How to Sync Dual Carburetorsby Michelle Schaefer
The procedure of synchronizing carburetors is relatively easy once the basic principle behind it is understood. In a dual-carburetor engine, both carburetors must be set to the same idle specifications and must open at the same rate. If one carburetor opens faster than the other, the engine may experience poor power or hesitation. The process of synchronization is a mechanical process that ensures not only that both carburetors open at the same rate, but also that they produce equal power.
Remove the air cleaner or air cleaners and set them aside. Start the engine and bring it to normal operating temperature. Check for vacuum leaks at the carburetors by spraying starting fluid around the base of the carburetors. A change in engine operation or idle speed when you spray the fluid indicates a vacuum leak. Repair any vacuum leaks before proceeding with carburetor synchronization. A vacuum leak will make it almost impossible to properly synchronize the carburetors.
Turn off the engine and disconnect the carburetor linkage from both carburetors. Most dual-carburetor engines use a 1/2-inch nut on a ball joint to attach the linkage to the carburetor throttle bar. Place the Dvorak Multiple Carburetor Synchronizer on the air inlet of one of the carburetors. Ensure the synchronizer float glass is vertical and the float ball is free to move. Restart the engine and allow it to idle. Turn the air adjustment screw of the synchronizer so that the float ball is centered exactly halfway up the glass.
Move the synchronizer to the other carburetor and place it on the air inlet. Adjust the carburetor until the float ball is in the exact same position as on the first carburetor by turning the carburetor throttle stop set-screw clockwise to increase the vacuum reading or counterclockwise to decrease the vacuum reading. Move the synchronizer back and forth between the two carburetors and adjust each one until the float ball reading is exactly the same on each.
Lower the idle speed of the engine by reducing the higher of the two carburetors to match the lower one. Increase the idle speed of the engine by increasing the lower-reading carburetor to match the higher-reading one. If the idle speed is correct but the carburetor synchronizer float readings do not match, turn one carburetor adjusting screw clockwise while turning the other carburetor's adjusting screw counterclockwise by the same amount.
Reconnect the throttle linkage with the engine still running. Loosen the lock-nuts on the ball joints of the throttle linkage. Turn the connecting rod in or out until the ball joint end fits into each carburetor's throttle lever without binding or changing the idle speed. Adjust both ends of the linkage and tighten down the throttle linkage lock nuts and throttle-to-carburetor linkage nuts. The throttle linkage must have a no-resistance slip-fit onto the carburetor throttle arms. Retest the carburetors with the synchronizer. There should be no change in the readings after installing the linkage.
Adjust the idle mixture by turning the idle mixture screws all the way in until they gently seat. Turn the screws counterclockwise three to four turns. Turn each screw clockwise an equal amount on each carburetor and note the engine idle speed. It is important that the mixture screw on each carburetor be turned an equal amount. Continue turning the screws clockwise about 1/4 turn at a time until a stumble in the idle is noticed. Back each screw out counterclockwise 1/4 turn to set the final adjustment.
Open and close the throttle to observe the linkage for symmetry of operation and for binding. Adjust the linkage by adding shims or by bending the rod if required. Observe the linkage at full and part-throttle openings to ensure equal operation of the throttle plates.
- Install fresh throttle-return springs on the carburetors. The carburetors must return instantly and positively back onto the idle set screws every time the throttle is released.
- When installing dual carburetors, use new base gaskets. Ensure the carburetors are pushed all the way to the rear of their mounting studs. This guarantees the carburetors are exactly equal on the manifold and that they cannot move out of relationship to each other.
- Engine exhaust fumes are toxic. Work only in a well-ventilated area.
- Use caution when working on a running motor. Keep hands, clothing and tools away from moving engine parts.
- A fender cover or old blanket will protect the finish of your car when working under the hood.
Items you will need
- Starting fluid
- Dvorak Multiple Carburetor Synchronizer
- Slotted (flat-head) screwdriver
- Combination wrench set
- "Motor's Auto Repair Manual 1968"; Motor; 1968
- car engine image by itsallgood from Fotolia.com