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Fuel System Basics

Whether idling through the no-wake zone or cruising across the bay at full throttle, any engine relies on a steady stream of clean fuel. Selecting the appropriate fuel components is easy when armed with a little knowledge of the engine requirements. Engine size mainly determines the size of fuel components required. Well designed fuel systems are engineered to deliver sufficient fuel flow through the spectrum of engine speeds. Improper components may reduce fuel flow, and like a dam in a river, everything down stream becomes fuel starved, resulting in a rough running engine. Engines excel with quality fuel components like those made by Sierra, Racor and Attwood.

Crucial to fuel flow is a well maintained fuel filtration system. Properly sized filters should extract contaminants without restricting flow. Marine systems have two stages of filtration, the primary or pre-fuel filter and the secondary, or engine filter. The primary filter is first in line. Its task is to extract water and large particle contaminants, and is therefore serviced more often. Primary filter systems offer dramatically improved protection over a single filter system alone. If your inboard or outboard does not currently incorporate a primary filter, an aftermarket model can easily be installed. Be sure to select a primary filter (also called a fuel water separating filter) that is sufficient to handle the engine flow rate at full throttle. Racor and Sierra each offer a range of easily installed primary fuel/ water separating filters. Next in line are Secondary filters, typically located just before the fuel pump, carburetor or injectors. Secondary filters function as a last chance trap to rid fuel of grit and debris, and are usually specific to OM (Original Manufacturer) and engine model.

To fairly evaluate filter performance, consider not only the smallest particle size a filter extracts, but also the efficiency at which it filters. For example, take two 10 micron filters. Each may filter out 90 percent of all particles 10 micron or larger at half throttle, but when fuel flow accelerates, a high quality filter maintains efficiency while a lesser filter allows far more contaminants to pass through. The efficiency also drops when filters are left in service beyond their usable life. Spent filters not only lose the protective filtration features but also dam the river, starving the engine of fuel. A vacuum gauge installed on the filter will detect the pressure drop caused by a clogged filter.

To maintain a solid stream of fuel minus air and obstructions, use marine grade fuel line components. AWAB clamps are the best choice for complete clamping force to secure fuel hose to barb fittings. A vibration resistant screw and a non-perforated inner surface with rounded edges help this clamp achieve a hose-friendly leak-free seal. Use these clamps in conjunction with brass fuel line fittings to connect tank vents, fuel fill hose, deck fill plates, and fuel feed lines. Any new installation must include a fuel line shut off valve. To achieve maximum performance at higher speeds or engine loads, all fittings selected should maintain adequate ID (internal diameter) throughout the fuel line. For outboard systems, Sierra offers complete replacement fuel lines to fit Mercury, Johnson, Evinrude, Honda and a host of others. They also offer replacement bulbs and connectors to keep outboard fuel lines primed and running.