NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Gasoline                                
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Use only fresh unleaded gas. Regular octane rating (87) is fine - small engines operate at relatively low compression ratios so knocking should not be a problem. There is likely no benefit to using anything higher. In fact, I've heard that small engines may actually be harder to start and run poorly on expensive high octane gas due to their low compression ratio. Gas that is over a month old may have lost some of its more volatile fractions resulting in hard starting, possible varnish formation, and other undesirable effects. At the end of the season, dump the unused (unmixed, not from a two-stroke engine!) gas into your car (preferably when its fuel tank is at least half full so your old gas gets mixed with fresh gas). This way you won't be tempted to use it at the start of the next season.

If the tank detaches easily as with many Tecumseh/Craftsman engines, just lift it off and drain the gas into a gasoline safety can reserved for this purpose. If the tank doesn't come off easily, I typically use a cooking baster to do this (you know, the thing you use when baking turkeys!) although the gas tends to destroy the rubber. There are similar devices or gas siphons available at auto parts stores that survive better.

For 4 stroke engines, gasoline is used as-is since there is a separate oil supply. For 2 stroke engines, you must mix the proper amount of the correct 2 stroke engine oil (outboard motor oil or whatever is recommended by your engine's manufacturer). Fill your '2 stroke mixture' gas can with about half the amount of gas you are preparing and add the proper amount of 2 stroke engine oil. Put on the top and slosh this around to thoroughly mix the oil in with the gas. Then add the remaining gas to the total amount for which your oil measurement was made.

How do you tell if a lawn mower you just inherited has a 2 stroke or 4 stroke engine? The vast majority are 4 stroke - look for an 'oil filler cap'. On many, this is clearly marked with words like 'oil' or 'oil fill' or with a suitably ambiguous icon. Removing it will reveal a dipstick. (Note that unlike the engine in your automobile, this is both the test and filler location.) However, on more basic models, it may be near the base of the engine and be unmarked. In addition, there will generally be markings as to the need for the gas/oil mixture somewhere on the cover. The only major manufacturer of lawn mowers I know of that has used 2 stroke engines in recent designs extensively is Lawnboy.

If you are unsure of the correct mixture ratio - they typically range from 16:1 to 32:1 gasoline:oil by volume - 16:1 is probably a safe choice. The worst that can happen is that the spark plug may be more likely to foul (and you will pollute more than necessary) but at least you won't risk damaging the internal parts from lack of oil. It is of course best to determine and use the recommended mixture ratio.

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