I must say I am less than pleased with my Stihl 170 chain saw. I bought one because of their supposedly great reputation. I didn't even run a gallon of fuel through the saw and the coil failed. Fortunately it was under warranty. I am probably just starting on my 3 gallon of fuel and the saw lost power. Turns out the spark arrestor screen was plugged with carbon. I am using Stihl Ultra oil. Is this normal for this saw?
Stihl 170 Chain Saw Carbon problem
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bertsmobile1
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The saw is either running very very rich or you have mixed the fuel with way too much oil.
non-adjustable carburetor and oil is 1 bottle to 1 gallon of gas and air cleaner is clean. Probably will try different oil.
BlazNT
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Some bottles are for 2 gallons of gas. For one gallon of gas it is like 6 oz.
The bottles (2.6 oz) I have say makes 1 gallon.
bertsmobile1
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As it is a new saw. take it back to the shop.
There could be a problem with the engine.
Chain saws spin very fast so a coil that only fires 3 out of 4 revs will not be noticed by the operator but make a big difference to the emissions.
The carb is adjustable, just not by the owner.
There could be a problem with the engine.
Chain saws spin very fast so a coil that only fires 3 out of 4 revs will not be noticed by the operator but make a big difference to the emissions.
The carb is adjustable, just not by the owner.
Well, it is not brand new and under warranty anymore. I bought it in 2012.
I emailed Stihl about the issue and below is their reply.
Two stroke engines like the one in your MS170 use an ignition principle called "loop scavenging". This means that when the fuel is cycled through the four transfer ports of the cylinder from the bottom to the top of the combustion chamber, it begins to swirl in a widespread pattern to insure that as much of the fuel burns as possible when the unit fires the ignition spark.
Unfortunately, it doesn't matter how efficient the design of the engine is in the two-cycle format because a small amount of raw fuel is going to escape with the burnt exhaust gasses each time the engine fires. This raw gas/oil mix tends to collect on the fire arrestor screen of the muffler, and over time it will accumulate and cause an obstruction. When this happens, the obstruction will not allow the exhaust gasses to escape, causing them instead to bounce off and push back through the combustion chamber and, if the blockage is severe enough, out of the carburetor venturi onto the air filter.
The issue can be compounded if the engine is feathered or run at part throttle for extended periods of time. The fuel cannot burn efficiently when the engine is run in this manner, so the screen can clog much faster.
The best way to cure this problem, as you know, is to remove the screen from the muffler and check it for carbon build-up. If carbon is present, simply pass a wire brush over the screen from all sides until the debris is removed. Then re-install it back into the muffler and try your starting procedures once more.
I think I may try another brand oil when the Stihl Ultra is gone. Not sure if my Tanaka string trimmer has a screen, but I have had that for almost 20 years and have never had that kind of problem and it runs 25:1 mix. And I could remove the screen. None of my other, vintage saws have that fine of a screen and I only cut wood on my property with my saws.
I emailed Stihl about the issue and below is their reply.
Two stroke engines like the one in your MS170 use an ignition principle called "loop scavenging". This means that when the fuel is cycled through the four transfer ports of the cylinder from the bottom to the top of the combustion chamber, it begins to swirl in a widespread pattern to insure that as much of the fuel burns as possible when the unit fires the ignition spark.
Unfortunately, it doesn't matter how efficient the design of the engine is in the two-cycle format because a small amount of raw fuel is going to escape with the burnt exhaust gasses each time the engine fires. This raw gas/oil mix tends to collect on the fire arrestor screen of the muffler, and over time it will accumulate and cause an obstruction. When this happens, the obstruction will not allow the exhaust gasses to escape, causing them instead to bounce off and push back through the combustion chamber and, if the blockage is severe enough, out of the carburetor venturi onto the air filter.
The issue can be compounded if the engine is feathered or run at part throttle for extended periods of time. The fuel cannot burn efficiently when the engine is run in this manner, so the screen can clog much faster.
The best way to cure this problem, as you know, is to remove the screen from the muffler and check it for carbon build-up. If carbon is present, simply pass a wire brush over the screen from all sides until the debris is removed. Then re-install it back into the muffler and try your starting procedures once more.
I think I may try another brand oil when the Stihl Ultra is gone. Not sure if my Tanaka string trimmer has a screen, but I have had that for almost 20 years and have never had that kind of problem and it runs 25:1 mix. And I could remove the screen. None of my other, vintage saws have that fine of a screen and I only cut wood on my property with my saws.
bertsmobile1
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OK,
I was under the impression it was a new saw.
Clean the screen and put it back.
If it is a problem then leave it off.
It is a spark arrestor so will not make much difference to the saw except a slight drop in back pressure.
A lot of owners toss them
The only big problem is the screen prevents mud pluggers nesting in the muffler so if you leave it off & mud pluggers are in your area tie a bag around the engine or store the saw in a box when not in use.
Still sounds like it is running rich or missing but you need some expensive gear to test it.
You can look the parts break down for your carb to find the hidden adjustments on the carb makers web sites not the Stihl one.
As it is out of warranty it will not matter that the tell tales are broken.
Just be ware of adjusting it too lean as slightly lean will over heat and wreck the bore in no time flat.
You already know the outcome of too rich.
I was under the impression it was a new saw.
Clean the screen and put it back.
If it is a problem then leave it off.
It is a spark arrestor so will not make much difference to the saw except a slight drop in back pressure.
A lot of owners toss them
The only big problem is the screen prevents mud pluggers nesting in the muffler so if you leave it off & mud pluggers are in your area tie a bag around the engine or store the saw in a box when not in use.
Still sounds like it is running rich or missing but you need some expensive gear to test it.
You can look the parts break down for your carb to find the hidden adjustments on the carb makers web sites not the Stihl one.
As it is out of warranty it will not matter that the tell tales are broken.
Just be ware of adjusting it too lean as slightly lean will over heat and wreck the bore in no time flat.
You already know the outcome of too rich.
Thank you for the input. I was under the impression there were no adjustments like some of our newer lawn mower carburetors. You have a point about being too rich. It should not have clogged that screen in less than 3 gallon of gas.