Thanks for endless entertainment every month. Your magazine simply rips! Now, can you please help explain a seemingly simple question that no one seems to have a simple answer to? When mixing oil and gas in my sweet YZ250 two stroke, is 40:1 leaner or richer than 30:1? The number of complex answers to this question baffles me when asking everyone from professional mechanics to local shop junkies to MX racers. Nobody really knows! Help!
Christian
St. Petersburg FL
The confusion arises because two-strokes have two types or richness and leanness (if that's even a word) going on. The jets in your bike's carburetor will only flow a certain amount of fuel mixture depending on the size of the jet orifices. Say, for example (and picking a number out of thin air) your bike uses 40 milliliters of fuel in one minute. On a two-stroke, that entire 40 ML of fuel is not gasoline. One ML is oil mixed in with the gas. The oil does not burn. If you had the oil mixed at 20:1, your engine would have 38 ML of gasoline and two ML of oil. So, as far as gasoline to the engine, 40:1 is richer than 20:1. The problem is that we describe oil mixtures as richer and leaner as well. If you are talking about the oil the engine is getting in one minute, 40:1 is leaner than 20:1, since the engine gets less oil. It is pretty easy to understand when you realize we are talking about two mixtures. Less oil equals more gas and vice versa. But now that you are educated on the intricacies of oil mixtures, other riders probably aren't. When a rider says, "I went leaner," you need to ask, "leaner on jetting, or leaner on oil mixture?" Then you will understand what he is saying. -Karel Kramel