About 4 stroking

[Buffco]

I've read a lot on the muffler mod threads and tuning threads....

I see a lot of mention about a chainsaw "4 stroking". Can someone explain why that is ideal or not, and how can you tell when the saw starts doing it?

I'm assuming there's a sound you listen for.....

 

[striperswaper]

do a search on here there is plenty, vids w/ sound...

 

[jus2fat]

The term is used as the sound of the saw while idling and warmed up and being used to cut. It's about the high speed setting adjustment. In tuning the carb, one would set the low speed adjustment first...just ever so slightly rich. This would be while at normal idle. Then you set the high speed while saw throttle is wide open (WOT)...Turn it clockwise (lean) until you get a high smooth rev...then turn it back counter clockwise just until you hear it change to a slight "gurgle" sound that is referred to as "four-stroking". It's not real smooth as when it was at high revving. Again, it is being set just ever so slightly rich. Now you turn the screw back toward clockwise (lean) until the carb just clears out again.
Now if you did this correctly...when you pull saw out of a cut and drop saw back to idle it will again "gurgle-four stroke"--but when you start another cut the carb will clear back out and rev smooth but is still set on the rich side.
The high speed needs to be set rapidly as you don't want to be holding the saw at WOT while not in a cut but for only a few seconds!!!!!!

This will explain it perhaps better for you than I did....

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


J2F

 

[bullittman281]

Hello,
I set my saw up so that as soon as I start cutting the engine "cleans up" and sounds really crisp. If its not in the cut I have it rich enough to burble. The best way to hear the change that the saw makes is to fire it up and get the high setting very rich. The saw will run poorly and sound rather rough. Start bringing the Hi screw in and the revs will come up and eventually you will find max revs and the engine sound will clean up. DO NOT set it for max revs! From the clean sounding max RPM as you turn the Hi screw out the engine note will make a distinctive change. That change is when the saw gets rich enough to start four stroking. At this point your fairly close to where you need to be. If in doubt make it a fuzz richer. your clue to being way too rich is the saw will try to four stroke in the cut. As soon as you start cutting the engine should clean up and sound crisp. If it still sounds rough you are way too rich. To my knowledge rich only fouls plugs and mufflers. I don't think that rich will cause any real engine trouble with in reason.

True story here. This fall I was helping a bunch of coworkers clear out a bunch of elm trees. One of them drug along their 372xp Husqvarna or something like that. They fried it cutting up this massive stump from not setting the carb for conditions. I had to fire up my PP 295 to finish the job because it had the longest bar on a running saw. I ran several tank fulls through the poulan at wide open throttle cutting this thing up. Thats wide open for and entire tank of fuel, not ever letting off. This stump was Huge! I set my carb up for the conditions of the day and keep and ear to it in case something changes. Talking with the guy later on I found out that he just stuck a replacement carb on it and ran it, never turning an adjustment screw. Somehow the carb was set close enough to at least run. He got away with it cutting up at high elevation but at 4,000Ft it killed his saw. DON'T let them run lean and you'll be good to go.

 

[bitzer]

Hello,
I set my saw up so that as soon as I start cutting the engine "cleans up" and sounds really crisp. If its not in the cut I have it rich enough to burble. The best way to hear the change that the saw makes is to fire it up and get the high setting very rich. The saw will run poorly and sound rather rough. Start bringing the Hi screw in and the revs will come up and eventually you will find max revs and the engine sound will clean up. DO NOT set it for max revs! From the clean sounding max RPM as you turn the Hi screw out the engine note will make a distinctive change. That change is when the saw gets rich enough to start four stroking. At this point your fairly close to where you need to be. If in doubt make it a fuzz richer. your clue to being way too rich is the saw will try to four stroke in the cut. As soon as you start cutting the engine should clean up and sound crisp. If it still sounds rough you are way too rich. To my knowledge rich only fouls plugs and mufflers. I don't think that rich will cause any real engine trouble with in reason.

True story here. This fall I was helping a bunch of coworkers clear out a bunch of elm trees. One of them drug along their 372xp Husqvarna or something like that. They fried it cutting up this massive stump from not setting the carb for conditions. I had to fire up my PP 295 to finish the job because it had the longest bar on a running saw. I ran several tank fulls through the poulan at wide open throttle cutting this thing up. Thats wide open for and entire tank of fuel, not ever letting off. This stump was Huge! I set my carb up for the conditions of the day and keep and ear to it in case something changes. Talking with the guy later on I found out that he just stuck a replacement carb on it and ran it, never turning an adjustment screw. Somehow the carb was set close enough to at least run. He got away with it cutting up at high elevation but at 4,000Ft it killed his saw. DON'T let them run lean and you'll be good to go.

Yep, always check your saw for tune after its warm especially if conditions are different from the last time it was run. It only takes a few seconds, just lift it out of the cut and drop it back in and it will save a major PITA situation later.

 

[bitzer]

When four stroking at WOT off load there is a little extra fuel that is not being used, when the saw is placed into the cut the fuel is used. If the extra fuel where not there the saw would be too lean in the cut and over heat.

 

[Brufab]

Great info and well explained thanks! I always wondered what that was. I will need to pay attention next time I run my old saws.

 

[Backstage]

Benefit of four stroking is that there’s too much fuel for a given load.
Understand that “four stroking” is when the engine can’t ignite the because there’s too much fuel there, so it skips a beat.

Consider that the throttle works exclusively by controlling air flow.

Obviously you want ideal fuel/air mix under load at WOT. But if it’s running just rich enough that there’s always a bit extra fuel at idle and WOT no load, you’ve got that extra ready to go in the cylinder when you pull the trigger. Remember what we said about the throttle? More fuel finally gets more air. Immediate power and throttle response.

 

[Old2stroke]

when you pull saw out of a cut and drop saw back to idle it will again "gurgle-four stroke

Not exactly right. You want it to 4-stroke at full throttle as soon as you take the load off the engine and clean up again as soon as you load it down again. 4-stroking at idle is no indication of correct high speed adjustment.

 

[Huskvarna hotellgäst]

I prefer the term "multi-stroking" to four-stroking.
It's what happens when a saw with a simple 2-stage carb (Walbro, Tillotson, Zama, etc) is running at a constant speed at full throttle without a load, because not all the fuel mixture is able to be burnt.
However, at idle, it might skip firing because the mixture is lean.

 

[rahtreelimbs]

Tuning a saw to 2 stroke/4 stroke is no big deal. Making sure you have full adjustment of the carb set it rich. With the saw warmed up start cutting. Lean the saw out a little until it burbles no load and smoothes out under load. After I get it tuned I take a quick no load rpm reading for future reference.

 

[rahtreelimbs]

This is a reasonable explanation.

https://en.wikipedia.org/wiki/Four-stroking?wprov=sfla1

 

[bwalker]

Tuning a saw to 2 stroke/4 stroke is no big deal. Making sure you have full adjustment of the carb set it rich. With the saw warmed up start cutting. Lean the saw out a little until it burbles no load and smoothes out under load. After I get it tuned I take a quick no load rpm reading for future reference.

Taking the no load reading after tuning is smart and it's what I do.
I basicly do the same thing except I use the cut and feel method instead of listening to the way the engine sounds. My hearing is pretty bad and with some engines especially with muffler mods I cant hear 4 stroking.
The thing to realise here is that a 2 strokes fuel requirements loaded and unloaded are different. The harder you load the saw the more fuel you need.

 

[Brufab]

Im I 4 stroking in the video? Trying to figure out if my saws are tuned correctly

 

[huskihl]

Im I 4 stroking in the video? Trying to figure out if my saws are tuned correctly

No, at 46 seconds (among others) you can hear it 4 stroke a little bit as you finish or start a cut. You are probably a hair leaner than what the how the saw was designed to run, but pretty close.

 

[Brufab]

No, at 46 seconds (among others) you can hear it 4 stroke a little bit as you finish or start a cut. You are probably a hair leaner than what the how the saw was designed to run, but pretty close.

Thanks huskihl. I appreciate the feedback. I will richen it up a tiny bit. Thanks again!

 

[Patrick1903]

Great info and well explained thanks! I always wondered what that was. I will need to pay attention next time I run my old saws.

Glad to see there are others that like to reopen really old threads besides me! . This is something I’m trying to understand and get better at (tuning). Have you watched the Tinman video on tuning (YouTube)? I posted a link to a video a couple months ago where I was trying to get opinions on how my tuning sounded. Feel free to comment on that!

 

[Brufab]

That's funny, I have been known to revive threads pre 2009. There's so much info on here that is helpful. That tinnmann guy is super smart. I have watched alot of his videos.

 

[Patrick1903]

That's funny, I have been known to revive threads pre 2009. There's so much info on here that is helpful. That tinnmann guy is super smart. I have watched alot of his videos.

I agree. I see a ton of threads that are very interesting to me, I read 40 messages, then find out there was no resolution/closure! Or I have the same question and don't think the OP from 10+ years ago would mind if I hijack the thread...I like the info (from old and new) to be consolidated in one thread.

This is the Tinman tuning video I watched that was helpful.

 

[bullittman281]

Holy thread revival batman!!!! I ain't seen this thread in years........ Its funny seen an old post revived. Keep up the good work and keep them saws tuned!!!!

Bullittman