New Stihl MS 170 stopped running, now hard to start & won't stay running

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Is this a bad sign? I removed the spark plug following a youtube guide and the manual; it was really hard to get off and took a lot of force. And it looks like this

sp1p.jpg sp2p.jpg sp4p.jpgsp3p.jpg

I watched Steve's small engine saloon's video about spark plugs and it looks like the middle part is dry and carbon like, but the silver part/the rings near the bottom were wet and left an oil mark on my finger.

Might be a dumb question but is it safe to leave the hole exposed to air like this, or assuming the spark plug is bad can I just plug it back in as long as I don't use the saw?

SteveSr said:
Gas looks good! Normally what you'll find is any water that is more than can be absorbed by the alcohol content in the fuel will separate out into a layer at the bottom.

What is REC90 gas and where did you purchase it? If it was at a high volume convenience store you are probably o.k. If it was at a non-busy (or late season) marina maybe not so much. Gas will still go bad sitting in a tank in the ground! Here is where you can find E-Free gas in your area:

https://www.pure-gas.org/
Click to expand...

REC-90 is just a name for a type/brand of 90 octane ethanol free gas I think. I actually used that site before buying the saw to see if they had this kind of gas where I live and got it from the only station in the city that has it, at dealer the guy even specifically mentioned going there to get the REC90 without me even bringing it up
 
wzd218 said:
Is this a bad sign? I removed the spark plug following a youtube guide and the manual; it was really hard to get off and took a lot of force. And it looks like this

View attachment 768992 View attachment 768993 View attachment 768995View attachment 768994
Click to expand...

You can put the plug back in and tighten it up. The black color says excessive idling or part throttle operation which is not good for the saw. The throttle should be WIDE OPEN when the chain is in the wood. Yes, this saw will scream - in delight! :)

AS far as starting goes are you following the proper starting procedure as described in the manual? There are actually 4 positions of the control lever as follows:

Starting from full down
1. full choke /part throttle - saw should "pop" but WILL NOT run in this position.
2. Part throttle / idle - Once the saw starts blip the throttle to go to the idle position. DO NOT leave the saw in part throttle with the chain brake on or you will MELT and RUIN the saw!
3. Idle - If the saw is warm it should start here.
4. Off.

If the saw has cooled off too much it may not want to start at idle. In that case go to part throttle/idle. If that doesn't work then choke it for a pull.
 
SteveSr said:
You can put the plug back in and tighten it up. The black color says excessive idling or part throttle operation which is not good for the saw. The throttle should be WIDE OPEN when the chain is in the wood. Yes, this saw will scream - in delight! :)

AS far as starting goes are you following the proper starting procedure as described in the manual? There are actually 4 positions of the control lever as follows:

Starting from full down
1. full choke /part throttle - saw should "pop" but WILL NOT run in this position.
2. Part throttle / idle - Once the saw starts blip the throttle to go to the idle position. DO NOT leave the saw in part throttle with the chain brake on or you will MELT and RUIN the saw!
3. Idle - If the saw is warm it should start here.
4. Off.

If the saw has cooled off too much it may not want to start at idle. In that case go to part throttle/idle. If that doesn't work then choke it for a pull.
Click to expand...

Yes that's how I start it, choke (unless it's just been used where I will start in the next one up instead), then pull on high idle until it starts (usually pretty quickly after 2-4 pulls), then switch it to regular idle by pulling the trigger immediately. When I wasn't using the saw I did let it idle between cuts, and also put it down for a few minutes at a time where it idled as well. I'll be very happy if that's what caused this and simply getting a new spark plug makes it run again. :)
 
Even just trying to get it going and not be able to work the engine will make a plug go black temporarily until it gets hot and burns off.

That plug doesnt look fubar'd. Some full tilt rips into the wood will clean that right up. Gonna say thats not your issue. If it will atleast try to fire or even idle for any amount of time, the plug is good enough. Just dont idle it so much. Properly running saw will flash up when warm even looking at the chord, may as well just shut er down if you're more than 30 seconds between rips.

Heres my MS250... it is a relatively low hour saw and I checked everything on it. Turned out the carb check valve was likely to be the culprit. Its been alleged that Zamas have issues...Binned the Zama for a Walbro...runs mint now.

If I was fast enough and played with the throttle enough and kept starting it, I would eventually be able to rev it wide open but would die immediately at idle.



But being that your saw is brand new I'd just take it back and tell them whats up. Doesnt sound like user error to me.
 
wzd218 said:
Yes that's how I start it, choke (unless it's just been used where I will start in the next one up instead), then pull on high idle until it starts (usually pretty quickly after 2-4 pulls), then switch it to regular idle by pulling the trigger immediately. When I wasn't using the saw I did let it idle between cuts, and also put it down for a few minutes at a time where it idled as well. I'll be very happy if that's what caused this and simply getting a new spark plug makes it run again. :)
Click to expand...

Don't bother with a new plug... Just run it like you stole it! The plug will clean itself if the saw is used properly.
 
husqORbust said:
Even just trying to get it going and not be able to work the engine will make a plug go black temporarily until it gets hot and burns off.

That plug doesnt look fubar'd. Some full tilt rips into the wood will clean that right up. Gonna say thats not your issue. If it will atleast try to fire or even idle for any amount of time, the plug is good enough. Just dont idle it so much. Properly running saw will flash up when warm even looking at the chord, may as well just shut er down if you're more than 30 seconds between rips.

Heres my MS250... it is a relatively low hour saw and I checked everything on it. Turned out the carb check valve was likely to be the culprit. Its been alleged that Zamas have issues...Binned the Zama for a Walbro...runs mint now.

If I was fast enough and played with the throttle enough and kept starting it, I would eventually be able to rev it wide open but would die immediately at idle.



But being that your saw is brand new I'd just take it back and tell them whats up. Doesnt sound like user error to me.
Click to expand...


spark2p.jpg

I cleaned about as much of that carbon off as I could get with tiny pieces of sandpaper, but it didn't seem to help. The saw started alright again but while it was running it sounded like it was running poorly and "changing speed" like the RPMs were going down at times, being unable to run at a steady speed. I managed to pull the throttle and cut through 3 small logs but while I was walking over to another one the engine cut out, after probably under a minute of running. Most of the times when giving it revs at the throttle it worked fine, a couple of times it seemed like it was having a hard time getting up to full speed. I looked at the spark plug afterwards and it looks the same as when I put it back in, not coated in carbon or anything like the pictures before, just a very tiny amount of wet oil visible but other than that exactly the same.
 
I'm not sure about what is covered under warranty or not but I'd imagine a saw with less than a tank of fuel through it should be no questions asked figure it out on the dealers behalf. You'll save yourself time and money by just taking it back and telling them it runs like garbage.

If it had no warranty and you really didnt want to pay for diag and shop parts, I'd dig into it starting with a carb tear down and checking of the high flow valve based on your running descriptions.
 
wzd218 said:
View attachment 769149

I cleaned about as much of that carbon off as I could get with tiny pieces of sandpaper, but it didn't seem to help. The saw started alright again but while it was running it sounded like it was running poorly and "changing speed" like the RPMs were going down at times, being unable to run at a steady speed. I managed to pull the throttle and cut through 3 small logs but while I was walking over to another one the engine cut out, after probably under a minute of running. Most of the times when giving it revs at the throttle it worked fine, a couple of times it seemed like it was having a hard time getting up to full speed. I looked at the spark plug afterwards and it looks the same as when I put it back in, not coated in carbon or anything like the pictures before, just a very tiny amount of wet oil visible but other than that exactly the same.
Click to expand...

This is beginning to sound like a bad carburetor with a sticky main nozzle check valve. While I have seen this on 10 year old carburetors I have no direct experience with new out-of-the-box failures.

I would describe these symptoms to the dealer and ask him to swap the carburetor. To insure honesty I would put a small inconspicuous scratch on the existing carb to make sure that the carb was actually swapped and not given the "sunshine" treatment!
 
and if that dealer was me, i'd say you idled the saw for how long?? Then administer the car-talk dope slap. Sounds like a carb issue tho. Let them sort it out for ya.

Saws are made to run, idle only for a little at a time. Say something is nutty with carb, and liquid fuel is going down the throat into the engine. Not properly vaporized. Puddling up in bottom of crank. Warm the saw up some, and the fuel boils off flooding the engine. eventually the oil starts to get hot enough to create smoke?
 
wzd218 said:
Is this a bad sign? I removed the spark plug following a youtube guide and the manual; it was really hard to get off and took a lot of force. And it looks like this

View attachment 768992 View attachment 768993 View attachment 768995View attachment 768994

I watched Steve's small engine saloon's video about spark plugs and it looks like the middle part is dry and carbon like, but the silver part/the rings near the bottom were wet and left an oil mark on my finger.

Might be a dumb question but is it safe to leave the hole exposed to air like this, or assuming the spark plug is bad can I just plug it back in as long as I don't use the saw?



REC-90 is just a name for a type/brand of 90 octane ethanol free gas I think. I actually used that site before buying the saw to see if they had this kind of gas where I live and got it from the only station in the city that has it, at dealer the guy even specifically mentioned going there to get the REC90 without me even bringing it up
Click to expand...

The carbon on your plug is most likely from the idling but if REC90 is 90 octane fuel it could have contributed to the soot and carbon as well.

I would clean the plug and try to start it again but in the future you should probably run the lowest octane fuel. Then only move up to the next octane rating if you have trouble with detonation. The higher octane fuel that you used may have cost you an insignificant amount of power but it should have run good enough. However, it will pollute more and it will carbon up your combustion chamber and spark plug. As you’ve learned, this is especially true at idle.

The owners manual will usually give a recommended octane rating but there are different ratings depending on the country of origin. The best bet is to start with the lowest octane available in your area and only move up in octane if you have preignition problems. This is true on any gas engine.

On the other hand, an engine in good condition and tuned we’ll will run pretty well even on fuel that has been sitting for several months. I doubt that your problem was caused by bad fuel. I would guess that your saw needs some attention but it could be as simple as cleaning the plug, not letting it idle for long periods, and running lower octane fuel.
 
Goofaroo said:
The carbon on your plug is most likely from the idling but if REC90 is 90 octane fuel it could have contributed to the soot and carbon as well.

I would clean the plug and try to start it again but in the future you should probably run the lowest octane fuel. Then only move up to the next octane rating if you have trouble with detonation. The higher octane fuel that you used may have cost you an insignificant amount of power but it should have run good enough. However, it will pollute more and it will carbon up your combustion chamber and spark plug. As you’ve learned, this is especially true at idle.

The owners manual will usually give a recommended octane rating but there are different ratings depending on the country of origin. The best bet is to start with the lowest octane available in your area and only move up in octane if you have preignition problems. This is true on any gas engine.

On the other hand, an engine in good condition and tuned we’ll will run pretty well even on fuel that has been sitting for several months. I doubt that your problem was caused by bad fuel. I would guess that your saw needs some attention but it could be as simple as cleaning the plug, not letting it idle for long periods, and running lower octane fuel.
Click to expand...
Why would Motomix and every other canned 2-stroke gas have a high octane rating if it was bad for saws?
 
Colt Marlington said:
Why would Motomix and every other canned 2-stroke gas have a high octane rating if it was bad for saws?
Click to expand...

I have no experience with any of those products but my guess is that they are blended for long shelf life and marketed as a convenience for people that don’t want to mix their own fuel.
Nothing will change the fact that the octane rating is simply about the volatility of the fuel and the higher the octane level the less volatile the fuel becomes. Therefore it won’t burn as completely as lower octane fuel in the vast majority of engines.
 
Goofaroo said:
I have no experience with any of those products but my guess is that they are blended for long shelf life and marketed as a convenience for people that don’t want to mix their own fuel.
Nothing will change the fact that the octane rating is simply about the volatility of the fuel and the higher the octane level the less volatile the fuel becomes. Therefore it won’t burn as completely as lower octane fuel in the vast majority of engines.
Click to expand...
See my previous post.
 

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