question for a mechanic about muff mods

[Chris-PA]

Since I've been home with a fever for two days and I'm bored I've had time to look into this a little more. It is hard to find pressure-volume graphs for gasoline 2 strokes, but I found a couple for diesels. It would appear that the cylinder pressure drops to atmospheric for the last 10 to 15 degrees BBDC. This effect would be going on before the MM, but with a muffler can most of what would get pulled back would be exhaust. If you imagine the extreme of removing the muffler entirely, then you would pull in oxygen, which will lean the mixture but I don't know how much. It would make sense that venting the can heavily would do that too.

Clearly there is a variation in experiences with tuning after a MM, but there is a lot of variation in how restrictive the original muffler is, the timing numbers of the engine, and especially what constitutes a Muffler Mod. It's pretty easy to confirm that increasing the air velocity through the carb makes the mixture richer. But if you pull oxygen through the exhaust the carb does not see it.

At this point I think that if you do a mod and need to lean the H a bit, you have succeeded in increasing the air velocity through the carb. If you have to enrich it then you're probably pulling oxygen into the can, and from there into the cylinder. So, for example, if the original muffler already flowed well and you cut big holes in it, then the air velocity through the carb won't be much greater but the mixture may lean out. I suppose this should make more power if you adjust the mixture for it (though I would worry it is not well controlled).

I never open mine that much - while I do use large openings, I always try to route the exhaust through the longest path through the muffler to keep it from being too loud. These mods have made more power and I have had to lean the H afterwards. I've certainly seen other approaches and some pretty big outlets.

 

[nmurph]

Or, following Occam's razor: they were already rich to begin, and you just leaned them up to where they needed to be.

 

[Chris-PA]

Or, following Occam's razor: they were already rich to begin, and you just leaned them up to where they needed to be.

Well, mine were not.

 

[MarcusB]

Now Im really confused cos you blokes use big words and stuff!! hehe. So what do we agree on on how things are tuned? WOT no load should be slightly four stroking? what about the low end, responsiveness needed etc?
Thanks very much for all the responses from everyone, chainsaws are unreal.

 

[Chris-PA]

The correct answer is to always listen to the saw and be ready to tune it regardless of what mod you did or didn't make. It should misfire when you lift and unload it and clean up under load.

In a nutshell, these carbs do not put out an approximately fixed fuel/air ratio like other carbs (even the cheap ones on a mower), rather the mixture is a constantly varying one that gets richer as the air velocity increases. You are trying to match a point on that curve so it is very easy to miss. The only real landmark you have is the onset of rich misfire (4-stroking) - if it goes away under load and comes back when you lift then you are about right.

I don't make mine as rich as some of these guys, but then my saws are all cheap.

 

[hamish]

There is a big difference in carburetor characteristic between 2 strokes and 4 strokes, plain and simple. So take 100% of any and all 4 stroke knowhow and toss it out the window. Have a pint, clear your head, and get onto 2 strokes.

 

[Chris-PA]

There is a big difference in carburetor characteristic between 2 strokes and 4 strokes, plain and simple. So take 100% of any and all 4 stroke knowhow and toss it out the window. Have a pint, clear your head, and get onto 2 strokes.

We'll have to disagree on that too I guess. The different characteristics of the pump that pulls the air through the carb are not very significant, chiefly in the pulsations you get in the intake. These are weaker in a 2-stroke. Piston ported designs have more reversion (but not read valve 2-strokes) and 4-strokes with long duration cams can have a bunch. Also, these carbs are nothing like the CV carbs used on 2 stroke bike engines.

There are big differences between all position diaphragm carbs and float bowl carbs designed for upright use, and I've been trying to help explain what the effects of one of those big differences is (the elimination of the air corrector jets). It's the main thing that governs the fuel system characteristics of chainsaws and controls how they must be tuned, causes "4-stroking", creates a max no-load WOT rpm that can be adjusted by the mixture screw, and tells you what you should expect from certain mods.

 

[Hunowie]

That was some really interesting info on howto do muff mod.
Care to give some photos of your MM Chris-PA? After and before mm?

 

[Chris-PA]

That was some really interesting info on howto do muff mod.
Care to give some photos of your MM Chris-PA? After and before mm?

I don't have that many good photos. This one gives one approach - the center baffle is cut out, but the opening faces downward when assembled to send the gases down to the bottom of the can. The outlet opening is up at the top, to preserve a longer path length.

I have spent a fair amount of time considering this since this thread, and I think this is a major variable in tuning. If you imagine a fairly well enclosed can on the outlet, with a volume maybe a few times the cylinder displacement, then it will contain mostly exhaust plus whatever fresh charge was lost. The cylinder will pull some of this back in. With no can, or a wide open one, then it is pulling in fresh air and the charge lost to scavenging is gone.

I still the scenario I described makes sense. If you open up the muffler to provide better flow, you'll get more air flow through the carb, and this will enrich the mixture. If you go beyond that and really open up the can you can pull extra oxygen in through the exhaust port, and then you may need to add more fuel. At that point your fuel use will probably go up, as any fresh mix that leaves the open exhaust port will likely be lost rather than partially pulled back in.

It will depend on how restrictive the original muffler was, and probably what its volume is and how it is constructed. I doubt this is strictly a function of how big and outlet you use, rather some combination of opening size and how short a path you have to the port.

With these RedMax style mufflers the outlet ends up being a pretty big percentage of the port size, but the exhaust is sent all the way down to the bottom, through a greatly enlarged hole in the baffle (bigger than the port), and back up to the outlet at the top.


Last, I'm still thinking about how strato plays into it. With a strato, what's going out the open exhaust port is mostly air. So losing it does not matter much - but pulling even more oxygen into the cylinder might make getting the fuel charge into the cylinder difficult, as that happens very late in a strato. I guess if you can tune it to 4-stroke out of the cut it's OK.

 

[Hunowie]

Looks rather stable and reasonably done.
Have you ever gone too far and had to wield some openings you did?