Saw Fuel – By Kevin Legacy

Issues with fuel has been a major concern of manufacturers of two-cycle equipment since I first started using, repairing and teaching on the subject in 1986. Using old or expired gasoline, improper mix and improper carburetor adjusting have cost manufacturers, equipment owners and operators tremendous amounts of money. I started researching this topic a few years ago. Upon talking with industry experts, manufacturers and mechanics, it has been explained that many of our problems stem from additives in gasoline, age of gasoline, fuel mix and the engine’s efficiency.

Gasoline, like anything else ages. As it ages it becomes less volatile and loses it’s initial octane rating. When aged fuel is burned in two-cycle machines, it burns erratically, causing carbon build-up (eventually leading to carbon-detonation) which results in engine damage. Additionally, the oxygenators and cleaning agents commonly found in today’s automotive fuels can react negatively to the efficiency of two-cycle mixed fuel. Remember, the automotive fuel industry does not spend time refining a better gasoline for the two-cycle equipment user. Pure gasoline, much like aviation fuel is more suited for the two-stroke fuel mix process.

Pay attention to the next automotive gasoline advertisement that you see on the television or in an ad. The automotive gasoline industry, promotes several detergents that clean or “scrub” the inside of your car’s engine. Those agents are somewhat beneficial to a car’s gasoline engine as they are designed to separate oil and grease from the surfaces of the engine block and valves. In layman’s terms, the detergent lifts and separates the oil film from the metal, and carries it off with the flowing oil and exhaust gases.

How do these detergents and today’s gasoline impact our two-cycle equipment? How does this affect the oil that is mixed with the gasoline to create the two-cycle fuel? The minute that you pour the mix into your two-cycle gasoline container, the detergents go to work. They immediately attempt to separate the gasoline and the oil. Our main issue…We NEED the oil to stay connected with the gasoline so we can conduct operations with the equipment that we must rely on to do our jobs.

Fuels containing alcohol additives are an additional problem with today’s automotive gasoline. For example, a two-cycle machine is adjusted and is running properly. The gasoline that was used in the current fuel mix (that you just finished) had little or no ethanol or methanol (both alcohols) added. Realizing that you are now out of fuel, a fresh can of two-cycle mix must be made up. You head out to buy fresh gasoline for the mix. The gasoline found at most pumps contain as much as 10%-15% of alcohol additives. Due to the addition of the alcohol additives, your two cycle engine will now run at a higher rpm and will idle differently. A carburetor adjustment will be needed, or you risk causing a permanent problem in a rapidly-wearing engine.

Why the carburetor adjustment? In my research and fact-finding, it has been explained to me that alcohol molecules require a larger hole to pass through than gasoline molecules. This explaines the “richer” adjustment needed for the fuel containing alcohol. For an example, we’ll say that your carburetor mixture screw setting is open two turns, and that there is an adequate amount of fuel passing by the adjusting screw. The “two-turns setting” was perfect for fuel containing pure gasoline. However, when you add gasoline containing alcohol to the fuel-mix, it does not allow the same volume to pass by the opening, thus the engine receives less fuel at that setting. Why is this important? A two-cycle engine receives lubrication from the fuel (oil mixed gasoline). If you limit the fuel flow, you also limit the lubrication to your engine. Machines today must be constantly watched to assure they are running properly. If they are not, and the machine continues to receive inadequate fuel (thus inadequate lubrication), you will have an increase in down-time. The greater the alcohol percentage in the gasoline, the greater the issue.

In some cases, we have heard that some members add a little extra oil to compensate for the deficiency in lubrication discussed above. This is also not a good practice, and also detrimental to the engine. Oil will retain heat, and if not burned cleanly will increase harmful emissions and increase carbon build-up within the engine. This unburned carbon will cause bearings to skate, piston rings to stick and the scoring of pistons and cylinders. Again, carburetor adjustment is very important and must be done to maintain the efficiency of the machine.

We must also remember that alcohol is an outstanding solvent. The presence of alcohol will adversely affect the fuel lines, seals and other rubber parts of our two-cycle machines. A solvent often softens rubber, causing additional problems. You may have noticed that the fuel lines on your saw have miraculously transformed from a rigid hose to a collapsible chewing gum consistency. Additionally, ethanol collects water very efficiently. This assists in removing moisture from gasoline. If a two-cycle engine contains water molecules in the fuel, corrosion of the aluminum and magnesium components may occur.

Almost always when conducting our saw operations courses, we discuss carburetor adjustment. We sometimes receive a word of caution from course organizers, supervisors or department mechanics at those training sessions. In many cases, they don’t want “just anyone” opening up a saw and adjusting the carburetor. While we do agree with the statement that IDEALLY only the saw mechanics or members trained in saw maintenance should be performing adjustments, we hear of just as many (if not more) two-cycle machines damaged due to lack of adjustment than from adjustment. If a two-cycle machine is not adjusted properly, you run the risk of engine damage. An operator must know when a piece of equipment is out of adjustment. It is important to the life of the engine, and operator safety.

Short of being the saw mechanic, what can we do to lessen this fuel problem as firefighters? One of the most important and easiest things that we can do is purchase and utilize a fuel with a higher octane rating (91) or use an octane booster additive. If multiple two-cycle machines are on your apparatus, sometimes each call for their own (sometimes different) fuel mixtures. If this is the case, a universal fuel mixture may be beneficial to your department. While there are several types of universal mixtures commercially available, assure that the one selected states “Good for ALL Air-Cooled Two-Cycle Engines” on the packaging. Additionally, check to assure that the oil mix can be mixed with gasoline containing ethanol. Most of the universal mixtures contain a fuel stabilizer. When utilizing these universal fuel mixtures there is less math to perform, and less chance of an error in fuel ratios. In short, only one can of fuel mix will be needed on the apparatus (instead of multiple cans), and the fuel will be stabilized as an added benefit. Several other variations of “pre-mixes” have also become popular. In our experience, most work very well but are more expensive than mixing your own fuel. Typically, those pre-mix containers come in small containers (32oz). Always conduct your own research, and figure out what works for your department and your tools.


  • Kurt Callisen says:

    Kevin, great post. Another thing to remember is that the spark plug can foul very easy. When the saw is first started you pull out the chock, this makes the air fuel ratio richer( more fuel). If the saw is not allowed to reach operating temperature the one spark plug has a very good chance of fouling.

  • Gary Lane says:

    Wow… great explanation. Im picturing your wife yelling at you to come in from the garage late at night because youve missed another meal with the family, having spent all afternoon tinkering with the saw, lawnmower, dirtbike etc… Awesome! The description of the detergent separating the oil from the fuel really breaks it down to the simplest form for guys to understand. Its hard enough getting guys not to over tighten a chain let alone adjust a carb! No wonder the mechanics are scared! Great job and keep up the good work! Love the saw pic with blades on opposite sides. Great idea!

  • Bretlle says:

    Fuel is an important factor for running a vehicle. Proper fuel can increase the engine’s efficiency. Using old or expired gasoline, improper mix and improper carburetor adjusting have cost manufacturers, equipment owners and operators tremendous amounts of money. These things should be maintained properly by which engine’s efficiency will be increased.

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