-sync (or similar), and the other is the “Snail Gauge” or “SK Tool“. The Uni-sync has been around for at least 50 years, the “Snail Gauge” is only 15 years old or so; it’s a more modern and more accurate tool in our opinion.
The Uni-sync 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-sync is placed on top of the carburetor throat, with NO air bypassing the Uni-sync). Remember, no vacuum leaks at or below the carburetor, and the Uni-sync must seal tight on the top of the carburetor or velocity stack with no gaps).
Another tool is the “Snail Gauge” or “SK Tool“, which is an airflow measuring device. The nickname “Snail Gauge” is because of it’s shape. The Snail Gauge measures the airflow into a carb barrel very precisely, and does so without restriction. If you have Kadron carburetors, the air cleaner stud gets right in the way of using the Uni-sync, so you have to use a Kadron Sync Adapter section of tube (I use a cut off Pringles’ can) to space the Uni-sync above the top of the air cleaner stud.
IMPORTANT! The Uni-sync is a restrictive device, it “Chokes” the carburetor. Because of this, the Uni-sync should NOT be used on Externally Vented carburetors, like the Weber IDF, Weber IDA, Dellorto DRLAs, Weber DCOE, etc. This is because when you put the tool on the top of the carb throat, it creates a high vacuum condition that sucks through the air circuits (which are NOT being choked), and this pulls a ton of fuel in that is normally not metered. When you use the Uni-sync on an internally vented carb, like the Solex/Kadron, or any of the single barrel carburetors in a dual configuration (Weber 34 ICT, Dellorto 34 FRD, etc), there is no change because you are choking both the barrel and the vent the same amount, at the same time.
The “Snail Gauge” or “SK Tool” is not restrictive, it measures air flow, so it can be used on both internally and externally vented carburetors. The Snail Gauge’s only requirement is that the gauge/carb connection must not leak air. This is the case with all carburetor sync tools. The Uni-sync sits flat on the TOP of the carburetor, the Snail Gauge inserts into the top. The Weber ICT has a non-round top, and requires an adapter to connect without leaks!
The third 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*. Tune for the “same sound” on each carb. Primitive, but works!
PROCEDURE: 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-sync on the carburetor. If the idle speed changes, either the Uni-sync’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. If you use the Snail Gauge just place it on the carb throat and read the #; a typical idle reading is 5.5-7 kg/min. If you use the “hose method”, you just need to listen for the pitch of sound you hear (you want them to sound the same)!
So 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 also evening out the two carburetors airflow values. 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 Syncronizing tool if needed) 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 lock nuts 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). I turn the engine off and activate the throttle by hand. Make 100% sure that both carburetors open at the same instant! If they do not, usually lenthening one rod or shortening the other, will accomplish this. At this point you have both carburetors flowing the same amount of air, and opening at the same time, so now you need to adjust your idle mixture.
The idle mixture 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. Play with it, and adjust to achieve maximum idle speed. The biggest mistake in this part of the adjustment process is not waiting 10-15 seconds after an adjustment of 1/4 turn at a time, for things to “stabilize”. Don’t rush it! At idle it takes a bit for fuel to get flowing, or stop/evaporate, don’t be in a hurry.
When you find maximum idle speed for that cylinder, I go 1/2 turn richer with Dellortos, and 1/4 turn richer with Webers. This will result in a more stable idle speed as temps and conditions change.
Since you are done with that cylinder, do the next cylinder, then the next, until you have done all the throats.
If you ever change the
Idle Speed
Idle Timing
Idle Jet
Sync
You must re-set the idle mixture screws, since engine vacuum conditions at idle have changed!
We are assuming the jetting is already in the ballpark for this application. If you can not get the cylinder to lean misfire (miss, turning the idle mixture screw in, leaning it out), 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 we 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!
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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 understand the principle behind it.
If you have any comments or questions, feel free to give us a shout. If we have missed something or we are not clear on a topic and you mention it (e-mail is best), we will edit this article incorporating your suggestions to make it even better. Let us know how things work out driveability and performance wise for you!
Thanks for reading! Aircooled.Net
Article Written 3/19/97
Updated 4/3/12