High Quality Chainsaw Bars Husqvarna Toys

Chainsaw Porting Theory

huskihl

Muh fingers look really big
GoldMember
Local time
6:10 PM
User ID
360
Joined
Dec 31, 2015
Messages
22,455
Reaction score
143,003
Location
East Jordan, MI
Country flag
Four stroking is when a 2 cycle is gagging on too much fuel. As soon as you apply a little bit of load it needs more fuel, hence cleaning up in the cut. Amount of work being done determines how much added fuel is needed. That's kind of how I look at it
 

shorthunter

Super OPE Member
Local time
5:10 PM
User ID
637
Joined
Jan 21, 2016
Messages
288
Reaction score
676
Location
West Central Missouri
Four stroking is when a 2 cycle is gagging on too much fuel. As soon as you apply a little bit of load it needs more fuel, hence cleaning up in the cut. Amount of work being done determines how much added fuel is needed. That's kind of how I look at it

Gagging
gagging
/ˈɡaɡiNG/
adjective
  1. characteristic of choking or retching.
    "Angela made a gagging noise"

:b1:
 

Moparmyway

Its just a saw
GoldMember
Local time
6:10 PM
User ID
21
Joined
Dec 4, 2015
Messages
5,222
Reaction score
27,996
Location
In a meeting
Country flag
What's in the crank case may or may not burn, but its pressure can escape through the transfers.
And push back the incoming charge that’s being pushed in by the piston ?

I imagine most of the mix is burned as it blows into the combustion chamber.
If this were happening, would you still have a motor that runs ?
 

Wonkydonkey

Plastic member
Local time
10:10 PM
User ID
3189
Joined
May 14, 2017
Messages
4,564
Reaction score
18,063
Location
Sussex, UK.
Country flag
Surly , 4 stroking is 4play.:rolleyes:. no really :D,,, thinking about it 4strokin is the mix is rich, we know this cos revs climb if lean.... so it’s a ratio of fuel and air... more fuel than optimal, but not lean... stick saw in a cut and fuel has a bit more time to burn and cleans up...

This is also dependent on fuel type and oil ratio and altitude...

I ask myself,,, how does mtronic work it all out.... the answer is there somewhere , stihl has worked it out.

Well I think they have...o_O
 

Chainsaw Jim

Con Artist LLC
Local time
3:10 PM
User ID
836
Joined
Feb 8, 2016
Messages
4,014
Reaction score
77
Location
Springfield Oregon
When free revving you get unrestricted flow, where the vacuum pulling fuel from the carb is at its strongest, which lets it gag on fuel or misfire every other stroke (4 stroking). When under load the rpm level decreases enough to lessen the vacuum on the carb and allows it to clean up.
 

Nutball

Here For The Long Haul!
Local time
5:10 PM
User ID
7732
Joined
Oct 31, 2018
Messages
4,038
Reaction score
11,009
Location
Mt. Juliet, TN
Country flag
And push back the incoming charge that’s being pushed in by the piston ?
I think you misread me.


If this were happening, would you still have a motor that runs ?
Yes, because it would only happen every other cycle not every.


I'm just trying to understand exactly how a 2 stroke can misfire when its getting the same quality mix every stroke. Maybe a rich mix won't burn well at all with a little exhaust mixed in, but burns very well with leftover rich mix from a failed previous cycle. I don't know. I do agree with the richening effect of max unloaded rpm, but what's the exact process going on with firing every other stroke?
 

huskihl

Muh fingers look really big
GoldMember
Local time
6:10 PM
User ID
360
Joined
Dec 31, 2015
Messages
22,455
Reaction score
143,003
Location
East Jordan, MI
Country flag
I think you misread me.



Yes, because it would only happen every other cycle not every.


I'm just trying to understand exactly how a 2 stroke can misfire when its getting the same quality mix every stroke. Maybe a rich mix won't burn well at all with a little exhaust mixed in, but burns very well with leftover rich mix from a failed previous cycle. I don't know. I do agree with the richening effect of max unloaded rpm, but what's the exact process going on with firing every other stroke?
They can be leaned out to where they 2 stroke. But that's too lean to put a load on it. More load, more fuel
 

Nutball

Here For The Long Haul!
Local time
5:10 PM
User ID
7732
Joined
Oct 31, 2018
Messages
4,038
Reaction score
11,009
Location
Mt. Juliet, TN
Country flag
Makes sense, that's why I tune them in a long cut if possible, though not practical, so I use 4 stroking to tell where the tune is in the cut and where to go. Some people seem to want to tune to get max 2 stroking unloaded rpm. Maybe they want to use the flywheel as a blower or the chain as an oil applicator.
 

bwalker

Pinnacle OPE Member
Local time
6:10 PM
User ID
523
Joined
Jan 12, 2016
Messages
1,403
Reaction score
3,166
Location
UP
I have had some saws that run strongest on the edge of lean, and others that pull harder a little too rich. I can't explain the "whys", I just try to set them to what makes em happy.
Four striking is a terrible way to tune IMO. Find the rpm that they cut the best at record and then tune to that point. Assuming your load stays more or less the same.
 

Terry Syd

Pinnacle OPE Member
Local time
9:10 AM
User ID
575
Joined
Jan 18, 2016
Messages
1,589
Reaction score
5,973
Location
Comboyne, NSW Australia
Country flag
I have had some saws that run strongest on the edge of lean, and others that pull harder a little too rich. I can't explain the "whys",

I expect it has to do with the two fuel curves of the carb. You have the low speed circuit in the straight section of the carb and then the high speed circuit in the venturi. Two different fuel curves that combine.

Getting the two curves to work together to get the best powerband is what some of the tuners on this forum are doing. For the average bloke that isn't going to go redesigning the circuits of a carb, he's limited to tweaks of the needles to get the best he can get from the stock carb.
 

XP_Slinger

They’re Just Saws
Local time
6:10 PM
User ID
845
Joined
Feb 9, 2016
Messages
6,089
Reaction score
27,519
Location
Central NY
Country flag
I have had some saws that run strongest on the edge of lean, and others that pull harder a little too rich. I can't explain the "whys", I just try to set them to what makes em happy.
Same here Mike. In my experience big saws like to run a little fat for max power while smaller saws seem happier at max rpm tune. I also wonder about that. Then there’s always a saw that doesn’t behave the way you expect it to.
 

Darryll

Well-Known OPE Member
Local time
11:10 AM
User ID
1886
Joined
Sep 23, 2016
Messages
73
Reaction score
132
Location
New Zealand
Country flag
Thanks to all who contributed to this thread. I picked up a couple of nuggets in among all 63 pages. I have the next victim sitting on the bench beside me, my excel spreadsheet dialed in and finished designing all the ports in Cad. Time to start machining and grinding.
 

Darryll

Well-Known OPE Member
Local time
11:10 AM
User ID
1886
Joined
Sep 23, 2016
Messages
73
Reaction score
132
Location
New Zealand
Country flag
This was the least amount of time I have ever invested in porting a saw. Did not cut the chamber so I still have that trick up my sleeve. Muffler mod was already done. Decked the cylinder for .6mm squish. Calculated all the port areas and crunched the numbers as per Jenning calcs but instead of using 11000-12000 as my target rpm I worked out all the port time areas biased on an ex of 107 which was the height I got after dropping the cylinder. Adjusted the transfers a little as per calc and left the Intake completely alone as the clacs said it was good.

This is by far the strongest 372 I have ever built. Had I not read all of this thread I may not have given the less is more approach ago.

It seems not right not moving the transfers more or not opening up the intake but it's running so strong I'm a little nervous about touching it. I'll put it to some hard use and see how it goes in the real world and decide if I want to go back in and have another go.

Once again thanks to all.
 

Nutball

Here For The Long Haul!
Local time
5:10 PM
User ID
7732
Joined
Oct 31, 2018
Messages
4,038
Reaction score
11,009
Location
Mt. Juliet, TN
Country flag
Four stroking is when a 2 cycle is gagging on too much fuel. As soon as you apply a little bit of load it needs more fuel, hence cleaning up in the cut. Amount of work being done determines how much added fuel is needed. That's kind of how I look at it

Gagging? More work = more fuel? Simple terms are good, but when I argue a point I try to include all necessary details to prove the general concept. That's where repetition of simple terms in an argument bugs me, because it's like the same exact explanation is supposed to change my opinion when no new supporting facts are presented. So, then I try to induce a more detailed response by representing my arguement in further detail to either convince, or to provide the precise details to be countered to convince me of something different.

Back to my 4 stroking theory, but first my response to More work = more fuel

Gas engines are not like electrics. They don't efficiently use as much fuel as is needed, they use as much fuel as you give them: whatever the carb needles are set to, and whatever volume of air the current throttle position allows, of course according to rpm as well.

Electric motors are kind of self governing in a way. Open the throttle circuit to max (full bore of solid wire / lowest resistance or 100% on duty cycle) and at 0 rpm the motor is drawing max power to push the magnets. The faster the rotor spins, the less it load it sees like a torque converter, until max rpm is reached. If there is no load at max rpm, other than various frictions, the load is minimal on the motor, it doesn't have to push the magnets as hard because they are nearly matching the speed vector of the electro magnet (I really wish I knew enough about them to explain more precisely), and so minimal amperage (fuel volume) is drawn. Spin the motor as fast as it wants to go, and it will draw no power at full throttle. Spin the motor faster than physics says it should go, and it will output power back into the battery.

Run a gas engine without load, and it pulls the same volume of fuel per revolution (actually rpm dependent based on several variables, but the example is plenty true enough). How much power is it putting out? Well, the gas is releasing most of it's potential energy depending on how clean it burns, *but considering the limited rate at which gas burns, and engine rpm, and richening of the mixture due to high rpm*, the power output at the shaft is lower than it would be under load.

Run a gas engine under load, and it does not draw any more fuel. Technically it should draw less volume over time due to a lower rpm than no load revving, but still roughly the same volume per stroke, a little on the leaner side now that the high rpm isn't demanding too much CFM through the carb.

As for more work = more fuel, I agree 2 stroking during a no load rev is too lean for loaded use, so use more fuel for more work, but as for 4 stroking cleaning up to 2 stroking in the cut, I have some pictures to better represent my theory.

Imagine the fuel air mixture burning at the exact same speed no matter the rpm.
upload_2020-7-30_1-39-5.png

You can look at the picture in reverse for the effect of 4 stroking cleaning up into 2 stroking as load increases and rpm decreases. It all has to do with the burn speed of the fuel air mix relative to the time that the transfers are shut base on rpm.

* * = more detail can be given if necessary.
 
Last edited:

Sawrain

Pinnacle OPE Member
Local time
8:10 AM
User ID
2614
Joined
Feb 11, 2017
Messages
302
Reaction score
1,263
Location
South.
Country flag
Interesting post, I like talking things out as well.


Four stroking is when a 2 cycle is gagging on too much fuel. As soon as you apply a little bit of load it needs more fuel, hence cleaning up in the cut. Amount of work being done determines how much added fuel is needed. That's kind of how I look at it

I put this one down to the fact that as soon as load is applied to a four stroking saw rpm drops, then.

1) The decreased rpm brings the saw into an area of increased volumetric efficiency, so the fresh air charge mass per cycle is increased, drop rpm enough and there will be sufficient scavenging/air to support normal combustion.

2) Less so maybe? change in air flow will bring carburettor curve to a leaner area, well more out of a rich area, again helping to resume normal combustion.

Sounds like you know about about induction motors, if you haven’t, look up back emf, it is the fact that the closer the motors rotor is to to the suppply/stator frequency the stronger back emf force it produces is, opposing the stator, so it in effect suppressor the current flow that created it in the first place.
 
Last edited:

cus_deluxe

terms of service violator
Local time
6:10 PM
User ID
393
Joined
Jan 3, 2016
Messages
5,195
Reaction score
24,216
Location
Michigan
Country flag
Sometimes you get shamed for trying to be precise & specific about the small things, making sure the small details are understood, also a great way of testing your own theories by exposing your complete ideas on a subject.
not trying to shame anybody, i just think that its not as complicated as the previous post implies.
The decreased rpm bring the saw into an area of increased volumetric efficiency
in my view, thats about as concise and accurate a statement as can be made.

Run a gas engine under load, and it does not draw any more fuel. Technically it should draw less volume over time due to a lower rpm than no load revving, but still roughly the same volume per stroke, a little on the leaner side now that the high rpm isn't demanding too much CFM through the carb.
this is assuming that the volume of air passing the venturi is already pulling as much fuel as possible through the main nozzle, when in reality, we are restricting the amount of fuel via the h-screw in this example. so even when the rpms drop a little under load, the carburetor still supplies the same amount of fuel, to a certain point.
 

Sawrain

Pinnacle OPE Member
Local time
8:10 AM
User ID
2614
Joined
Feb 11, 2017
Messages
302
Reaction score
1,263
Location
South.
Country flag
not trying to shame anybody, i just think that its not as complicated as the previous post implies.

I didn’t mean to direct that at you, your post came in between Nutball and mine while I was typing.

I deleted that part as soon as I saw your post to try and avoid this, oh well, guess I can put it back now.
 
Top