OK guys - 25:1 (or 32:1) is safe - and as recommended when the engines were designed 30 odd years ago. Oil technology has moved on since then, and 50:1 with a decent specialist 2-stroke oil provides better lubrication than 25:1 with oil that was available back then. Further, the engine will produce more power, be cleaner running, and run cooler (and therefore last longer) than it will with more oil in the mix.
Explanation :- The correct air/fuel ratio for an engine running on petrol is approx 15:1 BY WEIGHT (actually 14.6:1 for 95 octane unleaded). When oil is added (which does not burn), it REPLACES some of the combustible content of the mixture. At 25:1, that is 4% of the fuel content, which make the engine run 4% lean. Lean mixtures burn hotter. To compensate for this, the mixture has to be adjusted richer, which increases fuel consumption, in addition, there is now too much oil, which comes out of the exhaust as blue smoke.
Obviously, all the above also applies at 50:1, but the ratio errors caused by adding a non-combustible lubricant to the fuel are halved, and there is less excess oil to come out of the exhaust.
All simple (3-port) 2-strokes are design compromises, but as a general rule, if you can see more than a hint of blue smoke in a 2-stroke exhaust at full load and speed, either it needs adjusting or there's too much oil in the mix.
I have used 50:1 in all the 2-stroke machinary I have ever owned, including an ancient Flymo that belonged to my grandfather - and that says 16:1 on the tank! As a bonus I can still see the lawn after I've mowed it, which was not the case on 16:1 - and it doesn't foul plugs either!
phil_saunders@bigfoot.com
Phil Saunders, June 2008
