# What is the affect of temperature on A/F ratio?

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I've read a lot of posts about jetting for cold weather, etc and got to thinking about what exactly is the affect of temperature ALONE (lets say humidity and altitude remain the same) on the A/F ratio.

If my internet research is correct, it would appear that for every drop of 5 degrees C (or 9 degrees F), air density increases by about 2%. If I hold the density of fuel constant (which is incorrect, I know), then the A/F ratio would also increase by about 2% for the same temperature drop.

So, if a bike were jetted to 14.7:1 at 20C / 68F, it would run lean to the tune of 15.8:1 at 0C / 32F.

Is my thinking correct? I know I have simplified it a bit, but was just wondering if I am in the ball park?

Dan

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I have always heard, but never had confirmed, that each jet is worth about 1.5 units on the A/F scale. So, 13-1 with one jet is about 14.5 with the next. Eddie could tell you.

A main jet change is required for every 2,000 feet or 25 degrees. That would be pretty burdensome. Luckily, modern bikes can be off a little. You could go from 12-1 to 15-1 in one ride, and the bike would still run.

It is sure tough to be perfect all the time, because conditions change. We haven't even started to talk about humidity!

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I would love to see the math and / or charts that breaks everything down, including humidity and elevation; anything to enhance my understanding of the affects of these factors. It would definitely help in anticipating when a change is needed.

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Further searching provided good information re: air density calculations with respect to jetting changes.

One question I still have (after plenty of searching) is this:

If you've ideally jetted all 3 circuits, and then ONLY the temperature changed (let's say it dropped 30 degrees F, which would require additional fuel), wouldn't it be necessary to now change all 3 circuits?

I'm asking because I've read a lot about adjusting the fuel screw when the temp drops, but that is really only addressing the pilot circuit. Wouldn't the needle and main jets need adjustment as well?

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im super new at this whole jetting thing too.

one of the pros here on the forum has said to go 1 step on the main jet for every 20-25 degrees. also, said he can fake it in with the a/f screw for pilot so no change out is needed for him. will that work for you and me? idk.