Carburettor Deterioration
Symptoms
Poor idle, flat spots during acceleration, fuel leaks, difficulty starting, rich or lean running.
Cause
The Cosmo Sport uses a Hitachi carburettor specific to the model. After 50+ years, every gasket, O-ring, needle valve, and diaphragm in the carburettor has deteriorated. The float may be saturated with fuel (if brass, it can develop pinholes; if plastic, it absorbs fuel and sinks). Passages clog with varnished fuel residue.
Fix
Full carburettor rebuild with a complete gasket and seal kit.
Kits are available from Japanese specialists, though availability fluctuates.
An ultrasonic clean of the carburettor body is ideal.
Ensure the float level is set correctly after rebuild.
Replacement carburettors are effectively unobtainable, rebuild what you have.
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