oil and petrol mix?

Running a 2-stroke engine off of a 40:1 - 45:1 fuel mixture is safe using the newer technology oil that we have around these days. Lots of old engines say to use 25:1 because the newer oils that we have now weren't avilibale back when the engines were made. I usally use a 45:1 mixture even on the engines that say 50:1 for extra lubrication so the engine lasts longer.

Levi
October 2014

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

Do not mix the oil at fifty to one, Far from being universal that ratio is used in very few machines, The majority are as the first chap said twenty five to one. Even if i purchased a machine that was recommended to run at fifty to one i would still use twenty five to one to insure good lubrication.

Brian
May 2008

Appreciate what phils saying but i have a flymo that runs at 25/1 and a snapper with a two stroke engine that runs at 32/1
both as the factory recommended so as robert says proberbly best to run safe at 25/1

Dave
May 2008

If a 2 stroke engine, 50 to 1 using 2 stroke oil is almost universal. 25 to 1 if you have to use standard oil (not recommended - the engine will be fine but you may not be able to see it due to all the blue smoke you will create!).

If a 4 stroke, straight unleaded in the tank and SAE30 in the sump.

phil_saunders@bigfoot.com

Phil Saunders
May 2008

That should read 25 to 1

Robert redberry
May 2008

You will not go far wrong with 1 part oil to twenty five parts petrol/ IE 251

Robert redberry
May 2008