Chainsaw on the fritz, any Thoughts

[Brian Brown]

I came home from work a couple of days ago, and someone had gifted me a 8' section of flowering plum. I need to get it cut and sealed ASAP. I have a Stihl ms 170 (I think that's the model), and am having some trouble. The saw starts fine, and idles great. The problem is when I press the throttle, it sputters bogs down and dies. It acts like it just isn't getting enough gas. I described the problem to the dealer and they want to repair it by gutting it and replacing everything at maximum cost. I think the problem is more simple, but I can't seem to find the cause. There is a small breather in the fuel line that gets clogged with sawdust, but cleaning that hasn't helped. Any Ideas why this thing isn't getting any gas, and how to fix it?

[Steve Schlumpf]

Brian - it could be as simple as old gas, a clogged air filter or a fouled plug. Let us know what you have ruled out and we may be able to offer some additional suggestions. Good luck!

[Randy Red Bemont]

I would try a product called "mechanic in a bottle". See if that helps. I used it on an old weed wacker with similar symptoms and it worked great. You should be able to purchase it locally or online.

Red

[Barry Richardson]

That is how my saw behaves when the air filter is clogged, try blowing it out or replacing it.

[Reed Gray]

If mine hasn't been used in a while, some times it will do that. If it has run out of gas, as in shuts off because it is out of gas, then it does that as well. A tune up once a year is a good deal. If the repair place wants to do a total overhaul, then find another repair place. Ask some of the local tree services who does their work for them.

robo hippy

[Stefano Bastianelli]

Brian, if it sputters and bogs down, either is not getting enough gas or too much and the engine is not able to burn it. If it starts and can stay in idle, I am pretty sure it is not able to burn the gas when you give more of it. As Steve said could be old gas, bad plug or not enough air in the mix.
First thing to try is new gas. 2 cyles engines like high octane.
Try to clean the plug contacts or getting a new one.
Clean the air filter.
Did you leave gas in the chainsaw for a long time?

[Olaf Vogel]

I have a Stihl ms 170 (I think that's the model), and am having some trouble. The saw starts fine, and idles great.

I described the problem to the dealer and they want to repair it by gutting it and replacing everything at maximum cost.

My old Stihls had similar issues, and taking it to the dealer produced similar results.

The solution was to talk to my landscaper buddy. They use tons of small gas engines, need maintenance and often dont want to do it themselves.
He connected me with a retired guy, who's hobby is servicing these engines. That guy did an awesome job, very cheap.

One saw was a dirty carb and it needed adjustment.
The other had parts in the carb put in backwards - by the dealer!
Two big saws repaired for about $60.

Overall (with cars as well) I found dealer support to be expensive and very lacking in expertise.
(they like to replace parts, as per manual, and hopefully that works. But they generally don't really look to understand the problems)

Just my 2c

And I can't wait til chainsaws are fuel injected. :)

[daryl moses]

Your high speed jet is likely the culprit. Stale gas the cause.
Sometimes an additive will clean it. Something such as Randy mentioned or a product called Sea Foam will sometimes work. Otherwise it will need to be taken apart and cleaned.

[Bob Bergstrom]

YouTube has some very good videos on Stihl chainsaw maintainance and repair plus total tear downs. I searched Stihl ms 170 and got numerous hits on reviews and repair. Here is one on fuel lines and carb problems by a very good small engine repair specialist. https://youtu.be/Z8Zc9oqJsGA. He is called Donyboy73 on YouTube and has a large following. If you are using Ethanol gas and left it in the saw you may have a crack in the fuel line or impluse line. Very common with the symptoms you have. Running pure gas in your saw is a diffident benefit. Ethanol and fuel lines don't get along. These small engines are made for world consumption and not just for America's ethanol.

[John K Jordan]

How long since it was last used? If it sat for months I'd dump the gas and try new gas/oil mixture. (If cleaning the air filter didn't help.) I had a Honda generator that acted like that and it turned out to be some buildup in the jet. Thing was, I could see light through the jet and it looked clean but it had some "varnish" inside from sitting unused with old gas. Cleaned the jet and it ran fine.

JKJ

[Brice Rogers]

I'd follow the previous advice first: dump the gas, put in fresh stuff, blow out the carb air filter, stick in a new plug.

Then, if it still doesn't run, something that I would do next ("low hanging fruit") is to note the position of the high speed jet (or both if you don't know which is which). Then I would tighten them, counting the number of turns (and fractions). Write it down. Then unscrew and remove them. Keep the jet screws separate (in case they are different). Then squirt in a carb cleaner like B-12 using the small tube nozzle stuck into the jet hole. Then put things back together, tighten the carb adj. screws and back them out to their original setting. That is likely to fix it.

But honestly the real problem could be a variety of different things. If that didn't work, I'd consider buying a Stihl carb rebuilding kit (they cost around $10-$15 on ebay) and also replacing the tubing and filter going into the gas tank. Sometimes the old tubing rots.

[Steve Nix]

I suggest rather than re kitting it go on eBay and but a complete new carb. I've replaced my chain saw , tiller and weed eater carbs for $25 of less.

[Brian Brown]

I had thought bad gas, but it hadn't been in long when the problem started. It still could have been old when I put it in. I checked the filter, and it looked terrible with it's puce yellow color. I cleaned it twice with compressed air (nothing came out), and washed it well in gasoline, and it looked the same. I went to the dealer and looked at a new one, and it looked exactly the same. Ran the chainsaw without the filter on, and same problem, I think that rules out the filter. I never thought of the fuel running too rich, the extra lean is the only thing I thought of. I checked and cleaned the plug, and it seemed OK. These and the breather are all I did. You have all given good advice, and I will look into the bad gas. Some questions:

Will the Mechanic in a bottle or sea foam clean out varnish deposits, and clean the carb without a strip down?
Assuming fresh gas when I buy it, how long after buying gas should it not be used?
Does mixing the gas with 2 cycle oil cause the gas to go bad faster?
When running a 2 cycle engine, are there symptoms that your gas is getting too old, before a major problem?

As you can see, I know almost nothing about small engines... or big ones for that matter. Thank you all for your advice.

[William C Rogers]

I only use my gas chain saw 6-8 times a year. So I use premix TruFuel in it. (I know, about $24 a gallon). But I have never had a fuel issue. I also use Sea Foam in all my gas engines. The TruFuel is 93 octane, saw loves it, but wallet doesn't. It is good for two years after opening, so I don't have to worry about it being old or me mixing it. I also have a small electric chain saw for those quick jobs at the shop.

[John K Jordan]

Assuming fresh gas when I buy it, how long after buying gas should it not be used?
Does mixing the gas with 2 cycle oil cause the gas to go bad faster?
When running a 2 cycle engine, are there symptoms that your gas is getting too old, before a major problem?


I try not to buy more gas than I can use in a month. Around the farm here, it gets used pretty fast!

Mixing with the oil should not accelerate aging. You can put some Stabil in the gas/oil mixture to keep it good condiition longer. But when it doubt, throw it out!

Note that I NEVER use gas that contains ethanol in my small engines. I use only high octane pure gas. New engines might be able to handle the alcohol but I refuse to take the chance. I've seen gaskets and fuel lines soften and disintegrate when used with gas+ethanol. I use the pure gas in all my small engines: chain saws, weed eater, leaf blowers, trash water pump, log splitter, band saw mill, little utility truck, 4-wheelers.

There are numerous places around here that sell pure gas but in some areas you have to hunt. I found compilations of stations that carry pure gas on the internet.

Also, I stick with Stihl 2-stroke oil and mix it 50:1 for all my 2 and 4-stroke engines that require oil-gas mixture. I buy it in quantity.

BTW, one of my Stihl chain saws gets cantankerous if not run regularly. Starts and idles fine but dies when given full power. What I have to do is start it (according to the Stihl instructions), let it idle for a moment, then gingerly baby the throttle until it gets to full power. Then I run it wide open for a few seconds while doing the required chain/bar oil check. Works fine thereafter until the next time I let it sit for a month.

Also, it's certainly your business, but I was taught to never use gasoline for cleaning. It is so volatile the chance of accidental ignition is high. Far better is kerosene. For Stihl air cleaners I use only compressed air from the inside as recommended in the manual. The manual also indicates to not use a flammable liquid but if needed wash it in warm, soapy water then dry.

JKJ

[Bob Bouis]

Stihl 2-cycle oil has fuel stabilizers in it already. A lot of them do. But even then the gas is only good for a year or so. Two tops before it starts to gunk up your carb. So if you store your saw for more than a season you should always pour the old gas out and start with fresh.

[Richard Dooling]

I also use TruFuel premix. I've been running a Stihl string trimmer on it for five years and no trouble. No ethanol. As a precaution I do use gas treatment in ethanol engines..

It's not cheap but I run both the trimmer and a chainsaw on TruFuel. So maybe depends on how much fuel you use in a month. No one in my area sells ethanol-free gas (not even marinas!). Well airports do but they won't deal in small quantities. Also I'm not sure how precise the fuel/oil ratio needs to be so I'm leery of me mixing small amounts with precision.

Just using new fuel may not help help any existing conditions but it may avoid future problems.

A note:
Ethanol seems okay in newer engines as long as the engine get very regular use. I had a Yamaha 100 the would run great right out of the mechanics shop ($$$$$) and as long as I ran it regularly. But let it sit for a while and the #4 carb would get sticky. This was despite religiously running it dry and draining the carbs after use. Sold it to someone who uses it at least a couple of times a week and no problem. I bought a small marine EFI.

[Mark Greenbaum]

My friends always say to run the saw out of gas, or drain it when done cutting to prevent gumming. I was going to suggest the diaphragm on the carburetor, or replace the fuel line and inline tank filter. HD sells a kit for about $10, and it worked on my weedeater. Also check that the chain is free to spin - sometimes a clinched chain on the rails can make the load bog it down.

[Bob Bergstrom]

I rebuilt a Stihl carb for a friend who was given the saw that sat for over a year. The inlet was closed with dried ethanol, had to use a drill bit turned by hand to clear it out. Ethanol leaked out the carb gasket, and both were almost like super glue. The fuel line was cracked. The inlet hose leaked, and the diagram in carb was warp and not working. All these problems were described on YouTube with how to diagnose them and procedures to follow.

[Brent Wells]

I also use Trufuel for my saw and weed eater. The 93 octane and no ethanol makes it start first pull every time. Like mentioned it has a 2 year self life when opened and a 5 year unopened. Small carbs do not like ethanol above 10% and I know stihl booklets will tell you to run at least 89 octane which will get you under 9%. If you mix your own gas they also seem to like the synthetic oil mix. Sea foam is one of the best carb/jet cleaners I have found but you may need to tear apart if fuel has started to shellac. Good luck.

[Dom Garafalo]

I've had the same symptoms with both my Stihl chain saw and Echo grass trimmer and it turned out to be a clogged spark arrester.

There's a good chance that's your problem. It is a simple fix. Take off the muffler, which will expose the spark arrester which is simply a small metal screen with very small openings which become filled with carbon. You can either toss the spark arrester or clean the screen and put it back in.

You should check to see if you have a clogged spark arrester before spending any money on other parts or small engine service. This problem is well known with shops that fix and service small 2 cycle engines.

Good luck

Dom

[Todd Mason-Darnell]

Since you can get it started, you try spray some carb cleaner into the air intake (with no filter on) will it is idling.\:

https://www.amazon.com/Gumout-800002231-Carb-Choke-Cleaner/dp/B005LD2GLW/ref=sr_1_1?ie=UTF8&qid=1490460264&sr=8-1&keywords=carb+cleaner+spray

(you can get this at any auto parts store)

If you have a problem with varnish build up, this may help.

[Tom M King]

My first guess would be the spark arrester screen. Been running the Stihl oil in orange bottles?

The spark arrester screen is right inside the exit hole in the muffler. It's held in with one screw. Take that screw out, and slide the little screen out. If you can't see through it very well when held up to the light because of black gunk, that's your problem.

To clean it, hold it with some junk needle nose pliers, and heat it to red hot with a propane torch. Let it cool, and wearing an old glove, use you fingers sliding back and forth over it to release all the ash. It should now be clear like a new screen.

Put it back in, and it should run like a new one.

The orange bottle dino oil is terrible for clogging these screens. I haven't had this trouble since switching all my small equipment to the synthetic oil in silver bottles.

[Scott DelPorte]

If you ruled out the air filter, I would try the mechanic in a bottle before tearing it down. I have had good luck with it bringing back gummed up 2 stroke engines in chainsaws, leaf blowers, line trimmers, etc. that had deposits from gas with ethanol. Follow the directions on the bottle.

I switched to stabilized ethanol free gas and haven't had problems. It is much more expensive but since I use these tools infrequently it is worth it to me.

[Wes Ramsey]

Sounds like either a blocked venturi from varnish or trash that got past the filter or just bad gas.

In my experience a varnished carb isn't easily cleaned without using something designed for the task. Carb cleaner won't even soften it. I buy rebuild kits online ($10-20) and soak all parts of the disassembled carb for an hour or so in a gallon bucket of Chem-Dip I got from the parts store. Rinse in water, blow dry with compressed air, reassemble with new parts and it should run like new. I've even gotten away with reusing all the gaskets before when they're still in good condition.

As far as ethanol gas, it is okay to use in 2-cycle engines. It got a bad rap with small engine owners because seals and other non-metal parts weren't designed to be in frequent contact with alcohol and they would turn brittle and fail. Regardless, it is highly hygroscopic and has a shelf life of about a month without additives. After a month or so the ethanol will have absorbed about as much ambient moisture as it can hold and will start to separate. Additives will help it last longer, but I don't know how much longer it will keep the gas 'fresh'. I've heard Seafoam recommended enough by mechanics and friends that it is all I use. That said, I avoid ethanol when I'm able to.

I've had the same symptoms as the OP in all my 2-cycle engines using old gas. If it is bad enough it won't burn you can pour some in a quart jar and let it sit for a few. You can watch the water settle to the bottom as it separates. Usually it will have a milky look to it.

[Dom Garafalo]

Brian, do yourself a favor, and check for a plugged spark arrester screen before you chase your tail.

I may be wrong, but I believe there's a 50%+ chance your problem is a plugged spark arrester screen.

Keep in mind, if it had bad gas it wouldn't start or idle.

Dom

[Jim Bell]

My first guess would be the spark arrester screen. Been running the Stihl oil in orange bottles?

The spark arrester screen is right inside the exit hole in the muffler. It's held in with one screw. Take that screw out, and slide the little screen out. If you can't see through it very well when held up to the light because of black gunk, that's your problem.

To clean it, hold it with some junk needle nose pliers, and heat it to red hot with a propane torch. Let it cool, and wearing an old glove, use you fingers sliding back and forth over it to release all the ash. It should now be clear like a new screen.

Put it back in, and it should run like a new one.

The orange bottle dino oil is terrible for clogging these screens. I haven't had this trouble since switching all my small equipment to the synthetic oil in silver bottles.


I've had the same symptoms with both my Stihl chain saw and Echo grass trimmer and it turned out to be a clogged spark arrester.

There's a good chance that's your problem. It is a simple fix. Take off the muffler, which will expose the spark arrester which is simply a small metal screen with very small openings which become filled with carbon. You can either toss the spark arrester or clean the screen and put it back in.

You should check to see if you have a clogged spark arrester before spending any money on other parts or small engine service. This problem is well known with shops that fix and service small 2 cycle engines.

Good luck

Dom



I'd check there first.