lean in a 2 stroke? what is it?


23 replies to this topic
  • Thumper1241

Posted February 22, 2010 - 10:24 PM

#1

ok what exactly is running lean in a 2 stroke?

is it the oil:gas ratio?

or the gas:air ratio?

thanks.

  • SS109

Posted February 22, 2010 - 11:43 PM

#2

Lean/rich always means fuel/air ratio.

  • huskymad127

Posted February 23, 2010 - 12:02 AM

#3

I wouldnt say always, more oil makes it run richer and can foul a plug and less oil means not enough lube and can seize the engine.

Thats what ive been taught anyway

  • matt3357

Posted February 23, 2010 - 12:06 AM

#4

fuel air ratio, don't try and jet using different oil mixes, it will barely make any difference.

  • rpt50

Posted February 23, 2010 - 03:03 AM

#5

This is a topic of endless confusion. Lean/rich should be used as referring to fuel/air ratio. Fouling plugs is not a function of how much oil you run! If you are jetted properly (fuel/air ratio!!!!), you can run just about any oil ratio. Kart racers run 16:1 and 20:1 with no fouling problems. I'm not sure what stimulated your questions, but I've always thought that the best advice for ratios is pick a good ratio (e.g., 32:1), and jet for it.

  • Charlie755

Posted February 23, 2010 - 06:35 AM

#6

A 2 stroke can be ran lean in two ways. If you are running to small of jets for the current climate and engine set up or if you run to much oil in your oil to fuel ratio you can run to lean as well. To much oil in the mix doesnt allow as much fuel which will cause it to run to lean as well. Running it to rich is just typically caused by to large of jets for the engine setup or current climate. Pretty simple really.

  • MrBlah

Posted February 23, 2010 - 06:41 AM

#7

I wouldnt say always, more oil makes it run richer and can foul a plug

Thats what ive been taught anyway


false

more oil makes it leaner

  • shagger

Posted February 23, 2010 - 07:45 AM

#8

If your bike is lean the power will feel soft and the bike will sound like it's bogging. If it's rich it will stutter and/or blubber.

I've accidentally double oiled my gas and run 16:1 with very little negative effects. But I jet just a little on the rich side because I feel that I get more consistent power in the woods. Were I spot on the extra oil would have put me on the lean side.

  • HNR_Engineer

Posted February 23, 2010 - 12:43 PM

#9

Charlie755 is correct in his explanation.

To add though, how sensitive your engine is to all this is dependent on quite a few factors like exhaust style, carb diameter to engine displacement ratio, head configuration, compression ratio etc. There are two rich/lean meanings, one is related to air/fuel ratio and one is to oil/fuel ratio and the common factor being the fuel.

I have had a few 2 strokes and they all were different and sensitive to different inputs.

  • shagger

Posted February 23, 2010 - 02:01 PM

#10

Perhaps. But if you take as a given that the rider has chosen a decent mix ration and is mixing his of her gas properly. Then it's all about jetting. Sure some mechanical swap can raise or lower tolerance. But the original question was the definition of "lean".

Throwing in things like tolerance and irresponsible fuel/oil mixtures will only serve to confuse.

Throw plenty of oil in your gas and be as consistent as is practical with the ratio. Then jet your bike so it runs crisp.

Some folks like to be stingy with their oil. Those are the guys who are going to eventually be generous at the parts counter.

Others will try to match their ratio down to some kind of nano gram per nano liter and with worry about how long their gas can was open and what was the temperature, humidity and wind speed at the time.

That's all rubbish. Be generous with the oil and slop it in there with reasonable accuracy. Then jet the thing so it runs the way you like it too. BE GENEROUS WITH TESTING. Try out things so you know what too far is and what it sounds and acts like.

Get your bike dialed in. Then go forth and slay the dogs! If some guy beats you its because he's more skilled at riding. Not because he's some sort of latent oil fetishist.

I'll shut up now.

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  • Charlie755

Posted February 23, 2010 - 02:04 PM

#11

Perhaps. But if you take as a given that the rider has chosen a decent mix ration and is mixing his of her gas properly. Then it's all about jetting. Sure some mechanical swap can raise or lower tolerance. But the original question was the definition of "lean".

Throwing in things like tolerance and irresponsible fuel/oil mixtures will only serve to confuse.

Throw plenty of oil in your gas and be as consistent as is practical with the ratio. Then jet your bike so it runs crisp.

Some folks like to be stingy with their oil. Those are the guys who are going to eventually be generous at the parts counter.

Others will try to match their ratio down to some kind of nano gram per nano liter and with worry about how long their gas can was open and what was the temperature, humidity and wind speed at the time.

That's all rubbish. Be generous with the oil and slop it in there with reasonable accuracy. Then jet the thing so it runs the way you like it too. BE GENEROUS WITH TESTING. Try out things so you know what too far is and what it sounds and acts like.

Get your bike dialed in. Then go forth and slay the dogs! If some guy beats you its because he's more skilled at riding. Not because he's some sort of latent oil fetishist.

I'll shut up now.


Actually he did ask about oil/fuel ratio as a cause as well.

"ok what exactly is running lean in a 2 stroke?

[COLOR="Red"]is it the oil:gas ratio?

or the gas:air ratio?[/COLOR]

thanks. "

  • shagger

Posted February 23, 2010 - 03:56 PM

#12

Well I guess if he isn't ironed out now he either crashed real hard on his melon or I want what he's having.

  • SS109

Posted February 23, 2010 - 05:28 PM

#13

Like I said in my original post, rich/lean refers to air/fuel ratio and nothing more.

Yes, if you change oil ratios without rejetting then you will make the mixture leaner (more oil) or richer (less oil) depending on which way you go. However, the changing of the oil ratio is not the actual cause of it being leaner or richer. It is the fact that you have changed the air/fuel ratio. For example, by adding more oil (say going from 40:1 to 32:1) you now have less fuel which leans the mixture which means you have changed the air to fuel ratio.

It is all about the air/fuel ratio.

  • MrBlah

Posted February 23, 2010 - 05:59 PM

#14

ss109, I thought increasing the oil in the fuel caused the fuel to flow slower through the jets, which made it leaner

  • SS109

Posted February 23, 2010 - 08:46 PM

#15

ss109, I thought increasing the oil in the fuel caused the fuel to flow slower through the jets, which made it leaner

Hmm, never heard that one before but, as far as I know, that would be incorrect.

To make things easy on me I will use ounces instead of gallons in this example. If you have one hundred (100) ounces of premix fuel mixed at 40:1. That breaks down to...

97.5 oz of gasoline + 2.5 oz of oil = 100 oz of 40:1 premix

Now, change that oil ratio to 32:1...

96.875 oz of gasoline + 3.125 oz of oil = 100 oz of 32:1 premix

The 32:1 mix would make the air/fuel mixture leaner than the 40:1 due to less gasoline in the mix when using the same main/pilot jets and atmospheric conditions (humidity, temp, air pressure, etc.) are the same.

  • kevin286

Posted February 23, 2010 - 09:30 PM

#16

Hmm, never heard that one before but, as far as I know, that would be incorrect.

To make things easy on me I will use ounces instead of gallons in this example. If you have one hundred (100) ounces of premix fuel mixed at 40:1. That breaks down to...

97.5 oz of gasoline + 2.5 oz of oil = 100 oz of 40:1 premix

Now, change that oil ratio to 32:1...

96.875 oz of gasoline + 3.125 oz of oil = 100 oz of 32:1 premix

The 32:1 mix would make the air/fuel mixture leaner than the 40:1 due to less gasoline in the mix when using the same main/pilot jets and atmospheric conditions (humidity, temp, air pressure, etc.) are the same.


I would agree if zero of the oil mix was combusted during engine operation.
I'm not sure it's quite as linear as that though. Because some of the oil actually burns along with the gas during engine operation, the air/fuel is not leaning out more in <i>direct</i> relation to the increase of oil to the gas mixture. I agree it is leaning with the addition of oil to the gas mix, just not on a linear scale. Where's the lurking engineers here?:thumbsup:

  • SS109

Posted February 23, 2010 - 09:41 PM

#17

I would agree if zero of the oil mix was combusted during engine operation.
I'm not sure it's quite as linear as that though. Because some of the oil actually burns along with the gas during engine operation, the air/fuel is not leaning out more in <i>direct</i> relation to the increase of oil to the gas mixture. I agree it is leaning with the addition of oil to the gas mix, just not on a linear scale. Where's the lurking engineers here?:thumbsup:

Yep, it is a very simplified example. If anyone needs a more in depth explanation then the engineering guys will have to take over. Though, is it really necessary? No one needs to be a rocket scientist to jet a bike.

  • kevin286

Posted February 23, 2010 - 09:52 PM

#18

Yep, it is a very simplified example. If anyone needs a more in depth explanation then the engineering guys will have to take over. Though, is it really necessary? No one needs to be a rocket scientist to jet a bike.


LOL certainly not. In fact I'm sure the change in air:fuel ratio is changed very little with gas:oil changes.
It would be something fun to figure out on a rainy day sometime though.
Fun that this all came from
"ok what exactly is running lean in a 2 stroke?
is it the oil:gas ratio?
or the gas:air ratio?
thanks."

</end thread jack>
answer = lean is too much air getting to the cyl. Need to add more fuel.

  • MrBlah

Posted February 24, 2010 - 03:20 AM

#19

Hmm, never heard that one before but, as far as I know, that would be incorrect.

To make things easy on me I will use ounces instead of gallons in this example. If you have one hundred (100) ounces of premix fuel mixed at 40:1. That breaks down to...

97.5 oz of gasoline + 2.5 oz of oil = 100 oz of 40:1 premix

Now, change that oil ratio to 32:1...

96.875 oz of gasoline + 3.125 oz of oil = 100 oz of 32:1 premix

The 32:1 mix would make the air/fuel mixture leaner than the 40:1 due to less gasoline in the mix when using the same main/pilot jets and atmospheric conditions (humidity, temp, air pressure, etc.) are the same.


that's what I've always heard, it makes the air/fuel leaner because it flows slower, google it

  • SS109

Posted February 24, 2010 - 09:40 AM

#20

that's what I've always heard, it makes the air/fuel leaner because it flows slower, google it

There is such a small part of the premix that is actually oil in the first place that I can't see a tad more than 1/2 an ounce (in the example I posted above) making any kind of flow difference that would be practical to note.





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