Did You Know? Engine Air/Fuel Ratio.

Question: This may be a ridiculous question, but I would like to know the correlation between air/fuel ratio and oil/fuel ratio rich/lean mixture. Is it acceptable to lean out the oil/fuel ratio from that of the manufactures recommendation to compensate for rich running conditions. I use Golden Spectro 2stroke oil in my ’99 KDX 200. Manufacturer recommends 32:1,I run the Golden Spectro at 40:1 and sometimes leaner and I find this to be acceptable by reading my spark plug. Please, could you inform me if I’m off track?

Answer: Its not a ridiculous question and you are right on track. Golden Spectro is ideal at 50:1 in the KDX 200 and the affect on fuel mixture is as follows: A change from 32:1 to 50:1 oil fuel mix = 1% in percentage of oil present. 50:1 = 2% oil and 32:1= 3% oil So, at 50:1 you are 1% richer on FUELfollow that? 1% less oil = 1% more fuel.

Mikuni jets are metered in percentage increments. So, a 170 main jet is 10% leaner than a 180 main jet. In order to compensate exactly for the oil ratio change, not accounting for air density, you would need to go leaner 1%. If you have a 180 main you would have to scrounge around for a 179 main jet. Good luck! They don’t make them.

More important though is what happens in the low and midrange because we can feel the bike running a little richer there when we go leaner on oil. Well, some of us can. So, the best course of action is to turn out the air screw a 1/4 or 1/2 turn to clean up the bottom to compensate for changing from 32:1 to 50:1. THAT’S ALL! (provided by Spectro)

Zipper's

2 Responses to “Did You Know? Engine Air/Fuel Ratio.”


  1. 1 Long Live The 2 Stroke Nov 3rd, 2011 at 11:00 pm

    Oil doesn’t burn as well as Race Gas.
    So the less oil you use the more Power you can make.
    But beware of the Seize Monster…always pushing the limits.

    P.S. They always run best before they Seize….

  2. 2 hdobob Nov 4th, 2011 at 6:32 am

    Amsoil @ 128:1

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