What Does Altitude Have To Do W/ Carb. Adjust

[StihlRockin']

I often see where people are selling chainsaws and they talk about the saw may need to be tuned for your area, season or altitude.

I don't mess with the carb setting too much. I can honestly say I've never had a Stihl break down on me in over 25 years. I've had minor issues like fuel lines, fuel filters and cleaning/tuning of the carb to get the saw running again and that's about the extent of my breakdown issues.(knock on wood) I prefer the mechanic guy do any adjustments for me. I often can get him to step out momentarily or bring it to his work station, etc, and he'll do the quick adjusting right there on the spot.

What is this thing with carb's having to be adjusted for altitude differences? How does this different alt. have to do with it and how does it affect the settings?

Thanks,

StihlRockin'

 

[PA Plumber]

Just from use the carb can go out of adjustment.

The situation I see the most is seasonal. If I have the carb adjusted right on for hot, humid weather, in the fall and winter it will be running high by a few hundred rpm. I think it has to do with the amount of oxygen needed for proper combustion.

 

[Freakingstang]

Density of air changes with drastic elevation.

Temp changes have a big effect as PA said. A saw setup in the winter will be rich in the summer, and a saw setup in the summer and ran in the winter will be lean (at same altitude. usually by the time you figure out it is lean, it is all over...

Air is denser when it is cooler, and requires more fuel to make it not lean.

 

[galde]

carb adjustments

Gas is formulated differently for season changes. This should require a fine adjustment also.

 

[StihlRockin']

I wanted to add one of the reasons I think I may have had luck with saws not breaking down seriously, is I let the pros tune it. I handle a saw very well, but I'm no mechanic. I recall several years ago I sold a 066 to a saw shop who gave me a fair price at the time and wasn't using it enough to miss it. Later when I went into the saw shop to look around, the owner said someone else bought it and then later came back to say the saw blew up! The owner told me at the time that he believes the guy was doing his own carb adjusting and set it too lean. LOL!

Thanks for the info. Makes sense the added moisture, etc, affects the combustion and everything else that goes on internally in the combustion process is changed.

StihlRockin'

 

[redprospector]

I live at 8700' elevation and do most of my cutting between 7000' and 9200' elevation. 16 miles to the west of me it is just under 5000' elevation. If you have been running your saw at 9200' (and it's tuned to run right) and then go to under 5000' and go to work without setting the saw richer, you won't be working with it for long. There is a big difference in air density in this kind of elevation changes. Sure the carb draws the same volume of air, but there is less oxygen, so at 9200' you have to run a lot leaner than at 5000'. That is why the dealer having to set up a new saw is silly here (the dealer being below 5000'), because when I get to the job I have to re-adjust it, and when I go to the next job I may have to re-adjust it again.
Back in the day's when you could buy a saw mail order (Nov. 07) most saw's that were shipped here wouldn't even start untill they were leaned way out.

Andy

 

[1953greg]

heres the scoop

http://www.2-stroke-porting.com/altiden.htm

 

[flea]

OK forgive me I is A pilot.
so I will go all aviation on you.

http://en.wikipedia.org/wiki/Density_altitude

the short of it is that the Higher you are in elevation the thinner the air is. The thinner air is important to internal combustion engines because you are burning a fuel air mix. if the air is thinner then you are not getting as much air in the mix.

Temp. has the same effect. The colder the air the thicker it is, the warmer the air the thinner. If you are at 8700' elevation and the temp is 105F then the air is very thin indeed. A Cessna won't fly worth a crap and your saw won't run worth a crap either. Same for cars too, however cars for the most part have computers to control this mixture for you but if you took a pure mechanical car engine it wouldn't run either.


BTW all plane references are about the wings ability to fly not the engines ability to run. An airplane has a manual mixture knob you pull to control the fuel/air mixture. pull to lean it out at altitude push to richen it up. your saw just has an adjustment screw that does the same thing. um I mean as far as the engine running not its (the saw's) ability to fly.

 

[pbtree]

Density of air changes with drastic elevation.

Temp changes have a big effect as PA said. A saw setup in the winter will be rich in the summer, and a saw setup in the summer and ran in the winter will be lean (at same altitude. usually by the time you figure out it is lean, it is all over...

Air is denser when it is cooler, and requires more fuel to make it not lean.

What he said.

 

[huskydave]

Density of air changes with drastic elevation.

Temp changes have a big effect as PA said. A saw setup in the winter will be rich in the summer, and a saw setup in the summer and ran in the winter will be lean (at same altitude. usually by the time you figure out it is lean, it is all over...

Air is denser when it is cooler, and requires more fuel to make it not lean.

Yes I know the Snowmobile racers set their carbs using a barometer as a guide.

 

[ShoerFast]

Just a rule of thumb,,,,

'Free-speed' (out of the wood speed) dose not have a lot to do with what is best for eny saw.

Whatever alititude, tempature or what kind of gas your running,,,,,
The very best chainsaw tuning device is a stop-watch. Starting with the 'H' too fat-rich or out (CCW) and timing cuts* in straight-grained consistent wood. Letting the saw do the work in the biggest log your saw will span. The best adjustment will be the fastest cookie-cut.

Just a note, this will sound slightly rich at 'WOT' out of wood or free-speed.

*Edit: From a too-fat adjustment, leaning maybe 5-minutes every timed cut till you get the fastest cut (Then maybe just a tad out for your Mother, as she wants you to be safe)

 

[oldsaw]

Elevation will make an engine run rich as the air thins out. 20 years ago I ran across a guy who had his car stall out about 3/4 or a bit more up Pikes Peak. He had run his battery down pretty good by the time I got to him. I had him pop his hood and pulled his air cleaner. The car started, I handed it to him and told him to put it back in at the bottom of the hill.

Air filters add restriction and act like a choke a bit. The change was just enough that the car would run, but not well.

Mark

 

[ShoerFast]

Elevation will make an engine run rich as the air thins out. 20 years ago I ran across a guy who had his car stall out about 3/4 or a bit more up Pikes Peak. He had run his battery down pretty good by the time I got to him. I had him pop his hood and pulled his air cleaner. The car started, I handed it to him and told him to put it back in at the bottom of the hill.

Air filters add restriction and act like a choke a bit. The change was just enough that the car would run, but not well.

Mark

The earliest of closed-loop/feedback engines needed to be turned off and restarted while climbing a mountain, as it reset the 'baro' sensor/AF-possessor.

Here is a link that I like to punch numbers into. Sort of interesting that on a bad air-day at the top of Pikes-Peak an MS-660 has less power then an 026 on a good day at sea-level!