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Stihl dealer employee standing in foreground of showroom display.

Old gasoline is the leading cause of small engine failures.

      Old gasoline, many small engine manufactures consider gasoline stored inside small fuel containers or fuel tanks to be old after three months. But it can also happen as early as fifteen days. Time can vary within geographic regions and storage methods. High air volume inside your storage container, high temperatures, wide temperature fluctuations, high humidity and direct sunlight can all accelerate the oxidization process. Ethanol gasoline can also attract water from the air during storage. Over time water accumulates and separates from gasoline. It settles to the bottom where it can corrode metal fuel system parts and fuel tanks. Water can cause running problems or corrosion that ruins an engine.

      Varnish can cause engine running problems or completely ruin an engine too. Gasoline evaporates and can leave behind a gummy varnish residue or dry particles which look like tiny flakes of rust. Either one can clog fuel filters, fuel pumps, injectors and carbs. This condition can cause an engine to run poor, lean and hot. Fresh new gas will not dissolve varnish or particles, a carburetor cleaning chemical is required. On the popular Honda EU2000 generator a clogged carburetor jet will usually cause the engine rpm's to fluctuate while running, plus the engine may die when a load is applied to the generator. Most of the time the fuel tank needs to be flushed out and the carburetor needs to be cleaned to remove the varnish. We use a heated sonic tank with cleaning solution for cleaning carburetors and a chemical for dissolving varnish in fuel tanks. Running old gasoline in an engine leaves behind varnish that can destroy it. In two cycle engines varnish gums up the piston, rings and crankcase. It can cause a loss of power, loss of compression or seize an engine. In engines with valves it gums up the intake valve and may cause the valve to stick while running. A stuck intake valve can destroy an engine.

      Acidic, gasoline becomes less volatile and more acidic as time passes. Acidic gasoline damages rubber fuel system parts like primer bulbs, fuel hoses, fuel pumps, grommets and internal carb parts. Some carburetors have small rubber check valves built into them that cannot be replaced, so the carb will have to be replaced if damaged. 

      Octane, gasoline will lose octane as it ages. Example, 91 octane may become 90 octane after 60 days. High octane fuels resist damaging pre-ignition & detonation better than low octane fuels. That's why hotter running high performance engines usually require 91 or higher, otherwise they may overheat. Some models of outdoor power equipment require 91 octane fuel, but most only require 89 or 87. During hot weather engines may need higher octane fuel than normally required or they may overheat. Every summer when our temperatures in Las Vegas rise over 100 degrees certain equipment types & models come in for repair because they're running hot, losing power, dying and making a strange noise. The strange knocking noise is caused by pre-ignition, that's when a red hot piston dome or very high temperatures in the combustion chamber ignite the incoming fuel before the spark plug fires. Pre-ignition & detonation can destroy engines. The very popular Echo PB-770 backpack blower is one example. They can run extremely hot, lose power, die, detonate or seize while using anything less than 91 octane during the summer months. But they may run fine on 87 through Fall, Winter and Spring. Sometimes a hot, lean condition can be caused by leaking gaskets, leaking seals, clogged fuel filters or badly adjusted, clogged or worn carburetor. A hot engine can also be caused by dirty and clogged cooling fins on the block or a damaged fan wheel. Just don't overlook old fuel, poor quality fuel or low octane levels. Expensive pre-canned ethanol free fuels with around 93 octane and 5 year unopened shelf lives are a better choice for multiple reasons. Beware, sometimes canned fuels don't hold up as advertised. We have seen brand new cans of fuel that would not ignite or burn in a new engine. We have also seen some cans turn bad just a few weeks after opening. Sometimes poor quality fuel can run ok in one type of equipment but perform very poor or not at all in another. If your small engine starts running hot, loses power, is stalling, dying or is making an unusual noise especially during hot weather we suggest trying a fresh tank of 91 octane fuel or higher. Do not use gasoline that contains more than 10% ethanol ( or ) E85 fuel, it's not gasoline and it contains 85% ethanol, they will most likely damage your engine.

      Diagnose, If you want proof it's overheating remove the spark plug and inspect it. The tip of the electrode where the spark occurs normally has some tan or black carbon buildup on it. If the carbon on the electrode tip has turned white and/or has tiny round ignition fouling deposits stuck to it, you have the proof. 

      Fuel storage, Store your fuel in a clean approved container and safe location. As a benchmark try to purchase a quantity you can use within three months and choose a container size that matches. A full container of fuel will stay fresh longer than a half empty container. That's because a half empty container has a greater volume of air exposed to the fuel. Keep the container out of direct sunlight and sealed as designed. Adding fuel stabilizer to fresh new fuel is highly recommended, it helps your gasoline last longer. Adding it to old fuel doesn't help.

      One last note, if you're filling a small gas can at the pump and need 91 octane, be aware the last person at your pump may have purchased 87. You may be getting some 87 octane in your gas can, maybe as much as a half gallon before the 91 your paying for comes in. To avoid this you can pump some into your vehicle before filling your gas can. Technical tip by Mark. 

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