question for a mechanic about muff mods

[Chris-PA]

If a carb had no limits on the amount of fuel it could pass through it (passage size, length of time the passage is open, pressure of the fuel in the carb), then yes, it would pick up an increased amount of fuel, the ratio would stay constant, and the saw would not need to be retuned. But changing the air flow only affects the pressure differential bw the venturi and the fuel passage. It doesn't change the size of the passage or how long it is open. The increase in pressure will deliver a limited amount of additional fuel, not proportinal to the additional air flowing through the saw, and WILL result in a leaner tune.

There are limits to the max air the venturi can pass and the max fuel the jet can pass (regardless of pressure drop), but the carb is nowhere near either of them. These carbs have no air corrector jets and produce a MUCH richer mixture with increasing air velocity. That is the behavior of a simple uncorrected fixed-jet venturi - it will give a fuel output that is roughly the square of the air velocity. That is what causes 4-stroking and why the engine reaches a terminal no-load WOT rpm that can be changed merely by changing the needle (which is the jet).

A carb responds only to throttle position (irrelevant at WOT) and air velocity. It does not know about load or rpm or anything else. Somewhere at the bottom end there isn't enough air velocity to draw fuel from the jet, and somewhere up top either the venturi or the jet will saturate (the saw would blow up long before it could pump that much air), and between those points the carb puts out a fuel/air mixture that gets richer with increasing air velocity.

 

[nmurph]

Yes, it gets richer with the increased velocity...I said that.

Folks, Chris and I are just going to have to disagree. I know a carb almost always has to have the H needle opened up whether you do a muffler mod or port a saw. If don't, you will lean the saw too much and score the piston.

 

[HuskStihl]

Yes, it gets richer with the increased velocity...I said that.

Folks, Chris and I are just going to have to disagree. I know a carb almost always has to have the H needle opened up whether you do a muffler mod or port a saw. If don't, you will lean the saw too much and score the piston.

OK so I have to know who's right! Neal is a university professor, but is likely still on Hydrocodone. Chris seems smart, but changed his username, indicating shiftiness.
Iocane powder, I'd bet my life on it

 

[HuskStihl]

My 385 needed to CCW after a MM. Surprisingly (to me at least) my 323L (I know, I know.....) had to go clockwise after gutting out the cat muffler. Like more than just a little.

 

[nmurph]

Nope, I can't stand the professorial types. Remember the old saying: Those that can't do, teach. Sometimes we can't dumb our internal documents down enough so the facutly can follow them. I mean simple A,B,C,...1,2,3...seriously, I ask my 10YO if he understands letters I send out just to make sure they are clear. If he gives me the thumbs up, I'm fairly certain at least a few academics won't be able to follow them!!

BTW, it's oxycodone. Hydrocodone makes me a little nauseous...still have to take a 1/2 tab to get eased off so I can sleep...just a little woozey first thing in the morning. By 6pm I'm clear-headed and can take care of the students...oh, that's when I leave...maybe it's not the meds!!

 

[RedFir Down]

A chainsaw carb responds to increased air flow (air velocity really) by delivering a richer fuel/air mixture. Just lifting in the cut raises the rpms and air velocity enough to make it so rich it misfires. At WOT the carb cannot tell the difference between higher rpms or a muffler mod - all it responds to is air velocity. Therefore if your saw was properly tuned before the mod, AND if the muffler mod was effective (i.e. it increased the air flow), then you will likely need to LEAN out the H needle to compensate.

If your carb has no accelerator pump (most), then you may need to richen up the L needle. Without a pump the L circuit must provide the extra fuel for acceleration (which is why you need to set the idle richer than optimum), and with the improved air flow you may need more for proper throttle response.

Strato engines do not seem to be as sensitive.

Im not trying to stir the pot here but Chris can you tell me where you have learned about this theory?
Website? Book? I would like to look into this.

 

[Chris-PA]

Neal knows we can disagree on technical points and that is never any reason for it to lead to personal animosity. Hopefully the discussion will help others to understand how these things work.

BTW - I would never recommend not tuning after a change. Heck, you often have to tune for seemingly no change at all. I just don't want people to think there is a set rule for what to do with the adjustments after some semi-random change to the pumping characteristics of the engine.

 

[nmurph]

Neal knows we can disagree on technical points and that is never any reason for it to lead to personal animosity. Hopefully the discussion will help others to understand how these things work.

BTW - I would never recommend not tuning after a change. Heck, you often have to tune for seemingly no change at all. I just don't want people to think there is a set rule for what to do with the adjustments after some semi-random change to the pumping characteristics of the engine.

Oh, don't think we can kiss and make up that easily

 

[Chris-PA]

Im not trying to stir the pot here but Chris can you tell me where you have learned about this theory?
Website? Book? I would like to look into this.

Some of it is from reading about the properties of venturis (there is some decent info on-line, but also a lot of pure nonsense info about carbs out there too). A little from fluid dynamics basics from way back in school. Most came from tuning Weber carbs for my modified Fiats and for my wife's SCCA race car. Webers have fully adjustable fuel and air jets, and a you can pick the two independently to get whatever variation of fuel/air ration vs airflow that you want. Once upon a time I had a book with a really good explanation of how that worked, but the last time this came up I looked through my stuff in the storage room in the barn and could not find it. I will look again, but some of that stuff was given away after kids and home ownership.

Unfortunately everything I see on line says that the air corrector jets are to improve atomization of the fuel, but this is not their primary purpose - it is to correct the natural characteristics of a fixed venturi/fixed jet. When I first started using chainsaws more often I could not figure out why my Mac kept misfiring and why I could not fix it. In trying to understand what was wrong with it I finally looked into the operation of these little runt all-position carbs. I could not find the air corrector jets on the diagrams, or on the carb, and I finally realized they were not there. After that the behavior of the saw made sense - in kind of a pathetic way.

The manufacturers are counting on the super rich fuel mixture at high air velocities to act as a crude max-rpm limiter, and to use a guide for setting fuel mixture (on a stock saw).

EDIT - That info may even have been in the Haynes Weber Carburetor Manual, which is one of the ones I'm missing (so I can't check).

 

[RedFir Down]

Some of it is from reading about the properties of venturis (there is some decent info on-line, but also a lot of pure nonsense info about carbs out there too). A little from fluid dynamics basics from way back in school. Most came from tuning Weber carbs for my modified Fiats and for my wife's SCCA race car. Webers have fully adjustable fuel and air jets, and a you can pick the two independently to get whatever variation of fuel/air ration vs airflow that you want. Once upon a time I had a book with a really good explanation of how that worked, but the last time this came up I looked through my stuff in the storage room in the barn and could not find it. I will look again, but some of that stuff was given away after kids and home ownership.

Unfortunately everything I see on line says that the air corrector jets are to improve atomization of the fuel, but this is not their primary purpose - it is to correct the natural characteristics of a fixed venturi/fixed jet. When I first started using chainsaws more often I could not figure out why my Mac kept misfiring and why I could not fix it. In trying to understand what was wrong with it I finally looked into the operation of these little runt all-position carbs. I could not find the air corrector jets on the diagrams, or on the carb, and I finally realized they were not there. After that the behavior of the saw made sense - in kind of a pathetic way.

The manufacturers are counting on the super rich fuel mixture at high air velocities to act as a crude max-rpm limiter, and to use a guide for setting fuel mixture (on a stock saw).

EDIT - That info may even have been in the Haynes Weber Carburetor Manual, which is one of the ones I'm missing (so I can't check).

Interesting... Every Chainsaw I have done a Muffler mod to needed to be richened up a little bit on the high side in order to tune the saw properly. Whether it be a Walbro, Tillotson, or a Zama carb.
That is just my real word experience.

 

[Chris-PA]

Interesting... Every Chainsaw I have done a Muffler mod to needed to be richened up a little bit on the high side in order to tune the saw properly. Whether it be a Walbro, Tillotson, or a Zama carb.
That is just my real word experience.

Well you have to go with your experience then - mine has been the opposite as described previously. I have 10 saws at the moment and all have muffler mods, and there have been a few others. For the ones where I did only that mod, none have needed H enrichment. What is important is to keep an ear out and adjust it as needed.

 

[banana boat]

I started reading this thread because I was nervous about my just broken in ms460 that I just did a mm mod on. The saw is almost new, all stock and running perfect before the mod and after the mod I was expecting it to run lean on the h but it was 4 stroking really heavy (in the cut too)and actually needed to be leaned out just a little on the h and richer on the L both adjustments were maybe 1/4-1/2 turn. I feel the saw is breathing better and "pulling" in more fuel at high rpms.

 

[DanS]

I'm new to this site, but am a long time motorcycle guy. I have been opening up air intakes, putting on free flowing pipes, and rejetting carbs for quite some time. I can tell you that introducing more air on either end will result in leanness. i.e. you need to richen up the mixture. The idea that the carb will become richer by itself is incorrect. yes you'll have more air flow, but carbs work with throttle position. For a given throttle position, the jets will pass a given amount of fuel. If you want to increase the air for a given throttle position, then you had better increase the fuel too. This can only be done with the H/L screws on a chainsaw carb. On motorcycle carbs you can change out the jets and raise the needle.

You can try it yourself, i did on my chainsaw. Sure enough, she was lean after a muffler mod.

 

[john_bud]

The discussion here had missed the point of WHY the carb must be adjusted. The small port muffler causes SIGNIFICANT amounts of exhaust with ZERO unreacted oxygen to stay in the combustion chamber into the next cycle. This fraction of the air in the chamber increases with rpm and acts like a car EGR circuit.

Free up the muffler and you can RADICALLY change the amount of free Oxygen in the chamber - especially at high rpms. The new O2 rich air mixture will result in a dangerously lean combustion process unless you add more fuel.

A "rule of thumb" is to open the H jet 1/2 to 1 turn. The rational is rich doesn't melt pistons.

Just my 0.02

 

[Chris-PA]

The discussion here had missed the point of WHY the carb must be adjusted. The small port muffler causes SIGNIFICANT amounts of exhaust with ZERO unreacted oxygen to stay in the combustion chamber into the next cycle. This fraction of the air in the chamber increases with rpm and acts like a car EGR circuit.

Free up the muffler and you can RADICALLY change the amount of free Oxygen in the chamber - especially at high rpms. The new O2 rich air mixture will result in a dangerously lean combustion process unless you add more fuel.

A "rule of thumb" is to open the H jet 1/2 to 1 turn. The rational is rich doesn't melt pistons.

Just my 0.02

EGR does not change the mixture, because it displaces both fuel and air. It effectively reduces displacement because a portion of the cylinder volume is occupied by inert gas (as well as lowering combustion temperatures). Even if the small outlets did act like EGR, freeing them up would allow more air velocity through the carb, picking up more fuel. Nothing can enter the combustion chamber without going through the carb (well, except stratos) - it is all about what the carb does in response to the air that goes through it.

I believe the small outlets are both to reduce noise and to turn the muffler can into a thermal reactor to allow the raw fuel from scavenging losses to be burned.

 

[nmurph]

...Nothing can enter the combustion chamber without going through the carb (well, except stratos) - it is all about what the carb does in response to the air that goes through it...

Chris, up to now I have agreed to disagree with you. But about the above statement, you are incorrect. Two stroke motors pull atmospheric air back through the muffler as the piston falls.

 

[Chris-PA]

Chris, up to now I have agreed to disagree with you. But about the above statement, you are incorrect. Two stroke motors pull atmospheric air back through the muffler as the piston falls.

OK, that's true (I shouldn't be so absolute) - after the combustion pressure dissipates it can pull some oxygen back in, and some more if there is a pressure wave that reflects back just right (kinda unlikely with a typical can muffler). Do you think that effect is enhanced enough to lean the mixture after a muffler mod?

 

[treesmith]

Haven't done many but mine too needed richening

Sent from my GT-I9210T

 

[gilfish]

Did my 046 and it needed leaning on the H and richer on the L

 

[albert]

Not all need richening of the H needle. All need richening of the L needle. So it pays to check and not assume. I checked the rpm on my partner 5000+ before pulling the baffle it was little over 13.5k after was very rich @ 11.8k It has the governor blocked on the carb. I have seeen the same results in many others. I believe when you go big on the mod you loose too much mix out the exhuast port before it closes and this requires a richenig of the H side. Chris-Pa is correct, the carb adds more fuel as the flow of air thru the venturi increases, and that is what causes 4 stroking unloaded.