What exactly is 4 stroking??

[matt9923]

What is the advantage of 4 stroking? Is this an advanage? Or is this just tuning up the saw?
you have to forgive me as I am learning also..

Fish good to see you back in form!!!

You could lean it out and it wont 4-stroke, You will burn up the saw. If its 4-stoking its limiting the rpm to a safe zone so you don't blow up the saw.

...... sorta.

 

[alpinecrick]

if a 2 stroke has a power stroke very time the piston reaches top dead or there abouts then logic wood have it 4-stroking wood be a misfire every other turn such as a 4 stroke with valves does.(the ex stroke not missing)
some exhaust gasses are pulled back into the intake mixture which displaces some available oxygen causing a mis fire when not under load.The slightly rich mixture causes this and will be correct when under load.
someone will correct me if this is wrong

Well, I for one think ya' did good with that explanation........

The sound four stroking makes is exactly how a hotrod will sound when running a bit too rich, it splutters at top rpm with wide open throttle with no load. In a two stroke, with WOT the slight splutter (too rich) goes away under load.


Casey

 

[scotclayshooter]

What is going on inside the saw to make it produce this unique sound???

Also, I have not attempted to try tuning my own yet, because I have difficulty distinguishing this sound. I've listened to the Madsen's link and it's still not very clear to me are there any vids available to compare the 4 stroking sound to one that is running lean???

Listen to the link again but with headphones on, It makes the sound much much clearer.

 

[angelo c]

Well, I for one think ya' did good with that explanation........

The sound four stroking makes is exactly how a hotrod will sound when running a bit too rich, it splutters at top rpm with wide open throttle with no load. In a two stroke, with WOT the slight splutter (too rich) goes away under load.


Casey

Casey,
Can you explain how loading the saw leans it? Something with the reduction in RPM ? haven't been able to grasp this yet.

thanks
Angelo

 

[FATGUY]

Listen to the saw in the wood and when it comes out of the wood. The saw should be be 4 stroking slightly before entering the wood, 2 stroking in the wood but go immediately back to 4 stroking when it exits the wood. If you really gotta lean on it to keep it from 4 stroking, it's too rich, if it's not four stroking at all, you're lean.

 

[angelo c]

Listen to the saw in the wood and when it comes out of the wood. The saw should be be 4 stroking slightly before entering the wood, 2 stroking in the wood but go immediately back to 4 stroking when it exits the wood. If you really gotta lean on it to keep it from 4 stroking, it's too rich, if it's not four stroking at all, you're lean.

NIk,
I can hear it. I can tune to it. But I don't understand where in relation to "peak" power the burbling is occuring. I can't separate all the Lean of Peak operations in the airplanes from the 2 stroke theory in the useless cavity of a brain I have on my shoulders.
I guess I need a graphic represention of the power curve in relation to peak or 4 stroking. And a Beer.

thanks though.
A

 

[ZeroJunk]

Listen to the saw in the wood and when it comes out of the wood. The saw should be be 4 stroking slightly before entering the wood, 2 stroking in the wood but go immediately back to 4 stroking when it exits the wood. If you really gotta lean on it to keep it from 4 stroking, it's too rich, if it's not four stroking at all, you're lean.

I like that. Good practical explanation.

 

[GASoline71]

Ho-humm... opcorn:

http://www.madsens1.com/saw carb tune.htm

Gary

 

[FATGUY]

NIk,
I can hear it. I can tune to it. But I don't understand where in relation to "peak" power the burbling is occuring. I can't separate all the Lean of Peak operations in the airplanes from the 2 stroke theory in the useless cavity of a brain I have on my shoulders.
I guess I need a graphic represention of the power curve in relation to peak or 4 stroking. And a Beer.

thanks though.
A

If I'm not mistaken, burbling occurs because of the presence of too much fuel. If this it the case, I would say and engine is begining to four stroke just before peak power.

 

[angelo c]

Ho-humm... Gary

Gary,

Something like this for a 2 stroke was what I was asking for. might be tough to actually produce considering the limited instrumentation available but thought I would ask.

Humm-ho back at ya !

A

 

[Jtheo]

There is some good information here for a guy that don't all that much, me.

But I do want to learn to keep my saws properly tuned. Thanks guys.

 

[ol'homey]

Angelo,
See if you can find a chart with cylinder pressure related to EGT and A/F ratio. The engine is your chart is under the constant load of the prop, whereas chainsaws go from unloaded to fully loaded. A properly tuned unloaded 4 stroking chainsaw at WOT might have an A/F of say 11-12:1 which is much too rich for complete combustion so the EGT is low due to the cooling effect of the unburned fuel and the RPM's are off peak due to the inefficient combustion. Drop the saw into the wood (now the engine is loaded) and the combustion pressure rises dramatically. The 11 or 12 AF that wouldn't burn very well at the lower pressure now burns almost completely at the higher pressure and gives max HP. EGT goes up due to more complete combustion leaving less unburned fuel available for cooling.
A lot of folks think max unloaded rpms= max loaded rpms and therefore faster chain speed. So they tune their saw to scream at max unloaded rpm with an AF of say 14:1. Then they drop the saw into the wood, the pressure rises and near perfect combustion occurs so there's no unburned fuel left over to cool the cylinder, EGT soars, HP drops. After a while the saw starts running like crap and the poor guy is baffled as to why so he takes it to THALL who pops the muffler off and thinks KACHING! another scored piston, that's the 3rd one today as he shows the guy the damage.

 

[angelo c]

ol' homey,
thanks for the explanation. I think I understand 'what' is happeneing just maybe not why or how. in a 4 stroke it's not the "cooling" effect of the fuel that matters most it's the timing of the flame front ignition in relation to the combustion stroke. the excessive fuel allows for delayed ignition of the unburned fuel and 'Slows' the burn. that's what the lead does, slows the flame front progression accross the combustion stroke( in effect cooling)

so it's richest at peak rpm which is burbling( 4 stroking) and when we load it in the cut that it leans to a more efficient burn ratio. in effect leaning on the saw or loading it up excessively is where the wood makes the saw go KABOOM because it's too lean a ratio and then the visit to Virginia ( Tommy's little Shop of Horrors)...sorry Tommy that we drug you in to this discussion.

if I got that right I am good. thanks.
A

 

[aldo]

Is it a wise move to do this tuning with the bar and chain off?

 

[tdi-rick]

Timberwolf's explanation from a previous discussion on another board on the specifics of four stroking.

"I don't think it is a misfire, that is a much more pronounced event.

What I think, though could well be wrong is going on is that as soon as the engine gains enough RPM for the given mixture the piston actually out runs the expanding charge and there is not the same pop when the port opens. As a result full burn and scavenging is hurt and the saw slows ever so slightly though it would not be a total misfire. The next cycle the incoming charge volume will be slightly lower due to losses in scavenging and RPM leading to a slight leaning of the mixture, and with the lower RPM you get a full burn the next time. The cycle then finds a balance of partial burns on 3rd, 4th, 5th... cycles."

 

[Moss Man]

Wooooosh.......

That was the sound of some of that techno stuff flying right over my bald melon!

 

[redunshee]

Listen to the saw in the wood and when it comes out of the wood. The saw should be be 4 stroking slightly before entering the wood, 2 stroking in the wood but go immediately back to 4 stroking when it exits the wood. If you really gotta lean on it to keep it from 4 stroking, it's too rich, if it's not four stroking at all, you're lean.

That's how I adjust carb! I also put a tach on it to see what the rpm's are. Finally to ease my mind I pull plug and check color. I do this after revving saw while at wot and shutting it off. If plug is a light brown I'm good to go.
Bob

 

[WoodViking]

Tach's?

Angelo,
See if you can find a chart with cylinder pressure related to EGT and A/F ratio. The engine is your chart is under the constant load of the prop, whereas chainsaws go from unloaded to fully loaded. A properly tuned unloaded 4 stroking chainsaw at WOT might have an A/F of say 11-12:1 which is much too rich for complete combustion so the EGT is low due to the cooling effect of the unburned fuel and the RPM's are off peak due to the inefficient combustion. Drop the saw into the wood (now the engine is loaded) and the combustion pressure rises dramatically. The 11 or 12 AF that wouldn't burn very well at the lower pressure now burns almost completely at the higher pressure and gives max HP. EGT goes up due to more complete combustion leaving less unburned fuel available for cooling.
A lot of folks think max unloaded rpms= max loaded rpms and therefore faster chain speed. So they tune their saw to scream at max unloaded rpm with an AF of say 14:1. Then they drop the saw into the wood, the pressure rises and near perfect combustion occurs so there's no unburned fuel left over to cool the cylinder, EGT soars, HP drops. After a while the saw starts running like crap and the poor guy is baffled as to why so he takes it to THALL who pops the muffler off and thinks KACHING! another scored piston, that's the 3rd one today as he shows the guy the damage.

So tuning with a tach is for guidance only? What I read out of this is it is recommended to tune by ear - and then maybe use the tach for double checking that you are not to far off recommended max rpm's?

Or would it be smarter to figure out at what rpm's the saw perfoms the best when in the wood - and then tune with a tach for that? - say if the max rpm out of the wood is 13000 but it perfoms at its best at 9000 rpms in the wood then you could just tune in wood and allways for the 9000 - or is this simplificating (is that a word?) things?

 

[redunshee]

Is it a wise move to do this tuning with the bar and chain off?

No! No!

 

[redunshee]

So tuning with a tach is for guidance only? What I read out of this is it is recommended to tune by ear - and then maybe use the tach for double checking that you are not to far off recommended max rpm's?

Or would it be smarter to figure out at what rpm's the saw perfoms the best when in the wood - and then tune with a tach for that? - say if the max rpm out of the wood is 13000 but it perfoms at its best at 9000 rpms in the wood then you could just tune in wood and allways for the 9000 - or is this simplificating (is that a word?) things?

I set rpm's out of the wood and try to keep at or near manuf specs. Its more for an "ease of mind thing" as I first tune by ear. Tach is to confirm my tuning steps.