How bad is it to run a saw out of fuel under load?

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HansFranz

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yes, you damaged any engine by being lazy, and no, pulling the muffler won't help.
Well, now ya got me all nervous and jerky. It seems to still have plenty of compression, but I'll have to fire it up today and try it.
 

cscltd

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heat is the #1 enemy of 2 strokes.
running it out of gas raises engine temp pretty quickl. Aluminum will melt at certain temp no matter what the oil is on it. It is the same as turning the high speed screw in. Newer saws running hotter than older saws for emissions, hence margin for error less on newer saws than older saws. Piston/cyl tolerances are tighter on newer saws too
 

HansFranz

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I'll have to fire it up today and try it.

Stihl runs like a
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Stihl runs like a
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raped ape!

Good to hear. I've never ran a chainsaw out of gas but have come close plenty of times. Only in some extreme condition would you have a wrecked the engine. Destroyed chainsaws would be very common, and create a fierce backlash in the community if running a saw to empty was a death sentence for the saw.
 

rogue60

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heat is the #1 enemy of 2 strokes.
running it out of gas raises engine temp pretty quickl. Aluminum will melt at certain temp no matter what the oil is on it. It is the same as turning the high speed screw in. Newer saws running hotter than older saws for emissions, hence margin for error less on newer saws than older saws. Piston/cyl tolerances are tighter on newer saws too
Seizing is still an oil problem no matter how ya spin it (putting detonation to the side different subject) pistons don't just melt on there own from heat in a low output industrial chainsaw they don't get that hot.
You need metal to metal contact for transfer of piston onto cylinder the friction and heat produced from this starts to melt the piston that's contacting the cylinder wall...
You get metal to metal contact from lack of oil film or oil film breakdown from heat (crap oil) leaving next to zero oil film between the piston and cylinder.
How you get to that point is many and varied but running a saw out of fuel isn't one of them, the process of seizing would have already started before the saw runs out of fuel even then running out of fuel isn't the root cause.
 

HansFranz

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You need metal to metal contact for transfer of piston onto cylinder the friction and heat produced from this starts to melt the piston that's contacting the cylinder wall...
You get metal to metal contact from lack of oil film or oil film breakdown from heat (crap oil) leaving next to zero oil film between the piston and cylinder.
Now ya got me wondering. I assume the skirt of the piston is gonna be undersize to the bore, so even if you have no oil, you shouldn't have skirt-to-cylinder wall contact.

My hunch is that in order to get piston skirt-to-cylinder wall contact, the piston needs to expand (from heat) more/sooner than the cylinder expands enough to accommodate it. I'm guessing that in a lean condition, without gasoline to carry heat away from the piston, the piston expands more/faster/sooner than the jug expands, leading to scuffing, friction, transfer and seizure...?

I'm not a saw mechanic by a long shot but I did stay in Holiday Inn once.
 

rogue60

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Now ya got me wondering. I assume the skirt of the piston is gonna be undersize to the bore, so even if you have no oil, you shouldn't have skirt-to-cylinder wall contact.

My hunch is that in order to get piston skirt-to-cylinder wall contact, the piston needs to expand (from heat) more/sooner than the cylinder expands enough to accommodate it. I'm guessing that in a lean condition, without gasoline to carry heat away from the piston, the piston expands more/faster/sooner than the jug expands, leading to scuffing, friction, transfer and seizure...?

I'm not a saw mechanic by a long shot but I did stay in Holiday Inn once.
No no oil is bad no matter what.
Pistons rock back and forth the intake side is the load side the bottom of the skirt gets pushed hard against the cylinder wall, oil is what stops it actually touching the cylinder wall.
Two or four stroke they will/should always have an oil film between moving parts if they don't metal to metal happens this is always bad.
In a four stroke vehicle most of the engine wear over it's life is on start up from no oil pressure and also a cold engine is putting in a lot of fuel (choke) this can wash diluted the oil on the cylinder wall.

Yes on cold starts the piston can expand faster than cylinder if the saw is put straight to hard work and the saw is not allowed to warm up slowly it's called a cold seizure.

When saws run out of fuel they bog splutter and don't take throttle ya have to shake em or pull the choke out in the hope they pick up a bit of fuel for like 10 more seconds cutting.
It's not like they go lean and rev to 15k in the cut for 5 minutes lol
 

PV Hiker

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I would be more concerned cutting in hot temperatures running out of fuel as you don't give it a chance to cool down, that could lead to vapor lock issues. You might have a problem getting fuel back into the cylinder and saw running like crap, especially a M-Tronic saw.
 

Cliff R

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I would try to avoid that scenario if you can. Most saws give you a warning that they are sucking the tank empty or below the pick-up. Running one till it dies out isn't going to destroy the engine but I've seen a few says that spend quite a bit of time in a lean condition allowing for multiple cuts before they completely run out of fuel. I suppose most of that deal comes from the shape of the tank and location of the filter in the bottom of it, plus whether you are cutting upright or on it's side, like cutting off a stump, etc.

Going lean reduces lubrication, higher EGT's and higher piston speed so it's never good for the P/C........Cliff
 

HansFranz

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...higher EGT's ....

That's kinda what set off the 💡 idiot light in my brain. I modded the muffler on my 660 with a big 3/4" pipe coming out the left side, and with the saw turned on its side, cutting the stump, my left hand was right next to the pipe, and the exhaust felt like a blowtorch coming out of there.
 

cscltd

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Seizing is still an oil problem no matter how ya spin it (putting detonation to the side different subject) pistons don't just melt on there own from heat in a low output industrial chainsaw they don't get that hot.
You need metal to metal contact for transfer of piston onto cylinder the friction and heat produced from this starts to melt the piston that's contacting the cylinder wall...
You get metal to metal contact from lack of oil film or oil film breakdown from heat (crap oil) leaving next to zero oil film between the piston and cylinder.
Yes on cold starts the piston can expand faster than cylinder if the saw is put straight to hard work and the saw is not allowed to warm up slowly it's called a cold seizure.
I’m confused on these statements. why doesnt the oil protect it from a cold seizure
 

rogue60

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I’m confused on these statements. why doesnt the oil protect it from a cold seizure

Oil does have it's limitations that’s why engines have tolerances that leave room for oil between moving parts from cold up to operating temperature. Yeah the smart dudes that design engine's know all about different expansion rates of all the different metals/alloys and crap generally that's why they recommended warming up an engine before ya give it the beans lol
A piston exspanding faster than the bore eventually it don't fit literally, I guess they came up with the name cold seizure because the engine isn't warmed up.
If you have an oil that could combat this I'd buy it 👍
 

HansFranz

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Yeah, the oil film prevents metal-to-metal contact. The main bearings for V8 engines only have a very thin "plating" of babbitt (or whatever soft metal they use) that would quickly be wiped off without an oil film keeping the hard steel of the crankshaft and the soft babbitt of the bearing apart...
 

sonny580

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I always end up runnin mine out of gas. Never in a felling cut since I do that first. On smaller trees I top off the tanks or grab a saw thats full. I take at least 6 saws with me and run one til its out or about out of gas then set it back on the truck and grab the next one and go. ---- Done that forever and the saws still run like the did when I bought them.
 
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