Two-cycle string trimmers, unlike automotive engines, don’t use an oil pump to lubricate the piston shaft during operation. With no pump, the engine must get lubrication another way: mixed fuel. These engines require you to pre-mix a specially blended oil into the fuel, which reaches the piston via the carburetor and lubricates the engine.
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The Required Gas
Regardless of engine size, these small engines are designed to run fast, hard and hot. Speeds can reach in excess of 10,000 rpm’s, which creates enormous friction inside the piston and crankcase. Since the operating speeds are so high, the engine takes a lot of abuse, so to reduce that impact you should use a higher grade automotive fuel for your string trimmer. Most manufacturers recommend 90-octane or above, purchased from a name-brand filling station, which will ensure the highest performance. Also, you should never use any blended or ethanol-based fuels for your mix.
The Required Oil
Two-cycle engines require two-cycle engine oil, and four-cycle engines require four-cycle engine oil. These different oils have been designed to function with the number of pistons, either one or two respectively, in the engine. These oils are different than two-cycle or four-cycle outboard engines. You should also never use leftover or reused automotive oil.
The Mixture Ratio
The mixture ratio for string trimmers will differ from model to model; there isn't a one-size-fits-all mix ratio. The reason is that these companies design and test their engines in the laboratory. They've found, through these performance tests, the fuel mixture that will achieve the best results with their engines. You can find the mix ratio in the operator's manual for your string trimmer. These ratios vary widely from trimmer to trimmer and from oil to oil. For example the ratios can range from 25:1 gas to oil and go up to 50:1 for certain specially blended oils for the high performance professional grade trimmer. If you can't find the mix ratio anywhere for your trimmer, start a mix at the lower end, around 30:1. Run through a tank of that while trimming, and take careful note of the acceleration and high-speed performance. Slowly lean out the mixture up to 40:1 and run another tank of gas. If the engine's slow or sluggish or has poor high-end performance, you can lean the formula up to 50:1, but note if your engine isn't designed to handle this, you can wear out seals, rings and other sensitive parts in your internal engine with a too lean mixture.
When mixing the fuel, you should pour the oil into the empty fuel container before mixing the fuel on top, which will ensure the fuel mixes correctly. You will also need to let the mixture stand for around 30 minutes to mix thoroughly. When mixing fuel, don’t mix more than you can use in a 30-day period as the fuel will start separating after this time. This old gas will damage your fuel system parts.