Premix theory...yall probably tired of hearing this...

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#1
So I'm transitioning from someone who only thought of rich/lean to do with the fuel/oil mixture to someone who is now trying to understand the process as a whole.

I'm looking for people to set me straight and also offer their opinion, but as of now, i understand that premix = the fuel/oil mixture. And when talking about premix, a rich mixture is one that contains a higher portion of oil.

So the less fuel you add to a given part of oil will result in a rich premix mixture (like 20:1). But the more fuel you add will result in a lean premix mixture (like 50:1)

However, the carb involves an air/fuel ratio. So what is considered to be a rich premix (20:1) actually translates into a lean air/fuel mixture since there is less fuel contained in the premix.

And it seems like you can somewhat compensate for improper jetting by adjusting the premix, but only within a small parameter. Since a WAY WAY lean premix will not contain enough oil to effectively lubricate the crank. And on the other hand, a way rich premix will just oil foul the plug quickly.

So what is important to focus on. Do you play with premix to try and get an ideal air/fuel ratio in the carb? Or do you jet the carb to get an ideal air/fuel ratio and then run a set premix ratio? And how important is the premix ratio? Seems like premix won't give you a whole lot of flexibility so why bother worrying about what exact ratio you are at?

Just wondering because I'm hearing people talk about rich and lean concerning premix and air/fuel and it gets so damn confusing I feel like I'm worse off after having read it all.

Micah
 

Rich Rohrich

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#2
After years of trying multiple combinations in search of the holy grail the best advice I can personally give is Pick an oil, pick a ratio, stick with the decisions and forget about them.

Jets are still the best way to JET :)
 
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#3
I whole heartedly agree with Rich's statement above. It sounds like you are now on the right track with understanding the whole rich/lean thing. I like to stay close to the bike manufacturer's recommendation for premix ratio and then jet my bike accordingly (Yamaha recommends 32:1 for my bike and I have been using 36:1 ever since I got it).

Happy jetting! :thumb:
 
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#4
fuel ratio

And it seems like you can somewhat compensate for improper jetting by adjusting the premix, but only within a small parameter. Since a WAY WAY lean premix will not contain enough oil to effectively lubricate the crank. And on the other hand, a way rich premix will just oil foul the plug quickly.


Micah[/QUOTE]

I have read on the Eric Gorr website that adding extra oil to set amount of gas will actually lean out the mixture! how do you explain that Rich? I took some chemistry, lay it on me..
 
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#5
I have read on the Eric Gorr website that adding extra oil to set amount of gas will actually lean out the mixture! how do you explain that Rich? I took some chemistry, lay it on me..[/QUOTE]

That's the thing. A lean premix = more gas/less oil. A lean air/fuel ratio = less gas/more air.

Both need to be taken into account. When someone talks about rich or lean, make sure you know if they are referring to premix or air/fuel as they are both kind of opposite in their own context.

Which is probably how so many arguments start.

To answer your question...more oil to a set amount of gas will reduce the amount of gas in the premix. Depending on ratio of course. By reducing the amount of gas, that make a rich premix, but lean air/fuel ratio. (less gas than if the premix were light on oil)

Micah
 
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#6
Lets Beat This Dead Horse..

Micahdawg said:
I have read on the Eric Gorr website that adding extra oil to set amount of gas will actually lean out the mixture! how do you explain that Rich? I took some chemistry, lay it on me..
That's the thing. A lean premix = more gas/less oil. A lean air/fuel ratio = less gas/more air.

Both need to be taken into account. When someone talks about rich or lean, make sure you know if they are referring to premix or air/fuel as they are both kind of opposite in their own context.

Which is probably how so many arguments start.

To answer your question...more oil to a set amount of gas will reduce the amount of gas in the premix. Depending on ratio of course. By reducing the amount of gas, that make a rich premix, but lean air/fuel ratio. (less gas than if the premix were light on oil)

Micah[/QUOTE]

so then, lets look at it from what the cylinder sees. Lets just assume that I have a properly running machine, at a set fuel/oil ratio, and the bike is jetted properly and the weather is constant and all that. our bike is running good. Now, we use twice the amount of oil in our premis as before. So now I have the same motor, and why would the cylinder see a fuel air ratio any different? It is still going to let the same amount of air as before, and the same amount of fuel, but now our fuel just has more oil, less gas and the same amount od air as before......so because the oil does not contain the same energy as the gas during the combustion process, the oil is taking up space that was occupied by the gas before, so there is a leaner fuel/air ratio as seen by the cylinder compared to the first case....
ok.
The lightbulb just finally went on.
Glad this site is here. thanks for your post Micah.
I'm going riding on my studded up KX250 this weekend on a frozen river in Fairbanks, Alaska down the Iron Dog Snowmachine Trail. You better believe I'll be correcting my fuel/air ratio for -10F...
 
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#7
Don't forget that when you add more oil to the mixture, the mixture becomes more viscous and doesn't want to flow through those small jets as easily. This has always been my interpretation of why more oil can create a more lean condition in the cylinder.
 
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#8
And that is why I was wondering the true signficance of the premix ratio. Because it seems like from what I've been reading in a short time, that your air/fuel ratio needs to be corrected through jetting. Because you don't have much wiggle room with the premix. If you are too heavy on the oil it will foul plugs (and as yz rider pointed out) it can be harder to get through the jets, but on the flip side, too little oil won't give enough lubrication.

I think I'll be hanging around 32:1 from here out and suggesting maybe 20:1 for my buddies 125.

Micah
 

Rich Rohrich

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#9
Micahdawg said:
Because it seems like from what I've been reading in a short time, that your air/fuel ratio needs to be corrected through jetting.
Absolutely, jetting is the key to performance.

Because you don't have much wiggle room with the premix. If you are too heavy on the oil it will foul plugs (and as yz rider pointed out) it can be harder to get through the jets
The too much oil fouls plugs line of thinking is really flawed. You can run a two-stroke at 14:1 ratio and not foul plugs IF it's jetted properly, you run decent fuel and it is ridden hard enough. Most plug fouling problems can be traced directly too poor jetting rather than oil ratio. I've run my last two two-strokes at 16:1 and 18:1 without spooge and never fouling a plug. Proper jetting and good fuel were the keys.

How much oil you need to run is pretty much a function of 3 things:

- Load - Higher loads demand greater amounts of oil. The amount of time spent under load is significant as well. More time under load requires more oil.

- RPM - Higher rpm will in many cases require more oil. RPM and load tend to be closely linked, so the same basic rules apply.

- Heat - To a lesser degree heat plays a role in determining oil volume requirements. Watercooled engines have made heat much less significant in terms of oil requirements but it's still something to consider. Engines that are subjected to high heat tend to live longer with more oil.

What this all means is you should base you premix ratio decisions on the intended use of the bike. YTrials riders run happily at 80:1 without issues, while super kart racers routinely run 14:1 with a perfectly dry exhaust.

The manufacturer's recommendation is usually a pretty good starting point for most of us. :thumb:

FWIW, running more oil in the mix does NOT make it harder to get the fuel through the jets. I have no idea where that notion came from but it's just wrong. The overall viscosity change that comes from adding a few ounces of additional oil to a gallon of gasoline is miniscule.
 
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