SU & Zenith-Stromberg Fuel and Carburetor Symptoms & Diagnosis : Moss Motors

Before adjusting the carburetors, verify that the following items have been checked for proper operation. Many suspected carburetion issues are actually caused by faults within the ignition system, making this the first place to look before making any carburetor adjustments. Random carburetor adjustments are useless and potentially harmful to the engine’s operation and should never be attempted.

Check these systems first

IGNITION SYSTEM

Check the condition and gap of the contact breaker points.
Check the tension of the contact breaker spring.
Check the condition of the spark plugs and verify that the gaps are correct.
Check the condition and integrity of the High Tension and Low Tension leads.
Check the ignition timing. Only small variations from the correct static timing are permissible.
Ensure that the centrifugal and vacuum advance mechanism is working correctly.

FUEL SYSTEM

Ensure that an adequate fuel supply is being delivered to the float chamber.
Examine the inlet manifold and carburetor flange for air leaks.
Ensure that the air filters are clean and allowing air to flow into the carburetor.
Check that the fuel pump output pressure is correct.

COMPRESSION

Check that the valve clearances are properly adjusted.
Verify the compression in each cylinder.
Ensure that the valves open freely and do not stick.

EXHAUST SYSTEM

Check that the exhaust system has not become blocked or damaged.

Fuel & Carburetor Symptoms and Diagnosis

LEAKING FUEL

Dirty or worn needle valve in float chamber
Punctured float
Blocked vent tube
Fuel lines broken
Excessive fuel level in float chamber
Defective gaskets

COLD STARTING DIFFICULTIES

No fuel in tank
Fuel pump fault
Metering needle sticks
Ignition and engine problems

HOT STARTING DIFFICULTIES

Fuel percolation in float chamber
Jet fouled
Idle mixture incorrectly adjusted
Idle speed is too low
Flooded inlet manifold
Ignition and engine problems

ERRATIC IDLE

Sticking piston
Dirty piston or suction chamber
Damaged metering needle
Idle mixture or idle speed incorrectly adjusted
Air leaks
Loose carburetor mounting
Sunken float
Ignition and engine problems

POOR ACCELERATION

Dirty piston or suction chamber
Piston is sticking
Bent metering needle
Jet is not centered
Perforated or hardened diaphragm (Z-S)
Dashpot oil level is low
Dashpot oil is too thin
Ignition and engine problems

ENGINE LACKS POWER

Dirty piston and suction chamber
Piston is sticking
Bent metering needle
Jet is not centered
Perforated or hardened diaphragm (Z-S)
Dashpot oil level is low
Dashpot oil is too thin
Ignition and engine problems
Fuel contaminated
Throttle not opening fully
Inadequate fuel delivery
Excessive ambient heat around inlet manifold
Ignition problems

OVERHEATING

Mixture of air and fuel is too lean
Inadequate fuel delivery
Inadequate fuel octane rating
Ignition, cooling and engine problems

PINGING

Inadequate fuel octane rating
Mixture of air and fuel is too lean
Excessive spark advance

EXCESSIVE FUEL CONSUMPTION

Loose jet
Dirty piston or suction chamber
Perforated or damaged diaphragm (Z-S)
Fuel level is too high
Clogged air filter
Fuel pressure is too high
Ignition and engine problems

MIXTURE IS TOO LEAN

Jet is fouled with dirt or varnish
Damaged metering needle
Air leaks
Weak fuel pump

CARBURETOR BACKFIRES

Mixture is too lean
Vacuum leaks
Too little oil in dashpot
Spark plugs are too hot
Excessively retarded ignition timing
Sticking intake valve
Broken valve spring

VARIABLE CHOKE CARBURETOR FAULTS