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Old 07-26-2011, 11:30 PM
Geo2199 Geo2199 is offline
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Purpose of exhaust screens on 2 cycle motors?

My Stihl FS-70Rc trimmer one day just shut off on me and wouldn't start about 3 weeks ago. That whole day the trimmer became less and less powerful the more I used it and it easily shut off on me and was very sluggish to rev whenever i hit the throttle. Later on I found out that the exhaust screen was clogged and once my friend removed it for me it crunk right up and ran like new. Haven't had a problem since.

What purpose does the exhaust screen have?

Is it something better left off?

Should I remove the exhaust screens on all my 2 stroke stihl equipment?

And I am not sure why my exhaust screen clogged up in the first place. I always run 89 octane gas and mix 2 bottles of Stihl 2 cycle oil with exacttly 2 gallons of gas measured out by the gas station pump. I put it in my 2 gallon gas tank and shake it well and it gives me a perfect 50:1 mixture everytime.
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Old 07-26-2011, 11:33 PM
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monoshock monoshock is offline
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Spark arrester, so no fires get started.
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Old 07-26-2011, 11:34 PM
Geo2199 Geo2199 is offline
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So without it on the exhaust my trimmer could cause a fire if left near grass or leaves?
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Old 07-26-2011, 11:39 PM
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monoshock monoshock is offline
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It could , but I take them out all the time.
Dirtbikes have had them for a long time.
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Old 07-27-2011, 01:59 AM
topsites topsites is offline
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The muffler on these engines is connected directly to the combustion chamber's exhaust port,
there is no manifold, there are no pipes, once that muffler is off you can see piston.

That screen you speak of is hardly two inches from where internal combustion is happening so if you were to take the muffler
completely off and start the engine and run it, you would be able to see sparks flying out of the exhaust port at any rpm but
the higher the rpm's, the more sparks would fly out.

Yes, sparks really do fly out of that exhaust port when the engine is running, at all times, and more than a few of them.

Once a muffler is installed there exist a couple of steel barriers but the air can still flow freely,
although it has to round corners it is not impossible for a spark to find its way through.

Without the spark arrester, sparks can fly out into the open air.
If such a spark were to land on dry tinder...
All it takes is one, by removing the spark arresting screen you are creating a fire hazard.

Do not remove this spark arresting screen, it exists to prevent fires.
I have never in 10 years had one clog but if your screen is clogging, try following manufacturer's recommendations regarding fuel oil mix.

Last edited by topsites; 07-27-2011 at 02:06 AM.
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Old 07-27-2011, 02:04 AM
Hell on Blades Hell on Blades is offline
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I only leave mine in when working in a National Forest or otherwise protected area.

By the way, they are cylindrical on a BG55 and much harder to remove (grinder with a cut off wheel did great)...
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Old 07-27-2011, 02:25 AM
madisonpressurewashing madisonpressurewashing is offline
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Quote:
Originally Posted by topsites View Post
The muffler on these engines is connected directly to the combustion chamber's exhaust port,
there is no manifold, there are no pipes, once that muffler is off you can see piston.

That screen you speak of is hardly two inches from where internal combustion is happening so if you were to take the muffler
completely off and start the engine and run it, you would be able to see sparks flying out of the exhaust port at any rpm but
the higher the rpm's, the more sparks would fly out.

Yes, sparks really do fly out of that exhaust port when the engine is running, at all times, and more than a few of them.

Once a muffler is installed there exist a couple of steel barriers but the air can still flow freely,
although it has to round corners it is not impossible for a spark to find its way through.

Without the spark arrester, sparks can fly out into the open air.
If such a spark were to land on dry tinder...
All it takes is one, by removing the spark arresting screen you are creating a fire hazard.

Do not remove this spark arresting screen, it exists to prevent fires.
I have never in 10 years had one clog but if your screen is clogging, try following manufacturer's recommendations regarding fuel oil mix.
Great post.
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Old 07-27-2011, 06:59 AM
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ALC-GregH ALC-GregH is offline
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Quote:
Originally Posted by topsites View Post
The muffler on these engines is connected directly to the combustion chamber's exhaust port,
there is no manifold, there are no pipes, once that muffler is off you can see piston.

That screen you speak of is hardly two inches from where internal combustion is happening so if you were to take the muffler
completely off and start the engine and run it, you would be able to see sparks flying out of the exhaust port at any rpm but
the higher the rpm's, the more sparks would fly out.

Yes, sparks really do fly out of that exhaust port when the engine is running, at all times, and more than a few of them.

Once a muffler is installed there exist a couple of steel barriers but the air can still flow freely,
although it has to round corners it is not impossible for a spark to find its way through.

Without the spark arrester, sparks can fly out into the open air.
If such a spark were to land on dry tinder...
All it takes is one, by removing the spark arresting screen you are creating a fire hazard.

Do not remove this spark arresting screen, it exists to prevent fires.
I have never in 10 years had one clog but if your screen is clogging, try following manufacturer's recommendations regarding fuel oil mix.
Dude, come on. Do you really believe what you're saying? There's no "sparks" flying out. If anything it would be a flame from the fuel burning. If it were sparks fling out, you'd have a very well destroyed engine. You are right with the rest of your post in WHY they have them.
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Old 07-27-2011, 07:20 AM
bmc1025 bmc1025 is offline
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I remove mine. I have also found the stihl oil in the orange bottle to be a bit dirty. It clogs screens and cause build up in the exhaust port if you don't keep the motor WOT all the time.
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  #10  
Old 07-27-2011, 07:32 AM
topsites topsites is offline
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Quote:
Originally Posted by ALC-GregH View Post
Dude, come on. Do you really believe what you're saying? There's no "sparks" flying out. If anything it would be a flame from the fuel burning. If it were sparks fling out, you'd have a very well destroyed engine. You are right with the rest of your post in WHY they have them.
There most certainly is, Guaranteed, try it yourself if you don't believe me.
You may have to remove the muffler and you may have to run it in the dark to see it, of course I believe what I am saying
because I have seen it with my own eyes, once the muffler is back on the sparks are a lot more sporadic but with the arrestor
gone they still come out once in a bit.

The sparks are a bright yellow at low rpm and then closer to a white or even an intense bluish sort of white at high rpm but if you
ever have the muffler off on one of these go ahead and find yourself a place out of the sun to run it, definitely wear ear protection
but start it up and you may have to wait until after the sun sets but tell me you don't see sparks flying out.

It's either pieces of still burning fuel, or perhaps tiny pieces of dirt in the air that catch it from the intense heat, I don't know
but I know for a fact that a trimmer with the muffler off throws sparks and if it does it without the muffler
then I am certain it can do it with it, again they didn't stick that screen on there for no good reason.

One might think they would, to comply with the EPA and the WTF but I can assure you the manufacturers don't stick that screen
on there to piss the end users off and if they didn't have to put it on there they probably wouldn't, but it is there for a reason,
and if you remove it you are creating a fire hazard.

Last edited by topsites; 07-27-2011 at 07:41 AM.
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