Running alittle rich

[stihlgoin]

Hey guys just a quick question about fuel mix... lets say I was to mix the fuel just alittle on the strong side... this would cause the saw to run on the rich side right? Saw ran fine til I mixed this last batch of fuel (12/30/05) so far I've ran 2 full tanks through the saw and half way through this tank of fuel... saw starts as normal, then idles normal (ie before cutting anything) but then say you go to a downed log and cut through it... when you're done/moving to next cut its like you're slightly holding the throttle open... if you tap the throttle and let go she'll run fine again (meaning normal idle speed) until you're done making your next cut then she'll do it again. Also a note here: It never does rev up real high on unless of course you're holding the throttle in all the way... but after making a cut, it will run as though you're slightly holding in, and does this til you tap the throttle... I did try waiting a couple minutes to see if it would idle back down to normal but it never did...make sense??

Any help you can provide is greatly appreciated.

 

[v8titan]

Sounds like you may have something binding up you throttle linkage...maybe a little shot of gumout will do the trick.

 

[TimberPig]

Hey guys just a quick question about fuel mix... lets say I was to mix the fuel just alittle on the strong side... this would cause the saw to run on the rich side right? .... .

If you put more oil into the mix, there is less fuel. This will lean the air/fuel mix the engine receieves, not richen it. The fuel/oil ratio will be more rich, but not the air/fuel ratio. The amount is normally small and relatively insiginificant unless you are talking going from 50:1 to 20:1 or something like that.

As to the hanging idle, sounds like it needs to be slightly richer on the low speed adjustment.

 

[mike385]

if it is a 372 watch for interference with the covering on the mixture screws, on the ones I have they have eaten right through the throttle cables.

 

[asb151]

if it is a 372 watch for interference with the covering on the mixture screws, on the ones I have they have eaten right through the throttle cables.

I am glad you mentioned that. I noticed the tight clearance between the 2 yesterday when I was in there grinding off the stops. I think I may go back and adjust the mixture screw cover so it doesn't rub.

 

[Max]

TP

How does it lean it out if you do not adjust the hi speed jet?

A richer fuel mix does not effect the jet setting.

I may be missing something here...if so please explain a bit more so it passes through my thick skull.

 

[TimberPig]

If you put more oil into the mix, it displaces gas, so for going from 40:1 to 32:1, you remove 8 parts gas from the mix in favor of 8 parts more oil. For a given volume of fuel/oil mix, there will be less gas in the mix. Since the carb draws a fixed volume of fuel/oil mix through the carb circuits, and then mixes it with the fixed volume of air, you have less fuel mixing with the air, which leans the fuel/air ratio.

 

[Max]

Thanks TP.

But...You say "the carb draws a fixed volume of fuel/oil mix"
If the volume stays the same, and the mixtures richens, how does the air increase to lean the saw out?

I'm not being a ****, I just don't understand how it leans out when the mixture richens and the voulume stays the same?

I hate to admit it but...Help me Simonizer!!!

 

[whatsnext]

Thanks TP.

But...You say "the carb draws a fixed volume of fuel/oil mix"
If the volume stays the same, and the mixtures richens, how does the air increase to lean the saw out?

I'm not being a ****, I just don't understand how it leans out when the mixture richens and the voulume stays the same?

I hate to admit it but...Help me Simonizer!!!

The air volume does not increase, the fuel volume decreases. If there is more oil in the mix there has to be less gas in any given volume. And, since oil is more viscous than gas your mixture will be slightly thicker and you will draw even less of it through the jet. If you don't understand this you probably should not be making any changes on your own.

 

[whatsnext]

It's not that big of deal really as 50-1 mix is about 98% gas and a 20-1 mix is about 95% gas but you can see that you just leaned your mixture by at least 3%
John..

 

[TimberPig]

As whatsnext added, the amount of air drawn through the carb does not change. By adding more oil to the mix, you get less gas drawn through in the fuel mix, because the oil displaces some gas. You then have less gas, added to the same amount of air, which leans the mixture out. The key is the oil in the fuel mix takes the place of what was previously gas.

The air fuel mixture and the fuel/oil mixture are two separate ratios. I think you are confusing a richer fuel/oil mixture with the air/fuel mixture. The fuel/oil mixture contains less gas as the amount of oil in the mix increases (numerically lower, ie 32:1 contains more oil than 50:1) this is then added to the same fixed volume of air that the carb is able to pass. You are effectively removing gas from the air/fuel ratio and replacing it with oil.

 

[mike385]

and more oil also lowers the octane of the fuel.

 

[Max]

Got It.

Thank you.

 

[Max]

whatsnext...thanks for your concern but it is not needed!

My request on an explanation of ratios/volumes has no bearing on my ability to tune my saws. I have been tuning my saws for 23 years and have not had a single issue with burning any of them up.

Maybe you should not give such pointed advice without asking or knowing someone personally before jabbing at them.

 

[THALL10326]

More Oil Means Leaner Mix, Huh

Max Im With You. If What We're Being Told Is True Then Just Add 1 Ounce Of Mix Oil To A Gallon Of Gas And See If The Mix Is Rich Enough To Keep From Scoring The Cylinder, I Think Not. As For The Thickness Of The Mix I've Never Seen A Carb That Couldn't Draw Enough Oil Out Of A Fuel Mix To Foul A Exhaust Port.

 

[TimberPig]

Max Im With You. If What We're Being Told Is True Then Just Add 1 Ounce Of Mix Oil To A Gallon Of Gas And See If The Mix Is Rich Enough To Keep From Scoring The Cylinder, I Think Not. As For The Thickness Of The Mix I've Never Seen A Carb That Couldn't Draw Enough Oil Out Of A Fuel Mix To Foul A Exhaust Port.

You are again confusing fuel/oil ratio with air/fuel ratio. Since the fuel part of the ratio is fuel/oil mix, and only a certain amount of that can pass through the carb, the more oil you put in that mix, the less gas is present. Oil is not actually fuel to run the engine on, it is to lube the engine. So the air/fuel ratio is the fixed volume of air that can be drawn through the carb, mixed with that fixed volume of fuel mix ( but really only the now reduced volume of gas). As you increase the amount of oil, you are mixing less fuel into the air fuel ratio, because the oil reduces the amount of gas in the mix for the given volume.

As to your hypothetical 126:1 fuel/oil ratio, yes that would be quite a lean (fuel/oil) mix and would likely cause engine failure if run hard ( but it would richen the air/fuel mix), unless you buy into the claims of some of the synthetics like Amsoil that spec 100:1 mix ratios, which isn't far off from what you are proposing.

It sounds contrary to what you might believe, but it is correct that if your carb adjustment does not change, more oil will lean the engine out.

 

[Lakeside53]

You just had to use that word "amsoil". Now Ben will get into this thread!

 

[04ultra]

Amsoil did I here Amsoil

 

[stihlatit]

Timber please edit that word amsoil.

Arnie

 

[Lakeside53]

I think it's on Begley's profanity list (can we see the list?), so he'll likely remove it pronto