Ideal fuel mix?

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MichMich

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Hi gents,

Now that both my Stihl MS461 and Farmertec/Stihl MS460 are 100% up and running strong (started second tank on both).
I've read somewhere that ideal mix through the first few tanks was 50:1, can anyone confirm?
On all other machines I own, manual talks about 40:1 and 25:1, but what do you guys recommend?

Thanks!
 
for 40 years I've run nothing but 50-1 XP Husqvarna mix, new and old saws....never had any saws go down or damaged( Except a Poulan /Craftsman 3,7 that I straight gassed cause the wife used the mix for the lawn mower then filled it with straight gas without mentioning it). This year after reading posts from this forum, I started mixing the oil at 40-1....I haven't done any heavy cutting so don't know if it is good or bad.....
 
thanks! My first saw was a Timberpro CS6150 and spark plug was always wet with the recommended 25:1, while it's perfectly dry now in 50:1, but top of the piston now has a brownish color, that's why I was asking
 
Don't overthink it, if the carb is properly adjusted for your fuel mixture you can run anything from 32:1 to 50:1 with no adverse side effects other than varying degrees of carbon deposits. The carb MUST be adjusted for the fuel mix though, adding more oil results in a leaner air/fuel ratio and less oil makes a richer air/fuel ratio.
 
thank.you very much for these infos!

sorry about the new fuel/oil ratio, didn't see it, must have overlooked the forum too fast...
 
Don't overthink it, if the carb is properly adjusted for your fuel mixture you can run anything from 32:1 to 50:1 with no adverse side effects other than varying degrees of carbon deposits. The carb MUST be adjusted for the fuel mix though, adding more oil results in a leaner air/fuel ratio and less oil makes a richer air/fuel ratio.
I totally disagree with that theory. The mixture is per air/ fluid, regardless if the oil mixed with the fuel. The idea that oil will replace air and lean a mix makes no sense to me. The oil with combust with the gas ( Why you get more smoke with more oil). I have never adjusted the air screw and was very critical on high performance two stroke dirt bikes and ran various mixtures from 32:1/ 40:1 and 50:1 depending on the oil manufacturer.
 
I totally disagree with that theory. The mixture is per air/ fluid, regardless if the oil mixed with the fuel. The idea that oil will replace air and lean a mix makes no sense to me. The oil with combust with the gas ( Why you get more smoke with more oil). I have never adjusted the air screw and was very critical on high performance two stroke dirt bikes and ran various mixtures from 32:1/ 40:1 and 50:1 depending on the oil manufacturer.
This has been addressed in many threads already. Do a search.
 
What's an air screw? I've got several pieces of air- and water-cooled 2T equipment and don't think I've ever run across an "air screw."
 
Bigbub - the oil in the mix replaces fuel, not air, thus the ratio of air to fuel increases (gets leaner) as oil percentage increases. The oil doesn't combust like the fuel, that's why you get the added smoke. I don't know that changing the oil ratio really effects the air/fuel mix that much though, at least for moderate changes like going from 50:1 to 40:1. Anybody know the typical air:fuel ratio for a properly tuned 2-cycle?
 
Hi gents,

Now that both my Stihl MS461 and Farmertec/Stihl MS460 are 100% up and running strong (started second tank on both).
I've read somewhere that ideal mix through the first few tanks was 50:1, can anyone confirm?
On all other machines I own, manual talks about 40:1 and 25:1, but what do you guys recommend?

Thanks!


1-gal 100LL AV/76--mL golden spectro
 
With older tech 2 stroke oil, the oil didn't burn(at least not completely) and wound up as spooge in the muffler. More oil for a given volume of fuel back then did mean less gasoline and a leaner air/fuel ratio.

I don't think that's the case any more. Modern oils burn with the gasoline, do contribute fuel to the air/fuel ratio, and don't wind up as spooge in the muffler. When I switch between 40:1 and 50:1, I have to richen the air/fuel mixture on the 50:1, not lean it out as the above wisdom would suggest.
 
Bigbub - the oil in the mix replaces fuel, not air, thus the ratio of air to fuel increases (gets leaner) as oil percentage increases. The oil doesn't combust like the fuel, that's why you get the added smoke. I don't know that changing the oil ratio really effects the air/fuel mix that much though, at least for moderate changes like going from 50:1 to 40:1. Anybody know the typical air:fuel ratio for a properly tuned 2-cycle?
Now you are into another issue that's no one will agree on. Stoichiometric ratio for complete combustion in gas engines is 14.7:1 (air:fuel), car engines often run lean for emission control at 23:1. For maximum power, ratios can go as low as 12:1 or more. HOWEVER, this is where it gets ugly as those ratios are based on mixing air and fuel based on their weight and carbs mix the two based on their volumes and there are just too many things that can affect the weight of air in a given volume. Maybe the lads who do dynamometer studies can join in with correction tables for altitude, temp., etc.
 
I totally disagree with that theory. The mixture is per air/ fluid, regardless if the oil mixed with the fuel. The idea that oil will replace air and lean a mix makes no sense to me. The oil with combust with the gas ( Why you get more smoke with more oil). I have never adjusted the air screw and was very critical on high performance two stroke dirt bikes and ran various mixtures from 32:1/ 40:1 and 50:1 depending on the oil manufacturer.

You're off base on this one on several points.
1. Chainsaw carbs don't use air screws like most dirt bikes do. The small diaphram carbs use fuel screws which are basically adjustable jets, there is a low speed screw for idle and just off idle and a high speed screw for mid throttle to wide open throttle. You aren't adjusting an air bleed and chainsaws are not a high performance engine, even the most powerful ones are making far less hp per CC than a dirtbike.
2. The oil isn't replacing any air, it is changing the ratio of the fuel in the air. More oil equals less fuel, it's minor and even though the oil does combust with the gas it is in no way the same. There has been a lot of experimentation that shows that a heavier oil to fuel mix does need to have the carb adjusted to avoid a lean condition. Contrary to your thought process you can actually burn up a saw by running too much oil and not adjusting the carb. It's unlikely to make much difference going between 40:1 and 50:1 but if your saw is already slightly lean (most are from the factory) and you decide to run 32:1 or even 40:1 then your slightly lean condition becomes a definitely lean condition and the saw will run hot even though it's getting more oil. The oil plays no part in cooling the piston crown (does not readily vaporize like gasoline) which causes the crown to expand more which can lead to a few failure modes. One is a soft seize where the piston expands enough to hit the cylinder but doesn't melt and deposit aluminum. Two is where the crown gets hot enough to melt and blow out or smears aluminum on the cylinder walls. Three it gets hot enough to soften and begin to smear aluminum on the cylinder and binds the rings up.
3. The extra smoke you get with more oil is actually un-combusted oil, if it had combusted then there would be no smoke. This once again reinforces the fact that more oil leads to a leaner fuel/air ratio as a portion of the fuel/oil charge isn't contributing to combustion or evaporative cooling of the piston.
 
I totally disagree with that theory. The mixture is per air/ fluid, regardless if the oil mixed with the fuel. The idea that oil will replace air and lean a mix makes no sense to me. The oil with combust with the gas ( Why you get more smoke with more oil). I have never adjusted the air screw and was very critical on high performance two stroke dirt bikes and ran various mixtures from 32:1/ 40:1 and 50:1 depending on the oil manufacturer.
Disagree all you want but he is right
 
You're off base on this one on several points.
1. Chainsaw carbs don't use air screws like most dirt bikes do. The small diaphram carbs use fuel screws which are basically adjustable jets, there is a low speed screw for idle and just off idle and a high speed screw for mid throttle to wide open throttle. You aren't adjusting an air bleed and chainsaws are not a high performance engine, even the most powerful ones are making far less hp per CC than a dirtbike.
2. The oil isn't replacing any air, it is changing the ratio of the fuel in the air. More oil equals less fuel, it's minor and even though the oil does combust with the gas it is in no way the same. There has been a lot of experimentation that shows that a heavier oil to fuel mix does need to have the carb adjusted to avoid a lean condition. Contrary to your thought process you can actually burn up a saw by running too much oil and not adjusting the carb. It's unlikely to make much difference going between 40:1 and 50:1 but if your saw is already slightly lean (most are from the factory) and you decide to run 32:1 or even 40:1 then your slightly lean condition becomes a definitely lean condition and the saw will run hot even though it's getting more oil. The oil plays no part in cooling the piston crown (does not readily vaporize like gasoline) which causes the crown to expand more which can lead to a few failure modes. One is a soft seize where the piston expands enough to hit the cylinder but doesn't melt and deposit aluminum. Two is where the crown gets hot enough to melt and blow out or smears aluminum on the cylinder walls. Three it gets hot enough to soften and begin to smear aluminum on the cylinder and binds the rings up.
3. The extra smoke you get with more oil is actually un-combusted oil, if it had combusted then there would be no smoke. This once again reinforces the fact that more oil leads to a leaner fuel/air ratio as a portion of the fuel/oil charge isn't contributing to combustion or evaporative cooling of the piston.

That is some well-stated two-stroke info, right there. It is polite, succinct and clear. And not buried 35,000 posts into the thread.
 
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