Rebuilt Carburetors

One Carburetor at a Time

Tuning Your SU Carburetor

Before adjusting your SU carburetor you should check the following first:

• Battery
• Coil
• Distributor
• Breaker Points
• Vacuum and Mechanical Advance
• Ignition Timing
• Spark Plugs and Gap
• Cylinder Compression
• Valve Clearance
• Eliminate air leaks in Inlet Manifold and Vacuum Hoses

Only adjust, or blame the carburetor for bad operation, only after doing the above items. Warm up the engine to it’s normal operation temperature.

Turn the engine off.

Unscrew the throttle adjusting screw until it is clear of it’s stop and the throttle is fully closed.

Set the throttle adjusting screw 1 1/2 turns open.

Mark for reassembly and remove the piston/suction chamber unit.

Disconnect the mixture control wire, i.e. the choke cable. Unscrew the jet adjusting nut (1) until the jet is flush with the bridge of the carburetor, or fully up if this position cannot be obtained.

Replace the piston/suction chamber unit as marked. Check that the piston falls freely onto the bridge when the lifting pin (6) is released. If it does not do this, then the jet needs centering. Turn down the jet adjusting nut (1) by two complete turns.

Restart the engine and adjust the throttle adjusting screw (5) to give the desired idling as indicated by the rev counter or by the ignition warning glow.

Turn the jet adjusting nut (1) up to weaken or down to richen until the fastest idling speed consistent with even running is obtained.

Readjust the throttle adjusting screw (5) to give the correct idling if necessary.

The graph Fig. 9 illustrates the effect on engine rpm when the lifting pin raises the piston, indicating the mixture strength. If the mixture is excessively rich, the rpm increases considerably. With the correct mixture, the rpm increases very slightly, and with a weak mixture, the rpm will drop and the engine will stall.

An audible and visual indication of mixture strength may be observed from the exhaust as follows:

Too Weak - Irregular note, splashy, misfire, colourless.

Too Rich - Regular or rhythmical misfire, blackish.

Correct - Regular and even note.

Readjust the mixture strength as necessary.

Reconnect the mixture control wire with about 1/16 in (1.6mm) free movement before it starts to pull on the jet lever.

Pull the mixture control knob until the linkage is about to move the carburetor jet and adjust the fast idle screw to give an engine speed of about 1,000 rpm when hot.

Finally top up the piston damper with the recommended engine oil until the level is 1/2 in. below the top of the hollow rod.

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Marine Carburetors

Rebuilt Carburetors

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Mike’s Carburetor Restoration

5733 188th AVE SW
Rochester, WA 98579
360 347-1077