Ratio changing tuning questions

[CoreyB]

Ok I have a question. I once was told that if you change from a 50:1 to a 40:1 with out adjusting the carb the saw would run rich. So tell me this if that is true wouldn't you have to richen the carb.
Because when I switched to 40:1 from 50:1 I had to lean out the carb.
I know tuning can be a challenge for a guy who has not done it but with info like this I am down right confused.
Anybody have an educational explanation they would like to share?

 

[Four Paws]

40:1 has more oil and less gas per unit volume than 50:1. If you go from 50:1 to 40:1, you will have to richen your carburetor H needle. This is required to give the fuel/air mixture the same ammount of fuel as it previously had running 50:1.

 

[huskihl]

The guy that told you that was misspoken. Think about it like this. Fuel mix has to pass through a hole, the size of which is dictated by your tuning screws. When your mix is thicker, not as much can flow through. Even though you have more oil, your fuel cools the engine and now you have less of it. Therefore you have to richen the screws to make up for it.

 

[huskihl]

Same thing happens with a temperature swing. At 40°, your mix is thicker than at 90°. So when it's colder, you should richen your needles.

 

[Moparmyway]

X amount of fuel is mixxed in with 50:1 (2.56oz of oil per gallon)
There is less fuel with 40:1 passing through the same opening, but more oil (3.2oz of oil per gallon)
So you would normally need to richen the mixture to replace the fuel that is displaced by more oil

 

[Keith Gandy]

The guy that told you that was misspoken. Think about it like this. Fuel mix has to pass through a hole, the size of which is dictated by your tuning screws. When your mix is thicker, not as much can flow through. Even though you have more oil, your fuel cools the engine and now you have less of it. Therefore you have to richen the screws to make up for it.

Best anwser

 

[cobey]

Same thing happens with a temperature swing. At 40°, your mix is thicker than at 90°. So when it's colder, you should richen your needles.

I was at some saw races
It went from 40 to 70 degrees
My saws were dumping fuel
Out the exhaust before I tuned them

 

[MustangMike]

Usually going from 40:1 to 50:1 does not make a big difference, but going from 32:1 to 50:1 (or vice versa) will really make a difference.

My M-Troinc saw took a while to get used to the difference between 32:1 and 40:1 (plus it involved a different make oil).

 

[Ozarker]

Don't forget to include air density in the discussion about temperature changes. There's more than the viscosity changes in play when discussing tuning due to temp differentials.

 

[CoreyB]

Don't forget to include air density in the discussion about temperature changes. There's more than the viscosity changes in play when discussing tuning due to temp differentials.

Yes we could open a huge can of worms but the answers above answer my original question.
I also find it interesting how just a different oil changes tune.

 

[species8472]

Thats y they have autotune now

 

[CoreyB]

Thats y they have autotune now

That would take some of the fun out for me. I enjoy that process.

 

[stihl saws]

I've been slowly switching equipment from Stihl Ultra at 50:1 to K2 at 32:1. It's taken a little carb tweaking but, not as much as I thought it would. I always tend to err on the rich side of perfect though.

 

[skidooguy]

I live at 4000 ft elevation so air density and temperature changes are always important factors for me to watch while using my saws. That is one of the reasons I choose amsoil at 40:1 it seems to be very consistent when tuning for me. Especially when I get up around 7000 feet and higher. I really did not notice as much tuning issues with the saber when I went from 50:1 to 40:1 ratios.