Boat Fuel: What You Need to Know

Unstabilized fuel is basically fuel as it comes out of the pump at the gas station. Unless you add a fuel stabilizer to it, fuel starts to deteriorate and lose its octane after about two weeks. Especially when exposed to the atmosphere or heat.

Carbureted fuel systems are vented to the atmosphere, leaving them susceptible to fuel degradation quicker than fuel injected systems. Fuel injected systems are pressurized, therefore closed to the atmosphere.

One of the biggest headaches for boaters and the marine industry in general is fuel problems. Fuel problems can be caused by a number of sources. 

I put them into 3 categories.

  • Unstabilized or stale fuel
  • Contaminated fuel: debris, water, etc.
  • Ethanol

Unstabilized fuel generally is not an issue with automobiles as they are usually driven on a regular basis. Boat motors are often inactive for weeks or even months at a time. Old degraded fuel smells a lot like varnish, and that is the term commonly used to describe the residue left in carburetors after all the volatile compounds have evaporated. If left sitting long enough, it will plug up every passage and pick-up tube.

Unless you have access to an ultrasonic cleaner, it takes some serious chemicals to dissolve this goo. None of them are good for you. It’s a job best left for the professionals. If you don’t use your boat every week, you should probably use a fuel stabilizer every time you fill up. Don’t overlook your motor manufacturer’s line of fuel care products. Most of them are excellent. My personal choice of aftermarket stabilizers is Startron made by Starbrite products or CRC stabilizer. They will keep your fuel good for up to a year.

Stabilizers will not recondition fuel that is already degraded. You need to add it to your tank when you put fresh gas in. If you have old fuel in your tank, you need to dispose of it before adding fresh. Some places that take waste oil will also take old fuel. Please dispose of old fuel properly.

After you fill your tank with fresh fuel and stabilizer, be sure to flush out the old fuel line and primer bulb before hooking the fuel line up to the motor. Take the fuel connector off the motor end of the fuel line. Hold the fuel line over a container and pump the primer bulb slowly until there is fresh fuel coming out. Running a motor on degraded fuel (if you can it started) can cause internal damage to the pistons and cylinders from detonation (low octane). Plugged carburetors can cause a motor to run lean from insufficient fuel, also causing piston and cylinder damage. This is especially critical in two-cycle motors as there will be a lack of lubrication.

What type of fuel should you use?

Almost all of the motor companies recommend 87 octane fuel, preferably non ethanol. If 10% ethanol is all that is available, that is acceptable as long as it has a minimum of 87 octane. Do not use fuel that has more that 10% ethanol. Why is ethanol not recommended for marine use? I will try and cover that subject at another time in more detail.

What about premium fuel?

If I had to choose between premium non-ethanol and 87 octane ethanol, I would opt for the premium. Some of the bigger four-stroke motors with variable cam timing call for a minimum of 89 octane for maximum performance. In other motors, the higher octane may produce more carbon buildup. If you know your carbureted small motor is going to be inactive for any length of time, unhook the fuel line and run it out of gas. However, do not run fuel injected or oil injected motors out of gas.

If you only use fuel stabilizer for winter storage, make sure you run the motor for at least 15 to 20 minutes to get the stabilizer circulated through the motor’s fuel system. With the high price of fuel, it makes no sense to let even six gallons of it spoil. You might save enough money to get in an extra fishing trip!

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