Long Cuts = Leaning Out?

[bwalker]

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.

K is right next to L on a keyboard and woukd is hard to misconstrue as wouldn't... once again you haven't a clue what your talking about.
Just like your hydrolocking comment. I've never had this happen. I have had a float needle not seat and fill the crankcase with gas, but the motor still turned over. On addition when starting a cold two cycle you always have some fuel pooling. Especially so when its cold out.

 

[rogue60]

The preasure inside the crankcase during thenuostroke is lower than atmospheric so yes, it is sucked into the crankcase.

Pushed not sucked the outside atmospheric pressure will equalize this lower than itself pressure void.
If we had an atmospheric pressure of zero naturally aspirated engines wouldn't even run.

 

[Vintage Engine Repairs]

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.

Thanks for the info, 350f+ is over heating, but what is the optimum temp for air cooled ope engine?

 

[Vintage Engine Repairs]

Here is a video, it’s not me, it’s a buddy Al @drf255 and his saw is m-tronic. Totally different in many ways, but I’m sharing it just so you can get an idea of what I’m describing, it’s EXACTLY the same symptoms. Just as a point of reference. Ignore the differences in saws and have a quick listen to the symptoms.

 

[bwalker]

Pushed not sucked the outside atmospheric pressure will equalize this lower than itself pressure void.
If we had an atmospheric pressure of zero naturally aspirated engines wouldn't even run.

Pushed not sucked the outside atmospheric pressure will equalize this lower than itself pressure void.
If we had an atmospheric pressure of zero naturally aspirated engines wouldn't even run.

Again.
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.

 

[rogue60]

Again.
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.

You can make a vacuum in an running engine yes by closing the butterfly.
A Diesel engine doesn't have a butterfly thus produces zero vacuum to get around this a vacuum pump is usually built into say the alternator like Toyota does that way ya at least have vacuum for brakes and other accessories.
This is basic naturally aspirated engine stuff is no sucking going on air is pushed in look it up It's a common misconception that naturally aspirated engines suck air in.

 

[huskihl]

Here is a video, it’s not me, it’s a buddy Al, and his saw is m-tronic. Totally different in many ways, but I’m sharing it just so you can get an idea of what I’m describing, it’s EXACTLY the same symptoms. Just as a point of reference. Ignore the differences in daws and have a quick listen to the symptoms.

I’ve had this happen several times on ported 261s. Which version M Tronic is it?

 

[huskihl]

If we are going to get sciencey, there is no such thing as a vacuum. Only positive pressure and negative pressure. But when the piston is on the way up and it closes off the transfers at 120-ish degrees before top dead center, it builds a negative pressure in the crankcase until the intake opens at around 75–80° before top dead center. It’s called primary compression. About 5 psi on the way down and -5 psi on the way up.

 

[Vintage Engine Repairs]

I’ve had this happen several times on ported 261s. Which version M Tronic is it?

No idea I’m afraid!

 

[huskihl]

No idea I’m afraid!

The v1 and 2 both had too small of an orifice in the brass nozzle/check valve in the venturi. The V3 orifice is drilled to the same size that we were drilling the v1 and 2.

I believe the Mtronic system has the ability to deliver enough fuel through the original orifice, but for whatever reason they don’t like to richen very quickly

 

[huskihl]

But to answer your original query, if there are no problems with the fuel system on a manually adjusted carb, the more heat you add into an engine the more fuel it needs to cool it down. Lean causes heat and heat causes lean. Once the case becomes heat soaked, the old settings no longer provide enough fuel. A simple H adjustment would fix it as long as it could provide extra fuel through the circuit.

 

[mattr66]

Check your fuel tank vent!

I second this.... What happens is that if the tank vent isn't letting enough air into the tank, it creates a vacuum that the pump diaphragm section of the carburetor has trouble keeping up with. It's like lowering the fuel pressure on a conventional carburetor system on a vehicle.

 

[Vintage Engine Repairs]

But to answer your original query, if there are no problems with the fuel system on a manually adjusted carb, the more heat you add into an engine the more fuel it needs to cool it down. Lean causes heat and heat causes lean. Once the case becomes heat soaked, the old settings no longer provide enough fuel. A simple H adjustment would fix it as long as it could provide extra fuel through the circuit.

Thanks mate good to know! Yeah just a slight tweak fixed it! Just wanted to understand why a hot engine needs more fuel!

 

[huskihl]

Thanks mate good to know! Yeah just a slight tweak fixed it! Just wanted to understand why a hot engine needs more fuel!

Simple answer, because it’s doing more work

 

[ZeroJunk]

K is right next to L on a keyboard and woukd is hard to misconstrue as wouldn't... once again you haven't a clue what your talking about.
Just like your hydrolocking comment. I've never had this happen. I have had a float needle not seat and fill the crankcase with gas, but the motor still turned over. On addition when starting a cold two cycle you always have some fuel pooling. Especially so when its cold out.

The reason you have never had it happen is because you don't work on anything.

I have not seen it on a chainsaw. But, see it literally every week on 2 cycle blowers.

Take the plug out and blow fuel two feet in the air. You can blow air through the plug hole and blow fuel out the exhaust. See it over and over. And yes, they were locked up.

And, it's not my problem that you can't type.

Funny thing me thinking you are actually that clueless about outdoor power equipment.

 

[ZeroJunk]

Again.
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.

Now, this is more your speed. Babbling about something.

 

[Tomos770]

Simple answer, because it’s doing more work

More work.....because it is overheating?

The amount of work (a wooden log....bar lenght....etc) stays the same in the/troughout the process....?

 

[Okie]

I see a slight tweak of the carb fixed it. (Your post #33)

You probably had to do a slight tweak richer. (maybe for a slight 4 cycle sound out of the cut)
But I've seen the fine screen in the carb restricted cause power loss, the grub screw vent not venting, old dried out carb internally.

I want to make sure that the engine is NOT EATING ITSELF while I'm trying to repair the saw or has not already got severe internal damage. (monitor it temp under load and look at the piston through the exhaust port)
That is a point I was trying to make. (engine leaning out and gets worse as it gets hotter under load and also the piston friction scuffing starts adding to the generated block heat) Most generally the carb is adjusted too lean, (H jet in just little to much, even 1/4 turn in on some saws is too much) but the Operator needs to be aware when he encounters a chainsaw doing as you describe that it can be ruined very quickly, scored piston and cylinder. I use a IR thermometer to verify that the saw is not overheating. If it's a new to me saw with those symptoms I remove the muffler and take a peek at the piston and if it's already scored, no need in proceeding, unless you just want to adjust the carb and see if that corrects the overheating issue. If you take the saw apart and replace parts and re-install the same carb and make tachometer adjustments the engine might do the same thing again fast. (and someone (new operator) not aware of such can destroy the saw fast by trying to force the saw to operate/cut correctly)
You might not ever notice lean and overheating when using the saw for trimming, on and off the throttle and the engine runs strong.

 

[Vintage Engine Repairs]

I see a slight tweak of the carb fixed it. (Your post #33)

You probably had to do a slight tweak richer. (maybe for a slight 4 cycle sound out of the cut)
But I've seen the fine screen in the carb restricted cause power loss, the grub screw vent not venting, old dried out carb internally.

I want to make sure that the engine is NOT EATING ITSELF while I'm trying to repair the saw or has not already got severe internal damage. (monitor it temp under load and look at the piston through the exhaust port)
That is a point I was trying to make. (engine leaning out and gets worse as it gets hotter under load and also the piston friction scuffing starts adding to the generated block heat) Most generally the carb is adjusted too lean, (H jet in just little to much, even 1/4 turn in on some saws is too much) but the Operator needs to be aware when he encounters a chainsaw doing as you describe that it can be ruined very quickly, scored piston and cylinder. I use a IR thermometer to verify that the saw is not overheating. If it's a new to me saw with those symptoms I remove the muffler and take a peek at the piston and if it's already scored, no need in proceeding, unless you just want to adjust the carb and see if that corrects the overheating issue. If you take the saw apart and replace parts and re-install the same carb and make tachometer adjustments the engine might do the same thing again fast. (and someone (new operator) not aware of such can destroy the saw fast by trying to force the saw to operate/cut correctly)
You might not ever notice lean and overheating when using the saw for trimming, on and off the throttle and the engine runs strong.

Yep spot on, literally 1/16 of a turn richer. My question wasn’t really about tuning, I’m happy with the process and know what to do, I was just really curious to know, why does a hot engine require more fuel to run correctly. I just don’t understand the science behind it that’s all.

 

[OM617YOTA]

Anyone have an unburned hydrocarbon tester? Wonder if a combustible gas detector would work, if applied to the exhaust. See what is getting out of the engine unburned when doing light trimming, vs in a saw that goes lean in a long hot cut.