Carb adjustment for altitude

[superduty5.4guy]

Just curious, and I know theres no 'exact' answer, but what type of altitude change would warrant an adjustment of the carb and when adjusting for altitude, do you richen/lean only H or L or both. (i do know that you lean it when moving to a higher altitude and richen when going to a lower altitude... i think). The relevance to this question is that I am having a ms660 shipped from california where it is at about 250 feet to where I am in ohio at about 1000 feet which would make a difference of about 750 feet going up in altitude. Just curious as to what adjustments I should be planning to make when it gets here. Thanks for the help.

 

[Evan]

shouldnt make any difrence because your going to tune it yourself to run at 1000ft

thats not realy enough to matter

 

[Freehand]

:agree2:When you're going above 2-3 thousand feet,maybe then.750 feet is insignificant.You'll love that saw,enjoy

 

[Cliff R]

It's more about the "density altitude" than just the distance you are above sea level.

The quality of the incoming air greatly effects the mixture(s) required for best power and low EGT's.

I carry a small screwdriver in my chaps for fine adjustments throughout the year. In "pea-soup" humidity and 90 degree plus outside temps, the DA can go over 5000' here in Ohio at 1200' above sea level. This typically requires a minor "tweak" in the lean direction with the "H" speed screw on most saws.

It's the reverse in the cool/cold Winter months. Low humidity and cold incoming air typically require a screwdriver blade or so movement to add some fuel with the "H" speed screw.

Bottom line here, er on the rich side with noticable "four stroking" at no load but still good power in the cut, and things will be fine. The "trump card" in that deal is these new saws with rev-limiters. They can be difficult to set correctly, as the rev limiter fools the tuner into thinking you have plenty of fuel from the high speed jet. With those saws, I like to start out plenty "rich", and sneak up on the most ideal setting by leaning them up slightly and making a heavy cut, etc, until you find the best power at the richest setting(s).....Cliff

 

[Ptarmigan]

Altitude

The lodge that I work at is at 9500'. Before I found this website we used to have our chainsaws serviced and tuned in Bishop. The elevation there is 4500'. The saws were always a major frustration, hard to start, would shut down if you let off the throttle real fast, and foul alot of plugs. It could take about half an hour of pulling to get it to start after a refill of gas and bar oil.

When I decided I wanted a bigger saw of my own I was leaning toward a stihl because of the problems we were having with the husqvarnas. Then I found this site and realized they were just running too rich and picked up a tach. I ended up having to lean out the high and low from 1/8 to 1/4 turn to put the rpms to stock spec. Now they start quick and will refire normally on the first pull.

I think you'll be ok with a 750' elevation change, but I do like having a tach just for the peace of mind in knowing just where it is set. When I was tuning by ear (inexperienced) I was erring more towards rich and safe and setting the saw about 1500 rpm low.

Sean

 

[BobL]

Atmospheric pressure changes constantly everywhere on the earths surface by around +/- 2.5% over periods of about a week. Chains saws have to cope with that kind of pressure fluctuation anyway so that is why they have an altitude tolerance.

To observe a 2.5% change in pressure due to altitude alone then (at low altitudes) that is equivalent to a change of altitude of around 750 ft. Around here I see pros move their saws over altitudes of 1500 ft and do not worry about clanging their retuning settings.

BTW going up in altitude is not going to damage your saw as the reduced pressure just makes the saw run richer. Going down is potentially worse as the higher pressure makes the air/mix ratio leaner.

 

[J.W Younger]

adj carbs is a lot like maintaining chains, your're never going to get top performance until you can do it yourself.
I've seen the density/altitude change by 1000ft over the course of a day.

 

[litefoot]

I understand that density altitude can change dramatically, but I for one am not going to call the airport for flight conditions everytime I run my saw. I tune my saws to run at my woodlot at 5300'. I harvest the wood at 8000'+. They run rich, but I'd rather not lean them for altitude then forget to richen them back up when I get to the house.

 

[gallegosmike]

I live at 7500ft and cut from 8500ft to 11000ft. I just tune a bit rich(100-200 rpm or so) for those elevations. THAT WORKS for me, others it may not... I carry a tach with me to make sure that I am not running lean. So far Ive been pretty close on my adjustments. The area I live at has pretty low humidity year round. And I try to cut wood only in the fall to early winter.

 

[J.W Younger]

I understand that density altitude can change dramatically, but I for one am not going to call the airport for flight conditions everytime I run my saw.

if you have to call anyone to tell when your saws not running right you are pretty much whupped

 

[superduty5.4guy]

Thanks for the replies fellas I appreciate the help. Everything you guys have said has definately educated me alot. I'm not exactly a :newbie: at this stuff but I'm definately still learning. Next thing I need to pick up is a tach from baileys.

 

[Evan]

gallegosmike
a raise in altitude means the air is thinner and there is less of it wich means if your rich at 7500 your getn even richer up higher.
so your saw is set 100 or 200rpm rich at 7500 then you take more air ayway up higher and maintain that amount of fuel so your richer.

i notice when i tune my saws at 3200 that theyre rich at 8-9000. but this doesnt matter because i always have a screwdriver in my pocket and tune for each day im cutting. very humid days are alot difrent then dry days to.

i tune by ear only. but a tach would be nice to have just see how many rrrrrs my saws are running while in and out of the wood

 

[Rounder]

Just curious, and I know theres no 'exact' answer, but what type of altitude change would warrant an adjustment of the carb and when adjusting for altitude, do you richen/lean only H or L or both. (i do know that you lean it when moving to a higher altitude and richen when going to a lower altitude... i think). The relevance to this question is that I am having a ms660 shipped from california where it is at about 250 feet to where I am in ohio at about 1000 feet which would make a difference of about 750 feet going up in altitude. Just curious as to what adjustments I should be planning to make when it gets here. Thanks for the help.

The higher you go, the richer it gets, and yeah, 750 ft makes a difference on your H-jet, i.e.-turn it in- a daily annoyance falling in Montana - Sam

 

[gallegosmike]

gallegosmike
a raise in altitude means the air is thinner and there is less of it wich means if your rich at 7500 your getn even richer up higher.
so your saw is set 100 or 200rpm rich at 7500 then you take more air ayway up higher and maintain that amount of fuel so your richer.

i notice when i tune my saws at 3200 that theyre rich at 8-9000. but this doesnt matter because i always have a screwdriver in my pocket and tune for each day im cutting. very humid days are alot difrent then dry days to.

i tune by ear only. but a tach would be nice to have just see how many rrrrrs my saws are running while in and out of the wood

Thats why i tune a little bit rich(minus `100-200 rpms). Meaning that when I go up in altitude the little bit rich I set it at, it evens out. Remember I am at 7500 going to 8500 -11000 ft.

I do NOT tune by ear. I need to save what little hearing that I have left! I do it by tach and plug reading. Like I said in my last post. I cut during the fall to early winter. It is a time of low humidty and cool temps.

You said that you go from 3200 to 8-9000'ish ft. Gary has made comments about this subject in the past. You to also take account for temp and humidty
That is a different animal from me. Your going from low to high altitude. I am going from high altitude to a really high altitude. So my setup is going to be different from yours.

 

[J.W Younger]

Thats why i tune a little bit rich(minus `100-200 rpms). Meaning that when I go up in altitude the little bit rich I set it at, it evens out. Remember I am at 7500 going to 8500 -11000 ft.

I do NOT tune by ear. I need to save what little hearing that I have left! I do it by tach and plug reading. Like I said in my last post. I cut during the fall to early winter. It is a time of low humidty and cool temps.

You said that you go from 3200 to 8-9000'ish ft. Gary has made comments about this subject in the past. You to also take account for temp and humidty
That is a different animal from me. Your going from low to high altitude. I am going from high altitude to a really high altitude. So my setup is going to be different from yours.

Mike, anytime you have less dense air it will have less oxygen. If you have less o2 you can't burn as much fuel, so a correct mixture at lower alt will be rich at higher alt if all other conditions stay the same.