Gasoline fuel flow is simply engine fuel
consumption. The rule of thumb for flow is
gpm =max. engine hp ÷600, or L/min. =max.
engine kW ÷ 118. So a 140 hp (104 kW) gas
engine would require a 0.23 gpm (0.88 L/min.)
filter (14 gph or 55 L/hr.). These fuel-demand
flow estimates are usually generous, but the
engine manual is the final authority; don’t fail
to consult it. (Note that some fuel-injected
gas engines have a return line just as diesel
systems do. These engines will have higher
fuel flow rates and will require diesel-like re-
turn piping.)
NOTE: All gallons are U.S. gallons. To
convert U.S. gallons to imperial/UK gallons,
divide U.S. gallons by 1. 2. Gasolinein the
United States is usually termed petrolin the
United Kingdom.
To See Through or Not to
See Through
Fuel filters, of course, are really fuel filters
and water separators combined. To see the
water level that has accumulated—as well as
any sludge or sediment—fuel filters are avail-
able with clear glass or plastic bowls. On
diesel installations, these see-through bowls
are Coast Guard–approved only if they have a
metal flame shield around the bottom of the
bowl. (The shield enables the filter to pass
the required 2^1 / 2 -minute burn test.) Gasoline
inboards, however, cannot have any form of
clear bowl; only solid metal bowls will pass
muster. (See-through bowls are OK for out-
board gas engines, presumably because any
breakage would leak overboard.) Some clas-
sification society rules do not allow clear
bowls on either gas or diesel installations.
Such metal-bowl filters must be fitted with
a water-probe indicator/alarm and with a
vacuum gauge.
Filter Height and Location
It’s poor practice for a filter to be at the high-
est point in the fuel system or even above the
engine. If the filter is located high, air bubbles
tend to collect in it or fuel can drain out, leav-
ing an air bubble. Either way, this results in
frequent and annoying bleeding of the sys-
tem. If a filter must be located very high, then
at least be sure to install a solenoid shutoff
valve in the line to keep fuel from draining
out. Ideally, filters should be about level with
the fuel pump on the engine. The maximum
lift (vertical height) from the takeoff at the
tank to the inlet port on the engine is
48 inches (122 cm). Even that is a bit high.
Less is always better. Lift heights approach-
ing or more than 48 inches (122 cm) require
installation of a booster pump.
Whenever the filter is above the tank
level, a hand wobble pump (or similar)
should be installed in the feed line so that the
filter and the entire feed line can be purged of
air and filled with a solid slug of fuel.Fuel-Filter Micron Rating
(Sieve Fineness)
Fuel filter elements come in various mesh or
sieve sizes, or finenesses. The finer the fil-
ter, the finer the grit and sediment it will
remove. Fineness is measured in microns—
the nominal diameter of the minuscule open-
ings in the filter element. A micron is roughly
40 millionths of an inch (one thousandth of
a mm). The primary filter (the fuel/water
separator with filter element; see Figures 4-3
and 4-10) should usually be fitted with a
30-micron filter. This catches the larger par-
ticles of sediment. Some prefer a 10-micronChapter 4: Fuel Piping and Fuel System Bonding
Carburetors on Gasoline Engines
Under CFR Yacht and Commercial, each carburetor must leak no
more than 5 cm^3 (0.3 cu.in.) of fuel per second when- the float valve is open
- the carburetor is at half throttle
- the engine is cranked without starting
- the fuel pump is delivering the maximum pressure specified
by its manufacturer
Updraft and horizontal-draft carburetors must have a device
that - collects and holds fuel that flows out of the carburetor
venturi section toward the air intake - prevents collected fuel from being carried out of the car-
buretor assembly by the shock wave of a backfire or by re-
verse airflow - returns collected fuel to the engine induction system after
the engine starts