please explain four stroking to me in laymans terms

[rusty shackel]

please explain four stroking to me in laymans terms , and what it does for the saw ?

thanks ,
rusty

 

[bitzer]

A saw four stroking is one that is tuned correctly. Out of the cut (at WOT) it has a little extra fuel running through it and kind of flutters when the fuel ratio is just right. Too little fuel and the saw sounds like it is screaming(lean). Too much fuel and it sounds rough or kind of barks(rich). You will hear the saw flutter at wide open throttle and when you put it into wood the engine should clear up and make a consistant note or sound. The way I set a saw by ear is start it rich and slowly lean out the mix with the high speed screw until it just flutters at WOT. If you turn it farther in then that it will start to scream and run lean. Its the equivalent to a four stroke engine having a little extra fuel and then cleaning up when it is doing work. Thats the way I understand it anyway. Hope this helps. I think if you do a search for saw tuning you can get some sound bites that can give you an idea of what I mean.

 

[bitzer]

Heres a site that will explain it better with the sound bite. http://www.madsens1.com/saw carb tune.htm

 

[howellhandmade]

A two stroke engine has a power stroke every revolution of the crank. When the spark ignites the mixture, the piston is pushed down by the expanding gases, which exit through the exhaust port on the piston's way down. On the piston's way back up, it picks up fuel/air mixture from the intake port, compresses it, and bang, repeat. A four stroke engine has separate compression and exhaust strokes, so each piston makes power every other revolution of the crank.

When a two-stroke engine makes the "four-stroke" sound, it just SOUNDS like a four-stroke because the mixture is too rich to ignite every single stroke without a load at high rpm, and the miss gives it another harmonic that makes it SOUND like a 4-stroke. Because you want the 2-stroke engine to have enough excess fuel for cooling, the sound of a rich miss, or 4-stroking, is desirable to hear when the engine comes off load at high rpm. If it did it all the time, you would be too rich in general. If you don't hear it ever, the engine is running too lean.

Jack

 

[spudulike]

All good explanations, just to add if your saw doesn't four stroke at the top end, the lubrication and cooling that this provides will not be present and your saw will probably fry it's piston - it will melt the exhaust side of the piston and leave a smear of aluminium on your cylinder and weld your piston rings to the piston.

You will then only have around 70PSI compression and an expensive repair bill

Not good

Spud

 

[rusty shackel]

You will then only have around 70PSI compression and an expensive repair bill


this is why i'am asking , thanks for helping me out .

rusty s

 

[scotclayshooter]

I dont think it has been explained fully by anyone yet.

My take on it is that theres too much fuel in the cylinder and its still burning when the exhaust port opens, Thats where the odd noise comes from.

When you then load the engine the piston is travelling slower and the charge has time to burn fully.

 

[WalterWhite]

Hi Rusty,
Both layman and noob here...

I bought a Poulan 4620AVX that was a Sears return and was using it after the seller had gone through the carb to get it running (close to) right. I still wasn't sure if it was right or not but felt it should have more power. I also noticed that when the saw was running out of fuel, it would really scream (*) and that indicated to me that it was probably running a bit rich. Point is, I had reason to believe it may bot be properly adjusted.

When I got the tool to adjust the carb, I leaned it out a bit and found I haditmuch more power. I had listened to the descriptions and .WAV files on four stroking but those never quite clicked with me. However, when I started cutting, I found that the saw had a kind of buzzing sound at full throttle, no load and that smoothed right out when the chain dug into the wood. That was an ah-ha moment and at that point I recognized what four stroking sounded like. I think it's the kind of thing that you'll recognize when you hear it. Someone with a well tuned saw could probably demonstrate it in a few seconds once their saw is warm.

(*) I also learned (from others here) that it is normal for a saw to speed up as it leans out prior to running out of gas. And it is bad to continue to operate at full throttle under that condition. It leads to excessive piston/cylinder heat that cannot easily dissipate when the engine stops. It is better to release the trigger at that point and allow the saw to run a bit at idle to cool it off before you shut down to refuel.

HTH,
walt

 

[sefh3]

Buy a tach. Go to Acres and get the operating RPM's and set your saw.

 

[TraditionalTool]

I dont think it has been explained fully by anyone yet.

My take on it is that theres too much fuel in the cylinder and its still burning when the exhaust port opens, Thats where the odd noise comes from.

When you then load the engine the piston is travelling slower and the charge has time to burn fully.

This is one of the simplest and understandable descriptions I've heard.

 

[scotclayshooter]

Buy a tach. Go to Acres and get the operating RPM's and set your saw.

Nope it dont work like that im afraid!

Its better to tune THEN check the rpms as a baseline for checking the tune in future.

The minute you do a muffler mod or any porting the manufacturers max rpm are worthless.

My 242XP has max rpms of 15,500 but runs best when set for 14,800

My ported 262XP 4strokes big time at 13,900 but any leaner or richer cuts slower!

If you have trouble with the madsens link and cant hear the difference's try it wearing headphones.

 

[sefh3]

Nope it dont work like that im afraid!

Its better to tune THEN check the rpms as a baseline for checking the tune in future.

The minute you do a muffler mod or any porting the manufacturers max rpm are worthless.

My 242XP has max rpms of 15,500 but runs best when set for 14,800

My ported 262XP 4strokes big time at 13,900 but any leaner or richer cuts slower!

If you have trouble with the madsens link and cant hear the difference's try it wearing headphones.

I assuming the saw wasn't modded. Once you tune it at your baseline you can tell the differene but I do it with headphones on.

 

[scotclayshooter]

I assuming the saw wasn't modded. Once you tune it at your baseline you can tell the differene but I do it with headphones on.

The tuning for peak rpms can be thrown way off at altitude.
Set a saw with a dirty air filter then clean it and it could go lean.

Theres a lot of scope for error just relying on a tach.
Im sure you would get away with it in most cases though.

 

[drkptt]

I dont think it has been explained fully by anyone yet.

My take on it is that theres too much fuel in the cylinder and its still burning when the exhaust port opens, Thats where the odd noise comes from.

When you then load the engine the piston is travelling slower and the charge has time to burn fully.

This is one of the simplest and understandable descriptions I've heard.

It is, however, not correct.

There is a limited range of air/fuel mixtures that will ignite--very rich mixtures won't burn. You can richen a two-stroke near this point and there will be occasional misfires.

A typical 2-stroke does not have perfect scavenging--there will be some residual exhaust gas trapped with the fresh mixture.

4-stroking occurs when:
-a charge near the rich limit burns.
-the residual exhaust in the next charge upsets the balance (displaces some oxygen) and causes a misfire.
-the combustion chamber is flushed by this unburned charge.
-the next charge is all air/fuel and ignites.
-the next charge misfires.
-etc.

4-stroking is a consistent fire/misfire behavior--one power stroke per every two revolutions. You can put a saw on a constant-speed dyno and richen it up under load until it will hard 4-stroke. It will make significantly less power. Measured cylinder pressure will show the fire/misfire cycle. I've been told some engines can be induced to 6-stroke, where it takes 2 revolutions to flush the exhaust residual out enough for the charge to ignite, but I've never actually observed this.

No-load WOT is not a happy place for a saw to operate. The power is so low it's only equal to the friction/windage load from the chain, fan, bearings, etc. Power at this maximum speed is low because the port timings are too restrictive (engine can't breathe) and the spark timing needs more advance. Under these conditions combustion stablility is poor; the range of acceptable lean/rich mixtures is narrower than at the peak power speed. This would make the engine start 4-stroking earlier (not as rich), but due to the unstable combustion you may not get a hard, consistent 4-stroke sound. I think this may explain why some have theorized that it's not cyclic misfire.

 

[RVALUE]

Is there a saw size too small to 4 stroke? audible?

 

[056kid]

A 4 stroking saw goes "waa naa naa naa naa naa naa naa naa" at WOT



A non 4 stroking lean saw goes "Weeeeeeeeeeeeeenn" at wot (like a 2 stroke bike jetted right..)


So to tune properly, get the saw hot, the hotter the better, open it up,
turn the H clockwise tunill the "naa naa naa" goes away, then go counter clockwise till the "naa naa naa" just comes back, now your done..



If you are curious yet, slap a new plug of correct temp range in the hot saw & hold her wide open for about 10 seconds. this is important, hit the kill switch & let off the throttle at the same time and then see what color the plug is.

hope that helped

 

[Arrowhead]

A 4 stroking saw goes "waa naa naa naa naa naa naa naa naa" at WOT



A non 4 stroking lean saw goes "Weeeeeeeeeeeeeenn" at wot (like a 2 stroke bike jetted right..)


So to tune properly, get the saw hot, the hotter the better, open it up,
turn the H clockwise tunill the "naa naa naa" goes away, then go counter clockwise till the "naa naa naa" just comes back, now your done..



If you are curious yet, slap a new plug of correct temp range in the hot saw & hold her wide open for about 10 seconds. this is important, hit the kill switch & let off the throttle at the same time and then see what color the plug is.

hope that helped

Great explanation!

 

[ty1220]

I wasn't EXACTLY sure what the guys were referring too either.
Until I tuned my 371 spot on and got it to four stroking. You will hear it when you get it there.


I like hearing it, it lets me know I don't have it too lean, the way this saw will scream I worry at times about it over- revving. I guess I keep it a bit rich mostly to guard against that.

 

[TraditionalTool]

It is, however, not correct.

Darn, it was simpler and easier than your version...

Not to worry, I can follow yours also...

4-stroking occurs when:
-a charge near the rich limit burns.
-the residual exhaust in the next charge upsets the balance (displaces some oxygen) and causes a misfire.
-the combustion chamber is flushed by this unburned charge.
-the next charge is all air/fuel and ignites.
-the next charge misfires.
-etc.

So, from what you say here, as soon as the saw is adjusted to provide less fuel, it will burn what is in the cylinder, hence quit the 4 strokin'...that makes sense.

4-stroking is a consistent fire/misfire behavior--one power stroke per every two revolutions.

I guess that does make sense since a 4 stroke only fires once every other stroke.

No-load WOT is not a happy place for a saw to operate.

I think this is one reason why many folks claim to only tune the saw in the cut.

 

[056kid]

No load wot wont hurt your saw.

people just think it will.....