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A method, fuel system, and components for facilitating the delivery of liquid fuel from a fuel tank in fluid communication with a float bowl carburetor of an internal combustion engine, wherein evaporative emissions of the fuel from the fuel tank and float bowl carburetor are mitigated. During opera

A method, fuel system, and components for facilitating the delivery of liquid fuel from a fuel tank in fluid communication with a float bowl carburetor of an internal combustion engine, wherein evaporative emissions of the fuel from the fuel tank and float bowl carburetor are mitigated. During operation of the engine, fluid communication is permitted between the fuel tank and the carburetor, and fuel vapors at a predetermined threshold superatmospheric pressure are permitted to vent outwardly from the fuel tank. During inoperation of the engine, fluid communication is prevented between the fuel tank and the carburetor, and fuel vapors at a predetermined threshold superatmospheric pressure are permitted to vent outwardly from the fuel tank.

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1. A fuel system for supplying fuel from a fuel tank to a carburetor for an engine having an intake manifold, comprising: a fuel filler cap for the fuel tank having a body removably engaged sealably to the tank, a pressure relief valve integrated into the body for relieving superatmospheric pressure

1. A fuel system for supplying fuel from a fuel tank to a carburetor for an engine having an intake manifold, comprising: a fuel filler cap for the fuel tank having a body removably engaged sealably to the tank, a pressure relief valve integrated into the body for relieving superatmospheric pressure from the tank, and a vacuum vent valve integrated into the body for relieving subatmospheric pressure from the tank;the fuel tank having a vapor dome and liquid fuel is gravity fed from the fuel tank to the carburetor;a biased closed pneumatic fuel shut-off valve orientated between the fuel tank and the carburetor, the shut-off valve having a housing, an inlet chamber defined by the housing and a cooperating inlet diaphragm, an outlet chamber defined by the housing and a cooperating outlet diaphragm, a dynamic chamber being defined axially between the cooperating inlet and outlet diaphragms and communicating with the intake manifold, a flexible third diaphragm disposed operatively in the dynamic chamber and being responsive to a vacuum pressure from the intake manifold, and a valve head engaged to the third diaphragm; anda conduit communicating between the inlet and outlet chambers which are exposed continuously to internal pressure of the fuel tank. 2. The fuel system set forth in claim 1 wherein the pressure relief valve is a diaphragm-type. 3. The fuel system set forth in claim 1 comprising: a cover engaged to the body of the filler cap;a resilient diaphragm of the pressure relief valve having a peripheral edge engaged sealably between the body and the cover, the diaphragm having a first side exposed to atmosphere and a second side exposed to a storage cavity of the tank when the body is engaged to the tank;a compression spring of the pressure relief valve engaged between the cover and the resilient diaphragm for biasing the pressure relief valve closed;a valve seat of the pressure relief valve carried by the body;a valve head coupled to the diaphragm and biased sealably against the valve seat by the compression spring; anda vent passage defined by the body, communicating between the storage cavity and atmosphere, and interposed by the valve head at the valve seat when the pressure relief valve is closed. 4. The fuel system set forth in claim 1 wherein the carburetor is a float bowl-type. 5. The fuel system set forth in claim 1 wherein the third diaphragm is a dividing diaphragm spaced axially between the inlet diaphragm and the outlet diaphragm. 6. The fuel system set forth in claim 5 comprising: a first side of the dividing diaphragm facing the inlet diaphragm and exposed to dynamic vacuum pressure of the intake manifold; andan opposite second side of the dividing diaphragm facing the outlet diaphragm and exposed to atmospheric pressure. 7. The fuel system set forth in claim 6 wherein the valve head is engaged to the inlet, dividing and outlet diaphragms for simultaneous motion responsive to the intake manifold pressure and non-responsive to the fuel tank pressure. 8. A fuel system for supplying fuel from a fuel tank to a carburetor of an engine comprising: a fuel filler cap for the fuel tank having a body removably engaged sealably to the tank, a pressure relief valve integrated into the body for relieving superatmospheric pressure from the tank, and a vacuum vent valve integrated into the body for relieving subatmospheric pressure from the tank;the fuel tank having a vapor dome and liquid fuel is fed by gravity from the fuel tank to the carburetor;a biased closed pneumatic fuel shut-off valve orientated between the fuel tank and the carburetor of the engine, the fuel shut-off valve having a housing, an inlet chamber defined by the housing and a cooperating inlet diaphragm, an outlet chamber defined by the housing and a cooperating outlet diaphragm, a dynamic chamber being defined axially between the cooperating inlet and outlet diaphragms and communicating with an intake manifold of the engine, a flexible third diaphragm disposed operatively in the dynamic chamber and being responsive to vacuum pressure from the intake manifold, and a valve head connected with the third diaphragm; anda conduit communicating between the inlet and outlet chambers which are exposed continuously to internal pressure of the fuel tank. 9. The fuel system set forth in claim 8 wherein the carburetor is a float bowl-type. 10. The fuel system set forth in claim 8 wherein the flexible third diaphragm is a dividing diaphragm spaced axially between the inlet diaphragm and the outlet diaphragm; a first side of the dividing diaphragm facing the inlet diaphragm and exposed to dynamic vacuum pressure of the intake manifold; andan opposite second side of the dividing diaphragm facing the outlet diaphragm and exposed to atmospheric pressure. 11. The fuel system set forth in claim 10 wherein the valve head is engaged to the inlet, dividing and outlet diaphragms for simultaneous motion responsive to the intake manifold pressure and non-responsive to the fuel tank pressure.