Page 151 - REPOWER REFERENCE GUIDE (2020)

Fuel System
• The tank breather pipe must have an inner diameter of at least 13 mm (1/2 in.) and must be fitted with a swan neck to
prevent water from entering the tank.
It is recommended that the exact route and length of the fuel lines be established prior to installation of the engine to prevent
problems later in connecting them to the engine.
All fuel lines must be well secured. The holes where the lines run through the bulkheads should be carefully rounded off or
protected with rubber grommets. This prevents damage to the lines from abrasion.
The following additional fuel connection‑related points are among those that must be considered. They apply to all engines
unless otherwise noted. Refer to boating standards (NMMA, ABYC, etc.) and Coast Guard regulations for complete guidelines.
1. The fuel pickup should be at least 25 mm (1 in.) from the bottom of fuel tank to prevent picking up impurities.
2. The maximum measured vacuum at the engine's fuel inlet must not exceed 6.9 kPa (1 psi) at 600, 3000, full throttle RPM,
and idle RPM.
IMPORTANT: A vacuum reading higher than specified can cause vapor locking with some fuels. It can also cause poor
engine performance because of fuel starvation.
3. Fuel lines used must be Coast Guard approved (USCG Type A1).

Description Specification
Minimum fuel line diameter on single‑engine gasoline installations 10 mm (3/8 in.)
Minimum fuel line diameter on multiengine gasoline installations 13 mm (1/2 in.) ID or larger
4. On Multiengine gasoline installations: Use a fuel pickup and fuel tank supply line for each engine. If a single pickup and
fuel tank supply line is used, the line must be 13 mm (1/2 in.) ID or larger.
5. Larger diameter (than previously specified) lines and fittings must be used on installations requiring long lines or numerous
fittings.
6. Fuel lines should be installed free of stress and firmly secured to prevent vibration or chafing.
7. Sharp bends in fuel lines should be avoided.
8. A flexible fuel line must be used to connect fuel supply line to fuel inlet fitting on engine to absorb deflection when engine is
running.
Fuel Tank
• The fuel tank must be properly vented to allow easy filling of the tank and prevent a vacuum or pressure from being
generated during normal operation. ABYC standards recommend that the vent pipe and associated hardware be 11 mm
(7/16 in.) ID minimum.
• The vent and fuel fill systems must be carefully designed to inhibit water intrusion, a common source of engine problems.
Proper location of the fill and vent fittings and the use of a good sealing fill cap is critical. A goose‑neck in the vent hose is
also recommended to inhibit water entry.
• The fuel pickup tube and fittings must be at least as large as the fuel lines to minimize fuel restriction—the larger the better.
A filter or sock on the end of the tube is recommended to prevent debris at the bottom of the tank from clogging the other
fuel system components (especially the antisiphon valve). Tank connections must be a minimum of 10 mm (3/8 in.).
• A tank with baffles or a sump is recommended for applications that will be used for aggressive boat maneuvers (for
example, ski boats) to prevent fuel aeration and starvation in turns.
• On carbureted engines, the fuel tank should be mounted below carburetor level, or gravity may unseat the carburetor fuel
inlet needle and cause flooding.
• The fuel tank should be mounted as close to the level of the fuel inlet connection on the engine as possible. Excessive tank
height can cause difficulties in finding a suitable antisiphon valve. Conversely, the fuel tank distance below the fuel inlet
connection has a major effect on fuel system restriction and should be kept as close as possible to the inlet (see chart).
Fuel tank bottom versus fuel inlet height
(Measured from the bottom of the fuel tank to the height of the fuel inlet fitting)
Less than 50.8 cm (< than 20 in.) Good
50.8–101.6 cm (20–40 in.) Marginal
Greater than 101.6 cm (> 40 in.) Undesirable
• The amount of fuel tank surface area that is exposed to the engine compartment can have a significant effect on fuel
temperature. It may be necessary to insulate the tank if fuel inlet temperature is excessive.
• Contaminants can clog the fuel filters and cause poor performance and possible engine damage, particularly on fuel
injected models. The tank should be thoroughly flushed by the installer or tank supplier prior to use to avoid getting
contaminants in the engine fuel system. All fuel tank openings should be sealed until fuel connections are made, to avoid
getting debris (fiberglass dust, etc.) in the tank.

90-8M0149179 eng NOVEMBER 2018 © 2019 Mercury Marine Page 6B-3