Small Engine Repair

Fuel Carburetor And Supply Know How

Fuel tank

Routinely examine the tank for leaks and for loose or stripped hold-down
bolts. Mysterious engine shutdowns that elude obvious explanation may be caused by a failed tank vent, which can be integral with the fuel cap.

Fuel Filters

Fuel filters should be changed at least every 100 hours of operation and
Fuel delivery, signaled by no fuel to the carburetor or by a bleached spark-plug has been compromised,
Fuel quality may also be suspect. Use a hooked wire to retrieve in-tank filters of the kind used on handheld tools. Replace deformed or rusted hose clamps.

Fuel Lines

plastic lines used on portable equipment to transfer fuel
and energize the fuel pump with crankcase pressure pulses. Heat and vibration do harden the plastic line, which then leaks fuel and, what is more, difficult to diagnose, air. Pulse lines are also susceptible to clogging. Mobile phones are great if a camera is on it, take digital photos of the fuel line routing before undertaking any repair work, check for leaks before starting the engine and after 30 minutes or so of hard use.

Fuel pump

Most handheld engines employ a fuel pump, mounted on the carburetor body. The pump element consists of a nitrile diaphragm, with flaps cut into it that act as inlet and output check valves. The cavity cast into the pump body is a variable-displacement fuel reservoir, whose volume changes as the diaphragm, which forms the floor of the cavity, flexes.
As the piston rises toward the top dead center (TDC), it leaves a partial vacuum behind it in the crankcase. This vacuum, transmitted to the outboard side of the diaphragm by a drilled passage in the carburetor body or by an external line, pulls the diaphragm down, away from the fuel cavity. Because of the increase in its volume, a slight vacuum is created in the cavity. Fuel, under atmospheric pressure in the tank, rushes in past the open inlet valve to fill the void. The pump is now charged.

Fuel Supply To Carburetor

A few milliseconds later, the piston rounds TDC. As it travels down the bore, it pressurizes the crankcase to 5 or 6 psi above atmospheric. The pressure forces the pump diaphragm upward into the cavity. The inlet check valve closes to prevent fuel from cycling back to the tank. At the same time, the discharge valve opens, admitting pressurized fuel into the carburetor.


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