Outboard Fuel & Fuel Systems FAQs

How can I improve the performance of my two-stroke outboard?

Rejuvenate is a multi-purpose clean-up oil that is intended for use in older 2-cycle engines. Applications include lawnmowers, PWCs, snowmobiles and motorcycles - along with EFI and carbureted outboard engines. Rejuvenate was originally developed for U.S. military applications. It is designed to withstand variances in fuel and oil quality, while providing enhanced lubrication to critical engine components. Carbon is a natural by-product of the combustion process. Excessive build-up is caused by left-over fuel and oil contaminants that have not burned away - and that carbon build-up reduces the engine's performance over time. Rejuvenate not only prevents the formation of carbon build-up, it removes existing build-up. Rejuvenate offers superior engine-cleaning capabilities compared to other premium oils and engine-cleaning fuel additives combined.

What is the advantage of the OptiMax/DFI Synthetic Blend 2-Cycle Outboard Oil?

OptiMax and other direct-injected (DI) engines put increased demands on lubricants than carbureted and electronic fuel-injected (EFI) engines. Oils used in traditional two-stroke engines don’t provide the proper protection for the higher operating temperatures and reduced oiling rates of DI engines. Mercury’s OptiMax/DFI oil has additional additives that provide the extra protection needed to maintain peak engine performance and reliability. Simply put, your engine will run longer and stronger with Mercury OptiMax/DFI 2-Cycle Outboard oil.

My engine keeps stalling at idle, and I find the primer bulb soft. Why?

Several basic items should be checked. First, is the fuel tank full of fuel and vent open? Second, is the fuel line pinched from a heavy object sitting on the line, or is a tight connection restricting flow? Are there cracks in the line or connections, allowing air to leak into the fuel system? Finally, due to the changes in fuel composition, have your local Mercury authorized dealer inspect the fuel pump check valves for cupping and replace the primer bulb with the latest style.

What octane rating should the gasoline that I use in my motor have?

United States and Canada
Use a major brand of automotive unleaded gasoline with a minimum-posted octane rating of 87. Leaded gasoline is not recommended. Performance Products will have different requirements, consult your Operation, Maintenance & Warranty manual.

What can be done to maintain fuel quality during periods of storage?

Mercury Marine recommends the use of a fuel stabilizer to prevent fresh fuel from undergoing degradation and oxidation during fuel storage. Mercury offers Marine Fuel System Treatment and Stabilizer both under the Mercury Precision Parts or Quicksilver brands. Additional benefits of the product includes: easier engine starting after periods of storage, elimination of the necessity to drain and dispose of fuel, prevention of the formation of deposits that could clog filters and tiny passages in the fuel system; and elimination of corrosion and rust throughout the fuel system. It absorbs water and prevents fuel line freeze-ups.

How can I lower the possibility of water trapped inside the fuel system being drawn into the engine?

The first issue is to prevent the water from entering the system. The second is to determine how much is in the system; if over a quart, you cannot effectively eliminate the possibility without draining the tanks.

If the amount is small, Mercury offers Marine Dri-fuel both under the Mercury Precision Parts or Quicksilver brands. Dri-fuel and Marine Fuel System Treatment and Stabilizer are designed to absorb water, allowing it to pass through the engine.

Mercury Precision Parts offers other alternatives, water separating fuel filters and a Water Eliminator. The Water Eliminator uses polymer crystals encased in a plastic filter to absorb water from the fuel in the tank. It is supplied with a 2 ft. (610 mm) lanyard and is designed to drop into the fuel tank through the fill. You will need to check to ensure that your fill hose routing is straight; excessive bends will not allow the Water Eliminator to reach the bottom of the fuel tank.

How do I connect my kicker motor to the boat's main fuel tank?

First you must determine if both engines require the same type of fuel supply. Generally all four-cycle and larger two-cycle (oil injected) outboard engines require straight gasoline. Smaller two-stroke outboards may require a gas/oil mixture and will experience engine damage if operated on the straight gasoline used with larger engines. In these applications, two separate fuel tanks are required, one for straight gasoline and one for the gas/oil mix.

Two preferred methods are available when both engines require the same type of fuel. First, use a fuel tank with dual pick-ups, one for the main engine and one for the kicker engine. Both engines will have separate fuel hoses and primer bulbs. The second method requires a 2-way fuel valve, located between the primer bulb and engines. The fuel line from both engines is connected to the fuel valve, which is rotated to deliver fuel to the appropriate engine.

How can I calibrate a SmartCraft fuel tank?

There are three ways to calibrate a SmartCraft fuel tank.

1) Do nothing (the tank will show in percent)

2) Press "Default" (which will divide the tank amount into four equal quarters)

3) Add fuel method

Please refer to your SmartCraft owner's manual or contact your local Mercury Marine dealer for additional information.

What alternative/equivalent 2-cycle oils are available when I can't purchase Mercury Marine Precision Lubricants or Quicksilver brands of oil?

If Quicksilver or Mercury Precision Lubricants outboard oil is not available, substitute another brand of 2-cycle outboard oil that is NMMA Certified TC-W3. The use of an inferior 2-cycle outboard oil can reduce engine durability. Damage from use of inferior oil may not be covered under the limited warranty. Always consult your Operation, Maintenance & Warranty manual for additional information.

What alternative/equivalent 4-cycle oils are available when I can't purchase Mercury Marine Precision Lubricants or Quicksilver brands of oil?

If Quicksilver or Mercury Precision Lubricants 4-cycle outboard oil is not available, use a premium quality 4-cycle engine oil, certified to meet or exceed any one or a combination of the following American Petroleum Institute (API) Service Classification SH, SG, SF, CF-4, CE, CD, CDII. Always consult your Operation, Maintenance & Warranty manual for additional information concerning viscosity requirements and temperature ranges.

What type of fluid is used in my power trim system?

We recommend either Mercury Precision Parts or Quicksilver Power Trim and Steering Fluid. This fluid can be used in any of our power trim systems. Acceptable alternatives will depend upon the type and age of the system. If the fluid is yellow, use automotive engine oil -- the heavier weight oil will cause the trim system to react slower in cold water. If the fluid is red or clear in color, use Dextron III automotive transmission fluid.

Do I need to use a fuel demand valve?

Yes, when the tank is pressurized.

How will I know if I have a pressurized fuel system?

It will be indicated by the tank cap (will ratchet similar to automotive). 

Does the fuel demand valve need to be in that position in the fuel line assembly?

No, it can be anywhere as long as it's between the fuel tank and the primer bulb in the fuel line assembly.

If I want to fill the tank, what should I do if I notice the tank is under pressure?

Rotate the fuel cap 1/4 turn to release pressure (see the four-step process above or refer to your Operation, Maintenance and Warranty manual).

Does the fuel demand valve have to be in a certain position (i.e. always facing up) to work?

It will function in any position.

What is the purpose of the holes in the top of the fuel demand valve?

The purpose is for venting the back side of the valve and water drainage away from the fuel demand valve diaphragm; the valve can be opened by depressing valve mechanism through slot on cover (manual over ride).

Is it ok to use 10% ethanol fuel with the fuel demand valve?