Premix ratio vs. fuel/air ratio ?

[Trailryder42]

I hope this is the right forum to ask this.

I've been having a discussion with a buddy about premix ratios for 2 stroke engines and their effect on the fuel/air ratio.

It is my understanding that when we talk of richness or leaness, we're talking about the amount of "fuel" in the fuel/air ratio. Correct?

A premix ratio of 32:1 has more oil in it than a ratio of, say, 40:1. Correct?

It is my understanding that an engine can be fine tuned/dialed in for a given situation by altering the premix ratio. Going from a premix ratio of 32:1 to that of 40:1 means the engine is getting less oil and more atomized fuel from the premix charge from the carb, making the fuel /air ratio richer; and visa versa if you went from a 40:1 premix to a 32:1 ratio, more oil displacing fuel in the mix, making for a leaner fuel/air ratio. Is this correct?

Thing is, alot of my buds use the terms rich and lean to describe the air/fuel ratio as to how much oil there is in their premix ratios. 32:1 being richer than 40:1. When they foul a plug it's because they're too rich(too much oil). And I agree to a point. The engine can utilize only so much oil from a premix in the combustion process, depending on premix ratios and jetting.

It's the amount of atomized, or raw fuel for that matter, that the engine is getting and is able to utilize in the combustion process that determines how the oil in the premix is utilized. Whether it is burned and how completely. Is this correct?

Because it's not the volitility of the oil in the premix that creates an ignitable mixture with air for the combustion process, it's the fuel.

Finally, (SHEESH!, you say), you will foul a plug when you either have more oil than the engine can utilize or the fuel/air ratio is too "rich"(more fuel than the engine can burn, too saturated. In this scenario you actually have a combination of fuel/oil fouling). At the beginning I said the terms "rich" and "lean" were used to describe the amount of "fuel" in the fuel/air ratio. What other/separate terms to use to describe the under or over-abundance of "oil"?

Thanks

 

[FruDaddy]

Sounds like you have a pretty good handle on it, don't listen to your friends, and if they blow up due to a lack of oil because they were running rich, have plans for the day they plan to rebuild.
The only terms I know of for excess/inadequate oil in mix are, generously lubricated and blown.

 

[Rich Rohrich]

Because it's not the volitility of the oil in the premix that creates an ignitable mixture with air for the combustion process, it's the fuel.

That isn't necessarily true. The oil can have a significant influence on the combustion process. With some oils there is little to no influence even at 14:1 ratios (this is what we want), while other oils really disrupt the combustion process, and adding more oil just makes things worse. You can't make blanket statements about how more or less oil will influence things without LOTS of testing.

It's never as easy as it appears on the surface is it? ;)

 

[steve125]

while other oils really disrupt the combustion process, and adding more oil just makes things worse. ;)

Easy Rich, your just encouraging the 100:1 ratio posts with that.
:yikes:

:)

 

[Trailryder42]

Thanks Rich, I do realize that but wasn't gonna go there. I should have said "oil isn't the primary medium in the combustion process". Altho the oil isn't as refined as the fuel, it's still a petroleum distilate and if atomized sufficiently will contribute as you say.

That isn't necessarily true. The oil can have a significant influence on the combustion process.

It's never as easy as it appears on the surface is it? ;)

 

[Rich Rohrich]

Thanks Rich, I do realize that but wasn't gonna go there. I should have said "oil isn't the primary medium in the combustion process". Altho the oil isn't as refined as the fuel, it's still a petroleum distilate and if atomized sufficiently will contribute as you say.

My basic point was, just because you change the fuel/oil ratio does not necessarily mean there will be an intuitively obvious change in the working air/fuel ratio the engine sees. :cool:

Easy Rich, your just encouraging the 100:1 ratio posts with that.

You're right Steve, I'm sure someone is bound to use that as justification for doing something stupid. :bang:

 

[cujet]

I use the Mobil synthetic oil in my bikes. There is no question I can fine tune the A/F mixture ratio with the oil ratio. I can even tell the difference between 32 to 1 and 50 to 1.


Chris

 

[atomarc]

If makes it more clear for me when I think in terms of , "the ENGINE is rich or lean", and the " FUEL mixture is rich or lean"...two different things. One is dependent on the other, but for me, it simplifies it.

Stuart.

 

[oldfrt613]

Holy cow ! Nothing like stirring up techno guy's.

I use the oil mix ratio to tune for temperature extreams. So once I get a bike dialed in for my "normal" riding, I use the ratio change for temp extreems. For example, I tune in my normal temps at 40:1. Then it gets hot and steamy I will back off to 32:1 - in the winter I will run 50:1. I find this easier than screwing with the entire jetting system. It has worked for years for me.

So, I am adjusting the amount of fuel by adjusting the ratio, however, I am not "playing" with the ratio when I'm doing my tuning. In tuning you need to adjust out each circuit - changing the ratio ( for a given oil ) changes availible fuel across the board. PS - I only run race gas - because pump fuel is inconsitent - which is just another variable to deal with ( one you have no control over ).

OK Let the sparks fly !!!!!

 

[Trailryder42]

Works for me, as long as it's understood when talking "rich"/"lean" it refers to "fuel". If an engine is running lean, it's not getting enough "fuel". A premix ratio of 32:1 is leaner than that of 50:1. With 32:1 there's less fuel to oil.

If makes it more clear for me when I think in terms of , "the ENGINE is rich or lean", and the " FUEL mixture is rich or lean"...two different things. One is dependent on the other, but for me, it simplifies it.

Stuart.

 

[Trailryder42]

No sparks here. That's exactly the concept I've tried to get across to my friends. Things is, when talking with them and we're talking richness/leanness, they're thinking oil and I'm thinking fuel. Do you find you can do this for changes in altitude also?

Holy cow ! Nothing like stirring up techno guy's.

I use the oil mix ratio to tune for temperature extreams. So once I get a bike dialed in for my "normal" riding, I use the ratio change for temp extreems. For example, I tune in my normal temps at 40:1. Then it gets hot and steamy I will back off to 32:1 - in the winter I will run 50:1. I find this easier than screwing with the entire jetting system. It has worked for years for me.

So, I am adjusting the amount of fuel by adjusting the ratio, however, I am not "playing" with the ratio when I'm doing my tuning. In tuning you need to adjust out each circuit - changing the ratio ( for a given oil ) changes availible fuel across the board. PS - I only run race gas - because pump fuel is inconsitent - which is just another variable to deal with ( one you have no control over ).

OK Let the sparks fly !!!!!

 

[atomarc]

Don't want to flog the dead horse, but I'm still foggy on this. Rich/lean in regards to the engine, ie, is it overfueled-rich..or underfueled-lean is not the same thing as.."hey dude,what mix are you running", 32:1-rich, because we are taking rich on OIL, or 50:1-lean, because we have LESS OIL. These are two different systems, both using the same descriptors...but with opposite meanings. You can have a engine that is running RICH on a mix of 200:1...jets the size of a straw running fuel that has barely seen oil. Am I OTL on this theory?

Stuart.

 

[oldfrt613]

Since we are metering fuel ( the primary cumbustable ) richer has a higher ratio of fuel ( 50:1 is richer than 32:1 ). From all my dealings, tuners will use this convention. It's only my bone head friends who can't figure this out. They fowl plugs, reduce the oil thinking thats what fowled it ( richen the mix ) and realy get confused when it keeps happening. I go through this time and time again. The basic misconception is that the oil is the only thing that can foul a plug. It's hard for them to understand that that little spark can only ignite so much fuel.

 

[atomarc]

oldfrt13,
I think I finally got it. Always think of the FUEL as the component making the mixture either rich or lean.


Stuart.

 

[motometal]

Since we are metering fuel ( the primary cumbustable ) richer has a higher ratio of fuel ( 50:1 is richer than 32:1 ). From all my dealings, tuners will use this convention. It's only my bone head friends who can't figure this out. They fowl plugs, reduce the oil thinking thats what fowled it ( richen the mix ) and realy get confused when it keeps happening. I go through this time and time again. The basic misconception is that the oil is the only thing that can foul a plug. It's hard for them to understand that that little spark can only ignite so much fuel.

that theory of less oil to prevent fouling...sorta like putting less coolant in your radiator, because that's what's boiling. Heck, if you didn't use any coolant at all, there is no way it could boil over, right?

I'm not real wild about the idea of using the ratios as a tuning tool. It just seems a bit hokey to put 36% less (a lot less!) oil ("engine protectant") in the mix to change the effective mixture. Is jetting really that bad? How long does it really take to change jets? Most of the bikes i've worked on don't even require removal of the carb, so you can do it right at the track no problem.

 

[Trailryder42]

Exactly. You articulate it very well. My friends are the same way, except I don't consider them boneheads for doing so, just in case they happen to read this thread. LOL

Since we are metering fuel ( the primary cumbustable ) richer has a higher ratio of fuel ( 50:1 is richer than 32:1 ). From all my dealings, tuners will use this convention. It's only my bone head friends who can't figure this out. They fowl plugs, reduce the oil thinking thats what fowled it ( richen the mix ) and realy get confused when it keeps happening. I go through this time and time again. The basic misconception is that the oil is the only thing that can foul a plug. It's hard for them to understand that that little spark can only ignite so much fuel.

 

[oldfrt613]

No, jetting isn't that bad, but every time you open the carb you risk contamination - I hate dirt in my engine. My bike is not always spotless. While changing the main or pilot on my Honda only require the removal of the float bowl drain, the KTM and YZ in my basement require the whole bowl be removed. All of them require lots of figetting to get to the needle - lots of potential dirt. I do make minor jetting changes all the time - I'm kinda fanatical about how my bike runs, what I don't like is changing everything at once. Where I live it can be 35 deg in the morning and 70 in the afternoon. Today it was in the 80's, tomorro will only have a high of 60 and a low of 34. It's a lot easier for my to use the mix gig. PS I have yet to have an engine failure due to lack of lubrication - too lean, yes - overheating, yes - contamination,yes. Contamination has been the worse by far for me. It's sneaky, it gets my lower rod bearing - and when it comes apart it's expensive !

 

[motometal]

how do you know that contamination is what took out the bearing?

keep in mind that the oil helps cool parts as well. Part of the way it does that is to form a heat conductive film on parts. It also reduces friction. As parts start to get hot, most oils tend to migrate away from the hot part (castor oil is one exception), so the more oil the better.

there have also been studies suggesting the more oil produces more power.

I can relate to your concerns regarding contamination getting into the carb. Not criticizing your pickyness of jetting, most folks should be more picky, but on the other hand some day if you become a little less finicky about it you may get to spend more time riding. I know guys that ride for a few minutes then are back at the truck bleeding air from the forks and lubing the chain. This is a constant ritual. I would rather spend more time riding, or messing with the suspension or the controls. To each his own. :ride:

 

[oldfrt613]

I spend more time riding than most. Most of my fidgiting is at night. I probably ride 4-5 times a week, several hours at a time. I have my own motoxross track and 3.5 mile loop through the woods. As for the lower end bearing, my ex-tuner ( he's my ex tuner because he has turned to the dark side of 4-stroke world ) claims dirt is the number one cause, especially when the piston is ok. He claims that soft aluminum piston will be the first gotcha for a lubrication failure. As for increasing power with higher oil ratio's, it is true that increases have been proven due to better ring sealing in some tests, however there is a practical limit to how much oil can be added before spark plug fouling and premature power valve gumming. I belive most have agreed on ratio's of 32:1 to 50:1 are reasonable. My hopped up outboard runs 50:1 at sustained RPM's. Modern day lubricants can more than handle the task.

But I digress. The focus of the thread was to clear up the effect of gas:oil ratio on carburator jetting, at least that was the impression I got.

 

[joshp]

I'm currently running 32:1 but have recently thought about changing to 50:1 to save money on oil. I'm currently using Motul 600 which per liter runs around $11. The bottle lists a 50:1 ratio and I've heard that if it lists a ratio on the bottle the oil is safe and will still protect up to that ratio. I read another thread that said the more oil the more power you will have, but is this true with pure synthetics? Also what is a good starting point jet wise if I'm going to change from 32:1 to 50:1? Obviously I'll need smaller jets but how much smaller should I start with?