Stihl 170 Chain Saw Carbon problem

chzuck · Oct 4, 2016

Sorry to say, but those two holes are blank holes with no welch caps. There is an L & H in the casting at those holes. After your email, I removed the air box and took a good look and there just are not any plugs in those holes on my saw. Purchased it in July 2012. Right now the saw is running great. If it ain't broke, don't fix it.

stevestd · Oct 4, 2016

Tyy said:
Ok let me say this again , your carburator has High and Low Speed jets , they are hidden under the limitor caps otherwise know by mechanics as welch plugs .This was the model of Zama carb that Stihl used for a very short time back in 2011 . They are alluminium and must be drilled out carefully , then you would require a adjuster tool which is available via the dealer or online . Otherwise you could set the metering lever under the diaphragm a few thousands less to lean out the carb . Less complex solution invest $20.00 bucks on a fully adjustable Walbro Wt-215 carb online . The EPA guru, s do not want homeowners playing with these saws...duh ? Anyhow most Stihl that I tune regularly have a very lean setting requiring adjustment . As for your spark arrester screens get rid of them . Unless you cut within US Forestry areas lol . They will plug and cause overheating , stalling ..etc ! The MS 170 is a consumer , hobby saw not a professional grade unit , thus cheaper grade in many ways but a good saw for the average homeowner . I have ownered and still do 40 yr old Pioneers & new, Dolmar and Stihl and Husky Pro Saws. They are all getting more restrictive to maintain , between EPA Tuning and S**t Ethanol Fuel . I use nothing but Amsoil Sabre now rated for 50:1 to 100:1 oil Ratios , I never find carbon within my cylinder heads or spark plugs . However I will not bad mouth Opti 2 or Stihl Ultra when used properly . Anyhow if you can't figure the carb out just bring to a area small engine mechanic worth his reputation who will tune your carb properly , Hell with the Stihl Dealer Bullshit . If it was in my shop it would have been out the door within the hour ripping wood chips the size of nickles ! P.S. Nothing wrong with the Old Homelites of the 60's before Poulan bought them out lol .

I had a similar problem with both a Shindaiwa brush cutter and hedge trimmer about 8 years ago (both with the same engine). I removed the spark arrester, but the exhaust was also blocked/restricted which caused all sorts of problems such as engine difficult to start, ran rough and low on power. Cleaned the exhaust, stopped using ethanol fuel and replaced it with Shell V-Power 98 octane. Have not had one problem since.

Tyy · Oct 5, 2016

Ethanol fuel will not cause exhaust system blockage , excessive oil or over rich fuel mixture only causes that condition . Ethanol will however cause all kinds of carburation restrictions with these small engine units . If you use premium grade synthetic oils Amsoil ..etc they have fuel conditioner within the oil which protects the fuel from water entrainment and separation during storage . Otherwise , some form of fuel treatment is required , such as within small engine 4 cycle units . Some people like Stabile or Seafoam , I utilize Startron fuel conditioner when ever I must use Ethanol fuel in 4 cycle engines , good for 2 yrs of fuel storage . As for the Stihl 170 still have not heard the condition of the spark plug colouration . I agree if units is running fine now don't mess with it if it returns to previous condition you know a easy solution . I believe the carb is one of the chineese clone carbs that Stihl used for a quick epa compliance with fixed jets which ran rich from the factory during break in and stayed that way , causing the required screen cleaning or removal , do to excessive carbon fouling from the over rich condition .

bertsmobile1 · Oct 5, 2016

FWIW, I have been swapping out these cast in fixed jet carbs with older adjustable ones.
Ryobi has been using non adjustable carbs for decades down here.
I inherited a truck load of "dead" lawn hornets with the business.
An old bloke from the local mens shed put me onto this and since then I have offloaded around 100 trimmers.
The fixed idle circuit blocks up so they get hard to start .
New carbs are way too expensive and you need to get exactly the right one but with an adjustable carb the only worry is to get a carb for the same size engine or slightly bigger and I also inherited a 5 gallon drum full of old carbs ( Bert never threw anything out ).

So I would be inclined to try that with your chainsaw.
Also wash it out before you start.
Blue smoke oils work by coming out of solution under pressure ( much like squeezing water out of a sponge ) and condensing in the sump.
The bearings only require a drop or two of oil and I have found some with several ounces of oil in & under the bearings.
Back in the "old" days blue smokers were fitted with draining plugs to facititate draining the engine oil which never mixed properly with the gas .

Now fitting an adjustable "non EPA complient " carb is most likely illegal so you do it at your own risk.
Other than that just keep in reducing the oil till the screen stops clogging.
I run my lawnmowers at 50:1 using Stens oil in place of 25:1 with absolutely no problems lubrication wise.
The same 50:1 goes in my Stihl gear against the 40:1 recommended mix, again no problems.
Now if the gear was run 8 hours/ day continious in death valley it might overheat, but I take the care to keep my cooling fins & crankcases clean.

Tyy · Oct 6, 2016

Bert , I currently have both a MS 260 and a MS 361 both early 2000 vintage , both Pro Grade , unfortunately the newer 361 much like the current 261 are epa lean fully adjustable carbs . I had to remove the welch plugs and actually rejet the 361 . I have also rejetted every Echo product that has came in the door due to the high elevations here . Unfortunately Stihl moved from Keihien carbs to Zama's in the 90's , not bad carbs until the epa regs came out . Today most carbs are Chineese Clones junk but cheap to replace . I remember back when we used Sae. 30 for premix @ 16:1 on the Old Macs and Pioneers lol . Lots of Carbon fouling back then and routine Head and Cylinder decarbonizing one or twice a yr . Also all my saws have muffler screens removed , since only cut within private property , better chance of having the MNR show up during Kangaro Season then finding me cutting fire wood up here in the Great White North !

startrite · Oct 27, 2016

chzuck said:
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.

Sounds like the carburetor is running too rich. Unfortunately the carburetor on your saw is non-adjustable. The good news is you can buy a new one on eBay for around $15.00. Since the saw is out of warranty, I would use Amsoil Saber mixed at 80 to 1. I have a Stihl MS291 and use this oil at the 80 to 1 ratio and use this same mix with all of my 2-stroke equipment regardless of the manufacturers mix requirements. Amsoil claims that Amsoil Saber can be mixed all the way up to 100 to 1 and still have plenty of wear protection. I wasn't willing to go that far with it but I have yet to replace the spark plug or clean the spark arrester on any of them. They all run cleaner with little or no smoke and seem to have more power without any carbon buildup. Hope this helps.

Tyy · Nov 2, 2016

startrite said:
Sounds like the carburetor is running too rich. Unfortunately the carburetor on your saw is non-adjustable. The good news is you can buy a new one on eBay for around $15.00. Since the saw is out of warranty, I would use Amsoil Saber mixed at 80 to 1. I have a Stihl MS291 and use this oil at the 80 to 1 ratio and use this same mix with all of my 2-stroke equipment regardless of the manufacturers mix requirements. Amsoil claims that Amsoil Saber can be mixed all the way up to 100 to 1 and still have plenty of wear protection. I wasn't willing to go that far with it but I have yet to replace the spark plug or clean the spark arrester on any of them. They all run cleaner with little or no smoke and seem to have more power without any carbon buildup. Hope this helps.

Fully adjustable carbs available at half that price , Amsoil in my opinion is the very Best Oil for all 2-stroke applications designed to be used between 50-100:1 Ratio,s!

Stihl 170 Chain Saw Carbon problem

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