Stihl MS 170 bogging down, no power

[GerrySM]

I have big saws but now only use this small chainsaw because a back injury means I cannot handle the heavier ones.

MS170 is 2 years old, suddenly developed a bogging out problem in the middle of a cut.

1. Muffler screen is clean.
2. Spark plug and chain are new.
3. Bar nose sprocket turns easily.
4. Fuel is fresh and correctly mixed.
5. Fuel filter is clean.

My suspicion is contamination via the air intake

When I remove the cover (13), I find the upper surface of the filter (2) heavily contaminated (this is an aftermarket eBay filter, perhaps it is not sealing properly?). There is oily sawdust on part 4, and if I put my finger into the intake vent on 3, it comes out contaminated with fine, slightly oily sawdust as well. I found this a bit shocking, as this area is supposed to be pristine, right?

I cleaned it all out as best I could, but the contamination occurred again, and I'm worried that some of this gunk has got into the carb. The saw loses power and almost stalls in deep cuts now. (There is no way to alter the fuel mixture on this machine, btw).

Is my next step a carb cleanout? I hope none of this stuff has damaged the piston ....

 

[GerrySM]

Someone reviewed these typical aftermarket air filters on Amazon like this:

Filters are much too thin. Would take 2 to 3 together to work right. Even Stihl makes heavier filter to try and keep sawdust out of engine.

I suspect this is the problem for me too. The OEM filter did not allow sawdust through, but these did, because the edges do not seal properly (filter too thin to cause enough pressure against the cover at the edges).

I also saw this review on Amazon for a new MS170 carb:

I replaced the original carb because air leaked around the chintzy air filter when the saw was new. It clogged the carb up with saw dust and I couldn't spray it out to where it would work right. Kept bogging down at full throttle. This new carb cured that issue.

So now I wonder ... new carb, or risk just getting a carb kit?

 

[VinceGU05]

does it run without air filter? if all good then yr filter is choking it.

 

[GerrySM]

It runs fine with the new air filter I have inserted. It just bogs down in the cut.

I'm pretty sure the carb is fouled. Just want an opinion on what to buy:

1. Carb Kit? ($5 on eBay)
2. New carb? ($12-15 on eBay)

 

[mountainlake]

I'd just take it apart and clean it. If that doesn't work a new carb. Steve

 

[GerrySM]

Thanks Mountainlake, I'll do that

 

[HarleyT]

Look at the fuel line closely. Look for cracks, also look for it to become soft and gummy.

 

[CR888]

Clean out the screen in carb like moutainlake suggests. Bet there is some crud in there. Probably don't even need a kit.

 

[Chris-PA]

These kinds of carbs cannot hold a constant fuel/air ratio as air flow changes. So anything that causes a change in air velocity through the carb will change the mixture, but these carbs are non-adjustable. If the filter significantly changes the air flow, the mixture must change.

The people who designed this thing with a nonadjustable carb knew exactly what kind of a cheese they were doing. That they sold it that way anyway shows they simply didn't give a crap about their customers.

 

[GerrySM]

In all fairness to Stihl, I think I am to blame here for buying cheap-ass Chinese filters that are too thin to seal out the airborne sawdust. I am now using 2 of them at a time to create some sort of seal at the edges when the cover is put on and locked down onto them.

 

[GerrySM]

Excellent aid to anyone looking to clean their MS170 carb:

 

[sugarbush]

if particles are going into the carb through the air filter that won't plug up carb.

 

[GerrySM]

if particles are going into the carb through the air filter that won't plug up carb.

If you look at the messages I quoted above, you'll see that others have found the same issue. Where do you think the sawdust will go, just evaporate into thin air? They will plug the airflow or fuel flow, I will find out which today.

 

[GrassGuerilla]

Might want to get a look at the P&C before you buy anything else. If enough **** went through the filter, where do you think it went? Quite possibly right on through.

 

[Chris-PA]

If you look at the messages I quoted above, you'll see that others have found the same issue. Where do you think the sawdust will go, just evaporate into thin air? They will plug the airflow or fuel flow, I will find out which today.

Fine sawdust is very unlikely to build up in the parts of the carb exposed to the intake air. It will go through the engine, possibly causing wear on piston and some carbon buildup.

 

[TBS]

I had an 017 at work that would do the same but eventually just wouldn't start, I took the carb apart and cleaned it but had to throw it back together because more important things came up, sent it to a dealer who said it was dead, year or so later last month pulled the carb apart and found out needle valve lever didn't have enough bend to open the needle valve. Bent a hair and that little saw runs like a champ now.

As for the filter use oem or just go to the store and get a pack of green scotch bright pads because they cheaper, much stiffer, and filter as good as most aftermarket air filters. You can cut them to fit as well.

 

[sugarbush]

the sawdust will go into the engine not into the passages of the carb. those passages are affected by what is in the gas tank. buy an adjustable walbro carb from ebay for about $15.00.

If you look at the messages I quoted above, you'll see that others have found the same issue. Where do you think the sawdust will go, just evaporate into thin air? They will plug the airflow or fuel flow, I will find out which today.

 

[GerrySM]

Ah hell, got inside the carb but while removing the little brass jet, which was screwed in very tightly, the screwdriver slot on the jet just cracked under the torque pressure of my screwdriver, so I guess I'll have to buy a new carb anyhow. As far as I know, this little brass jet is not available separately.

One could almost say that these cheap little carbs are not made to be serviced, they are throwaway items; quicker and cheaper just to replace them. Got a new one on eBay for around $12 (can't see the advantages of buying an adjustable Walbro ... the unadjustable one I had was working great until recently).

Fine sawdust is very unlikely to build up in the parts of the carb exposed to the intake air. It will go through the engine, possibly causing wear on piston and some carbon buildup

I had a look at the piston from the connection tube to the carb, seems unmarked and smooth. But there could still be damage in there, we'll see with the new carb.

If the new carb fixes things, it means dirty air can affect the carb. If not, then it suggests the machine is f*ed.

 

[dugide]

 

[GerrySM]

Yup, looking at the old carb, cannot see any dirt in it, so unless there was tiny debris in places I cannot see, I suspect the engine has been damaged by sawdust debris, which means the machine could be kaput. I have to wait 2 weeks for the new carb to come from China, then I'll report back here.