Theorey?

IA Leo · Nov 15, 2011

I can tell when my Poulan chainsaw is about to run out of gas at no load, it briefly revs up then dies.
Is it because it is running too rich normally?
Leo

M-MAN · Nov 15, 2011

I doubt it. I've never had a two cycle that didn't do that.

Oldmax · Nov 15, 2011

When they run lean they will run faster just for a couple of seconds.

Zachary Hoyt · Nov 15, 2011

2 cycle will run faster lean, but they have no power, or so I have read. My saw does that too. It is a handy warning and I usually shut it off immediately so that I can fill it up and have it start again more easily.
Zach

JMOR · Nov 15, 2011

(quoted from post at 11:01:45 11/15/11) 2 cycle will run faster lean, but they have no power, or so I have read. My saw does that too. It is a handy warning and I usually shut it off immediately so that I can fill it up and have it start again more easily.
Zach

o, if it is 'lean' that results in higher speed, then why doesn't your tractor, truck, car, etc. speed up as they run out of gas??

Lloyd In South Carolina · Nov 15, 2011

It is due to the lean mixture just as it runs out. Two strokes do not run lean for long, they do however run great just before they destroy themselves. Running that lean all the time will burn the piston.

Remember, your base mixture is based on optimal for all conditions, cutting wood when its 90 degrees, and cutting when its 10 below. There are better settings for both, however you don't want to check the temp everytime you cut wood. Therefore a happy middle ground is found.

L.

oldtanker · Nov 15, 2011

(quoted from post at 08:22:43 11/15/11)

(quoted from post at 11:01:45 11/15/11) 2 cycle will run faster lean, but they have no power, or so I have read. My saw does that too. It is a handy warning and I usually shut it off immediately so that I can fill it up and have it start again more easily.
Zach

Click to expand...

o, if it is 'lean' that results in higher speed, then why doesn't your tractor, truck, car, etc. speed up as they run out of gas??

Because yer car/truck/tractor is a four stroke. 2 strokes run richer to get enough oil on the rings. If you set up yer 4 stroke to run as rich as the 2 stroke it would do the same thing as the gas ran out and it reached the best mixture for just a few seconds.

Rick

Sid · Nov 15, 2011

Some of them do,

Dave Warden · Nov 15, 2011

Got an 83 make Mccullouch, or however you spell it and has always done that. How would the gas go lean at the end? Same mix all the way through. Have bought a sthil a couple of years ago though. The old yellow has lost some zip.

farmer boy · Nov 15, 2011

Both our Stihl chainsaws and the weedeater do that. Never noticed a shortage of power though. They always have full power up till they ain't running anymore.

Indiana Ken · Nov 15, 2011

Most likely the saw speeds up because it is set rich, from best power. As it runs out of fuel it passes through the best power setting briefly before running out of fuel. I have two Poulans, one I bought in 2002, and set them for best power then go about 1/8 turn rich. I have never noticed them to speed up before running out of fuel.

I do not set my two strokes rich as they run much better with the proper mixture. That includes my lawnboy mower, leaf blower, weed eater, toro snow blower. For high output racing or modified two strokes running lean can/will result in problems. However, for standard two-strokes I have never experienced problems from "so called lean settings". Setting them rich results in plug fouling and poor running.

Tractors will not exhibit this since the speed is typically controlled by the governor. Cars could if they were running a wide open throttle however, again that is not the normal condition.

Jerry/MT · Nov 15, 2011

My Stihl does that too (speeds up). I always thought is was the start of a lean-surge cycle that didn"t finish because the fuel ran out.

Tony S. · Nov 15, 2011

All 2 strokes should be set a little rich at full throttle so they won"t overheat. Should get a little "break" at full throttle no load.

Bob Huntress · Nov 16, 2011

There is some debate between the theory already mentioned, and the breaking suction in the fuel tank. Fill an empty one gallon milk jug with water and turn it upside down. Notice how the fluid is slightly (I realize that "Slightly" is used liberally, here) held in the jug? Notice also how the last spurt of water, however, pours out at a much higher rate than any other amount. Many, including myself, feel this suction break is why a 2 stroke surges as it runs out of gas. Others do argue that, for some reason or another, more fuel is present in the last amount of fuel, as compared to oil. I feel that if that were true, the engine would gradually increase in RPM's as the fuel/oil mixture is dropping, but that is my opinion.

MarkB_MI · Nov 16, 2011

The mixture on your saw is deliberately set a bit rich. The idea is it's better to sacrifice a bit of power rather than burn up the saw, which could happen if the mixture is set at an "ideal" (stoichiometric) mixture which gives maximum flame temperature.

Prior to the advent of modern electronic engine controls, pretty much all gasoline engines ran rich. Nowadays, though, automotive engines run very close to a stoichiometric mixture and rely on electronics to keep the engine from burning up at high power settings.

Theorey?

IA Leo

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M-MAN

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Oldmax

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Zachary Hoyt

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JMOR

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Lloyd In South Carolina

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oldtanker

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Sid

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Dave Warden

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