361 muffler mod w/ spark arrestor.

[Crofter]

Ben I cant quote a source at the moment but I think there is a circumstance when decellerating from high rpm with throttle closed and flywheel driving the engine, that the exhaust port sees definite flow reversal. Some cases of engines swallowing caked carbon from the exhaust ports that are hard to explain otherwise. Honest, I didn't make it up myself.

 

[Lakeside53]

My 2c - I won't even take take the chance of FOD (foreign object damage) by running without a screen. Way too much filth, mud, saw "dust" and plain junk everyhere in this business... and my saws "occasionally" aren't sitting nicely the "right way up" either on the ground or in the pickup... If AS members are worried about screens reducing performance, just make the opening bigger to compensate for the screen obscuration.

In any case, out West you don't want to be caught without a screen...

 

[bwalker]

Way too much filth, mud, saw "dust" and plain junk everyhere in this business...

And you think a screen will stop much of the above from entering the muffler??

 

[Kneejerk Bombas]

This spring I had to dig a mud wasp nest out of our blower exaust, and last year there was one in the exaust of the gas hedge trimmer, and mud wasps nests are just one possibilty of stuff that could get in there.
Screens are can be used as a quick diagnostic tool too, you can see your carb setting by the color, and if it's getting clogged you know you got a problem.
What is the problem with screens?

 

[Lakeside53]

And you think a screen will stop much of the above from entering the muffler??

yes, much of it.

 

[bwalker]

Ben I cant quote a source at the moment but I think there is a circumstance when decellerating from high rpm with throttle closed and flywheel driving the engine, that the exhaust port sees definite flow reversal.

Frank, I dont see how this would be possible when I think of the flow schematics of a two stroke under throttle or not.

 

[bwalker]

yes, much of it.

A screen will not stop sawdust, sand, dirt etc. Use a exhaust screen for a air filter and see how long your saw runs.

 

[Lakeside53]

A screen will not stop sawdust, sand, dirt etc. Use a exhaust screen for a air filter and see how long your saw runs.

For once you're likely wrong One end sucks it in, the other end doesn't.

 

[bwalker]

For once you're likely wrong One end sucks it in, the other end doesn't.

thats a given. The example I provided as to show how poor a wire mesh is at keeping fine debris like sand, saw dust, dust etc out.

 

[Lakeside53]

Throw a shovel of sand at your screen door. A little, maybe, gets through. Throw it at your door without the screen - it all gets through. Just another silly analogy.

I'm staying with my opinion that the screen keep bad stuff out of the muffler - not all, but a lot. Given the crap that flys around the pickup bed at 60mph I'm happy my saws have that little extra protection. Never mind the odd 3/16 nut and small gravel that bounches off the freeway. Small chances, but it happens.

 

[fishhuntcutwood]

The screen will stop the chips my saw spits out. If my saw is making dust, it's time to sharpen the chain.

 

[Crofter]

Quote:
Ben I cant quote a source at the moment but I think there is a circumstance when decellerating from high rpm with throttle closed and flywheel driving the engine, that the exhaust port sees definite flow reversal.
Frank,

{ I dont see how this would be possible when I think of the flow schematics of a two stroke under throttle or not.
__________________
Ben Walker
Peace through superior firepower.}

Ben, I wonder if you had taken the position that screens on the exhaust were a good thing, that you might be able to see flow reversals when the engine is driven as a pump with the intake virtually shut off and the exhaust wide open!
You can take the screens off your equipment if you like, but I prefer them on. In the original restricted setup, removing the screen is a quick and dirty way of reducing restriction: I prefer to make the hole bigger and put a screen back on.

 

[DanManofStihl]

The muffler I got from walkers didn't come with a screen? When I am cutting with it it blows the chips like a leaf blower though I have never seen it "suck" any in to the muffler but I don"t stand their and watch it. I think some one would have complained if they had burnt up a piston and a jug from chips if they had chips come back in through the muffler.

 

[Lakeside53]

The "chips" and whatever else get in when the saw isn't running...

Not everyone stops the saws, washes them off and locks them in their nice Stihl cases

 

[Kneejerk Bombas]

It's time to post that cross section gif of a 2-stroke cycle. It shows the exaust vacume during the stroke, or at least helps you understand it.http://www.vf750fd.com/blurbs/Two_Stroke/two-stroke_c.gif

 

[Crofter]

Mike, you've gone and done it now!. That is a good animation of a tuned pipe, but Ben said you would have no backflow at the exhaust unless you had a tuned pipe on your saw. I feel that with a standard muffler you may have momentary backflow pulses when the saw is decellerating. There is no question that you have strong backflow with a properly tuned exhaust.
Try runnning a saw with no muffler at all and contemplate what the resulting performance change indicates.

 

[Kneejerk Bombas]

I did some timed cuts without an exaust. It was pretty fast, faster than with the stock muffler, but slower than the modified muffler I ended up putting on (simply enlarged ports with screens covering them).

 

[Crofter]

Mike I should have qualified it further. The idle with no muffler is just like having an air leak. Very fast and clutch engaged. Seems like air getting back in and diluting charge.