Engine System
24
Service Manual – PBU Propane Floor Burnisher
Fuel System
The PBU fuel system functionally consists of the propane tank, pressure regulator, carburetor, mechanical
governor, and operator throttle position selector The fuel (propane) is stored in the tank as a liquid, but
is dispensed into the fuel system as a gas The fuel pressure in the tank is much too high to be used by the
engine The pressure regulator reduces the pressure of the gas before it is delivered to the carburetor
The pressure regulator does more than just reduce the pressure, however It also meters the amount of fuel
being delivered to the engine based on the demand for fuel by the engine When the engine isn’t calling for
fuel, the diaphragm (valve) inside the pressure regulator stays closed, and no fuel is delivered When the
engine causes a small vacuum in the carburetor during the intake cycle, the diaphragm is opened and fuel
begins to flow. The amount of fuel flowing is proportional to the amount of vacuum from the carburetor.
In a sense, the pressure regulator is like a mechanical amplifier. A small amount of vacuum at the outlet is
able to control the large pressure at the inlet (the tank). The adjustments to the regulator fine tune the ratio
of this small vacuum to the control of the high pressure
The carburetor mixes the fuel and air in the proper proportion for
combustion by the engine A small metering jet in the carburetor
controls the ratio of fuel to air, and a butterfly valve controls the
total volume of fuel/air that is permitted to enter the engine The
position of the butterfly valve is controlled by the mechanical
governor
The governor is inside the engine and operates on centrifugal force
The faster the engine turns, the more the governor tries to close
the carburetor’s butterfly valve. Conversely, a spring connected to
the operator’s throttle position lever tries to pull the butterfly valve
open When these two forces are balanced, the engine will run at
constant speed even when the load on the engine changes When
the engine gets loaded, the speed drops and the governor doesn’t
pull the butterfly closed as hard, causing the valve to open more,
and more fuel/air to enter the engine
To prevent fuel from flowing to the engine when the engine isn’t running, there are two fuel lockouts. The
first is an electric valve that opens only when the ignition is turned on. The second is a vacuum lockout that
prevents fuel from flowing if there is no vacuum present at the carburetor.
Throttle Cable Position
The most common reasons for performing this adjustment are if the throttle cable has been replaced, or
if the cable clamp has loosened over time This procedure adjusts the effective length of the cable for full
throttle movement
1 Place the throttle selector into the Slow-position,
but do not force it if it doesn’t go all the way into
position
2 Loosen the cable clamping screw
3 Make sure that both the Throttle Selector
and the Throttle Link Arm are in their lowest
position, and then retighten the cable clamping
screw
4 Make sure the Throttle Selector can be moved
through its full range without causing the cable
to slip under the clamp
Butterfly
Valve
Metering
Jet
Vacuum
Lockout
Fuel
Inlet
Cable
Clamp
Throttle Position
Link Arm
Low Limit
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