Octane & Timing

Raymond Rexroad /
rarexro
Nov 22, 2007 09:18PM

I've read on this forum (many times) that most people recommend using premium fuel and going to 40:1 mix (or leaner).

My Hobbit loves 25:1 and 87 octane. With the 25:1 mix, I calculate that the octane is really 83.

Anyway, I wonder how many of you properly adjust your timing to account for the increase in octane.

For my Hobbit, I tried a 1/2 gallon with 92 octane at 40:1 mix, which is then about 90 octane. The motor ran worse (no timing changes). Then, I switched to 1/2 gallon of 87 octane at 25:1 - and the motor really woke up.

I can only attribute this to not having changed the timing to match the octane change.

Is there a rule a thumb that can be used - like for every two points, advance timing x degrees?

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Re: Octane & Timing

Warisesi /
Nov 22, 2007 09:55PM

do not put any advance on the timing it is a 2 stroke not a 4 stroke. most are factory over advanced which will put up the combustion temperature without enough RPM gain so it soaks the cylinder with too much heat and you will loose power or seize.

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Re: Octane & Timing

Jesse Stephenson /
jesses
Nov 23, 2007 01:01AM

changing your mixture like that drastically changes your amount of fuel that comes in from the jet, so that will actually change your jetting, which could explain the change in running. High octane fuel isn't any better for performance, it is only useful for high compression applications. If your engine is stock or at least stock head, higher octane fuel will give you no real change in the running of it.

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Re: Octane & Timing

maciek /
macelius
Nov 23, 2007 09:53AM

yah, you are changing two variables at once, try changing mix only or octane only, then there's jetting..

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