Fuel Mixture and Jetting 101 ?

MADisher

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#1
OK, so I understand the basic concept:

a) You need the proper jetting for your bike based upon altitude, and configuration (modifications, as in reeds, rad valves, pipes, etc).

But once you have that nailed down, does changing the mixture, say going from 50:1 to 32:1 effect it ? Do you need to consider the oil and the mixture too when jetting ?

If I've read previous posts right, going from 32:1 to say 50:1 puts more fuel in thus making you run richer and possibly causing you to foul plugs.

I'm not 100% clear on how the oil effects fouling. When you fould a plug on the trail the first thing everyone asks is what oil ya runnin ? What ratio ?

I use, blah, blah oil @ x:y raitio and have never fouled a plug.

Isn't it simply fuel ratio that causes fouling, and oil ratio that just makes it smoke ?

Someone enlighten me. Perhaps after my coffee I'll see the light.
:)

And make it a good answer, not too technical so we can make a FAQ out of it and post it on Just KDX or sumptin
 

Mac

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#2
Originally posted by 88RM250

But once you have that nailed down, does changing the mixture, say going from 50:1 to 32:1 effect it ? Do you need to consider the oil and the mixture too when jetting ?

If I've read previous posts right, going from 32:1 to say 50:1 puts more fuel in thus making you run richer and possibly causing you to foul plugs.

I'm not 100% clear on how the oil effects fouling.
Isn't it simply fuel ratio that causes fouling, and oil ratio that just makes it smoke ?
OK...You have a gas/oil mixture to make fuel right.
and you have that fuel mixing with air to give you an air/fuel mixture right.
Well the more oil you add to the gas/oil mixture the less gas you have for the same volume which will then make the a/f mixture too lean. Using less oil will means more gas in the gas/oil mixture which will richen the a/f mixture.:think
You see the oil takes the place of some of the fuel.

Too much fuel (gas and oil) is what causes smoke and spooge and fouling.
 
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MADisher

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#3
OK, But is the difference between say 32:1 and 50:1 really a big enough deal to consider when jetting ?

Even on justKDX, in the fredette area, they mention a particular jet based upon certain upgrades, but they don't ever mention you need x jet if you mix 50:1 and y jet if you mix 32:1

Do all those recomendations just assume the standard 32:1 ?
:confused:
 
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#4
Think of changing ratio's as fine-tuning for your jetting. More oil will lean out your jetting, and vice-versa. Personally, I won't run less than 32:1 anyway. If you don't want to constantly be rejetting, you should pick a ratio and stick with it.

And yes, going from 32:1 to 50:1 is a noticable difference, but not neccessarily enough to require changing jets, depending on the conditions. I used to run 52:1, and never could seem to get my jetting quite right. I switched to 32:1, and that was the key I had been missing. My bike runs great now.
 
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