Long Cuts = Leaning Out?

[Vintage Engine Repairs]

Hey guys, how come, when an engine is under prolonged load that it requires more fuel / can lean out?

Is it the heat increase? The way the fuel atomises?

I had it where it was tuned, be it on the very edge, but it would cut great for a few cuts, but then after a few minutes cutting a few decent sustained cuts, it would start to bog a bit, then it would be ok again if you let off, then you do another cut it would stumble.

Of course it’s too lean, and it’s an easy tweak of the carb, but I’m curious as to what is actually happening to cause it to happen after a few minutes of run time, rather than, say on the first cut under load.

Thanks!

 

[Old2stroke]

Probably just takes prolonged cutting at WOT to get all the residual fuel out of the intake/crankcase that has accumulated from idling with slightly rich low speed setting. If a "tweak" of the carb won't cure it, then it could be a fuel pump diaphragm that just won't deliver enough fuel or a restriction somewhere in the fuel supply path that limits max fuel supply.

 

[Vintage Engine Repairs]

Probably just takes prolonged cutting at WOT to get all the residual fuel out of the intake/crankcase that has accumulated from idling with slightly rich low speed setting. If a "tweak" of the carb won't cure it, then it could be a fuel pump diaphragm that just won't deliver enough fuel or a restriction somewhere in the fuel supply path that limits max fuel supply.

Thanks mate good points raised.

 

[MacAttack]

In my experience it's often a fuel pump diaphragm getting stiff / bad. At WOT they struggle to keep up. At half throttle or short burts they seem fine.

 

[rogue60]

I've never had a pro saw lean out after a long cut or working the guts out of em all day in summer heat myself. I don't even tune a saw very often set it and forget it. I've never had a big difference in altitude to warrant fiddling with the high or low all the time. Always worked around the 1000m mark not that that is very high above sea level. I've never run a saw at sea level personally always wondered if there would be much of a difference with higher atmospheric pressure and air density at sea level.
Anyways back to the question IDK too lean of a tune at the start? Tune is only a degree of heat fat is cold lean is hot manufacturers design engines to run at an optimum temperature thats somewhere between the above two extreme opposite tunes.

 

[Vintage Engine Repairs]

I've never had a pro saw lean out after a long cut or working the guts out of em all day in summer heat myself. I don't even tune a saw very often set it and forget it. I've never had a big difference in altitude to warrant fiddling with the high or low all the time. Always worked around the 1000m mark not that that is very high above sea level. I've never run a saw at sea level personally always wondered if there would be much of a difference with higher atmospheric pressure and air density at sea level.
Anyways back to the question IDK too lean of a tune at the start? Tune is only a degree of heat fat is cold lean is hot manufacturers design engines to run at an optimum temperature thats somewhere between the above two extreme opposite tunes.

Thanks rogue I appreciate your sharing!

 

[OM617YOTA]

I had a cheap Chinese carb rebuild kit on my 034S that would lean out more, the longer I was on the throttle. Genuine Stihl rebuild kit fixed it. Figuring the same thing, pump diaphragm couldn't keep up.

 

[Okie]

I really keep a heads up notice for a chainsaw acting like this:
Why: because the engine can be overheating and the piston swelling into the cylinder wall and it can happen fast. (most generally due to running lean.
Just a hint also about a chainsaw being in a long (about 30 seconds or more) full bar cut load (with a sharp chain) and the saw acts like it's bogging or losing power, take it out of the cut, rev it few times and it ready to go again and back into the cut and starts bogging again.
To confirm the running temp in a full bar cut I use a IR thermometer and monitor the block temp while in a full bar heavy load cut. If the block temp is getting to 350 or more it's overheating and the piston may be swelling into the cylinder and the engine starts bogging due to friction. I've seen saws that would be running lean and would be ok when used for light cuts and trimming but put it in a heavy full bar load and severe overheating and even engine seizures. (piston swelled into cylinder and engine locked up and might loosen up when it cooled down. Remove the muffler and see the piston scoring.

 

[rogue60]

30 secs thats fast I call a long cut a few minutes lol
Pro saws need no special treatment providing nothing is wrong with the saw and it's tuned correctly (ya don't even have to tune most of em anymore) they are designed to do constant work cut after cut with no overheating.
Even saw abuse like milling with a pro saw guy's know what they are doing is no overheating going on they get a slab to a tank with no stopping in some cases now that's hard work.
Yeah sure I've had saws lean out and do all kinds of weard things when there is something wrong with them from air leaks to needing carb kits. One 088 I have if I let it idle for like 15 seconds it starts revving high like it's running out of fuel but it runs fine in long cuts meh I'll get around to looking at what's wrong with it one day.
Some of the cheap Chinese rubbish saws I've messed around with don't like being pushed hard in summer heat regardless of tune adequate cooling is lacking in design but they can run fine in cooler temperatures.

 

[grizz55chev]

Check your fuel tank vent!

 

[Okie]

rogue60 asked:
30 secs thats fast I call a long cut a few minutes lol

Maybe I should have said 30 seconds or longer. I leave the saw in the cut and monitor for the temp to stabilize below 350F. I have a large log right outside the shop that I use as a test log. Easier to test using a log instead of a vertical tree.
I've seen the block temp start climbing fast and within first 30 seconds or quicker get to 350 and going right on up. This fast temp climb is a indicator that the engine is going to ruin itself fast when in a loaded cut.
The Operator when not familiar with such will usually think that it's the saw bogging down, maybe due to binding/pinching or woods chips in the kerf because when it's lifted out of the cut the saw can be reved up in few seconds, so back into the cut and the Operator not knowing it's overheating and the piston skirt swelling into the cylinder wall and adding more heat due to friction.
I've seen guys install new pistons and new blocks and do the same thing again. (then they usually start bad mouthing the replacement parts or the saw maker)
and a tachometer (that some tuners rely on) will not show such. Rpm's can be and usually are below specifications when the saw is under a full cut load.

 

[bwalker]

The longer your on the throttle and the heavier the load both need richer tuning.
If you have ever ran a two cycle with an EGT gauge it's pretty easy to see this.

 

[ZeroJunk]

I'm no expert on anything, but you can't tune a cold saw and hand it back to the user.

That's why I tune them rich. Most end users will never know the difference anyway and they don't end up with a burnt up saw.

I have no place or logs to tune in use at work. And, ain't taking the things home.


As to the original question, I have no idea what all is involved.
One would think it was fuel pooled up in the case or whatever. But, that would clear up pretty quick. Hell, I have blowers spewing raw fuel out the muffler and they will clear up in 60 seconds.
More to it than that.

 

[bwalker]

I'm no expert on anything, but you can't tune a cold saw and hand it back to the user.

That's why I tune them rich. Most end users will never know the difference anyway and they don't end up with a burnt up saw.

I have no place or logs to tune in use at work. And, ain't taking the things home.


As to the original question, I have no idea what all is involved.
One would think it was fuel pooled up in the case or whatever. But, that would clear up pretty quick. Hell, I have blowers spewing raw fuel out the muffler and they will clear up in 60 seconds.
More to it than that.

Fuel pooling woukd make the saw run richer as it made its way to the combustion chamber.
You must also consider that when charge enters the crank case under vacuum and exposed to heat it flashes to a vapor.

 

[ZeroJunk]

Fuel pooling woukd make the saw run richer as it made its way to the combustion chamber.
You must also consider that when charge enters the crank case under vacuum and exposed to heat it flashes to a vapor.

********. LOL it started with a B. May be the dumbest thing I have seen you post, so far.

 

[bwalker]

********. LOL it started with a B. May be the dumbest thing I have seen you post, so far.

This coming from the guy that thought "fuel pooling" would lead to a lean condition. Maybe, one day you will understand how a two cycle motor works...maybe.

 

[ZeroJunk]

This coming from the guy that thought "fuel pooling" would lead to a lean condition. Maybe, one day you will understand how a two cycle motor works...maybe.

How did you come up with that ? It will lead to hydrolock. See that about every day, but most always on 2 cycle blowers. Twice today. And, you can blow raw gas out of the bottom of the muffler and blue smoke out until it clears, but if you don't replace the carb it will come right back in. Promise you there is nothing lean about it.

I actually do this, not vicariously sitting at a desk.

 

[ZeroJunk]

Fuel pooling woukd make the saw run richer as it made its way to the combustion chamber.
You must also consider that when charge enters the crank case under vacuum and exposed to heat it flashes to a vapor.

I read that as fuel pooling wouldn't make the saw run richer. Makes no sense.

It's that leave half the letters out and you read it anyway sort of thing.

Woukd not being a word, in English anyway.

 

[rogue60]

The fresh change doesn't enter the crankcase under vacuum. In a two stroke a lower than atmospheric pressure is created as the piston moves up this low pressure in the crankcase will now be filled by the higher outside atmospheric pressure and at sea level that's about 14.7 PSI.
Air is pushed into naturally aspirated engines not sucked in.

 

[bwalker]

The fresh change doesn't enter the crankcase under vacuum. In a two stroke a lower than atmospheric pressure is created as the piston moves up this low pressure in the crankcase will now be filled by the higher outside atmospheric pressure and at sea level that's about 14.7 PSI.
Air is pushed into naturally aspirated engines not sucked in.

The preasure inside the crankcase during the up stroke is lower than atmospheric so yes, it is sucked into the crankcase. So yes this is a vacuum.
What your getting at is the differrance between an absolute vacuum and a relative vacuum. A two stroke crankcase is the latter.