Stihl MS170 carb issue?

[stagekraft]

Hey All,

Newbie to this forum, but hoping for a point in the right direction...

I have a small Stihl chainsaw, bought it used last year and it ran great until this summer.
I'm pretty sure it was some old/bad gas mix that created my problem.

The engine starts and idles fine, but when you try to rev up it falls on its face.
You can nurse the trigger and get it to rev up some, but no real power.

A search of the web brought be back dozens of references to this, but all seem to conclude that the problem can be resolved by;
Cleaning the spark arrest screen
Replacing the spark plug
Replacing the air filter
Replacing the fuel filter

None of which has resolved the problem...

What it really feels like to me is that the high speed mixture is off.
But this is a MS170 with the "018" engine, which does not have a high speed screw.
(according to the manual, the carb is so precisely engineered that it doesn't need a high speed adjustment).

So, I'm thinking, OK there is not an adjustment, but there has to be some sort of jet that meters the fuel,
perhaps the bad gas has gummed up this jet therefore throwing the mixture off.

Which leads me here to this forum, seeking some knowledge.

Anyone here familiar with this problem?
Am I thinking in the right direction?

Looking for some knowledgeable opinions before tearing into this little carb...

Regards,
JohnR

 

[Pincher]

By the symptoms you described, it seems you are on the right track. Hopefully someone with more experience comes along. My guess is that the metal screen inside the carb may have debris on it.

 

[DND 9000]

When these fixed jet carburetors don`t run right anymore (clogged, coroded etc.), it is better to replace them with a new one. You can try to unscrew the main jet and clean it, but in most cases it is better to replace the whole carburetor.

 

[066 redeye]

+1
Had nothing but trouble with fixed jet carbs.
Replace seems only fix.

 

[stagekraft]

I went ahead and pulled the carb apart, found the little fixed jet was indeed clogged up, I cleaned it up and re-assembled. the saw came back to life....
That gets me going for now, but I have ordered a carb kit, which includes an adjustable carb, spark plug, filters, fuel line, and even an ignition coil....
less than $15 on ebay

 

[av8or3]

I went ahead and pulled the carb apart, found the little fixed jet was indeed clogged up, I cleaned it up and re-assembled. the saw came back to life....
That gets me going for now, but I have ordered a carb kit, which includes an adjustable carb, spark plug, filters, fuel line, and even an ignition coil....
less than $15 on ebay

I suspect fir $15 you’ll be buying Chinese parts. Don’t use the carburetor, you can buy a brand new OEM carb for that saw fir less than $30. Just replace it.

 

[Nutball]

I'm curious what you saw mechanics charge. I've been doing more and more repairs, and I'm not sure what to charge. I don't want to clean out a guy's wallet, but I don't want to leave a lot on the table while still making them feel like they got a good deal.

Anyway, I just fixed a 170. It acted like the high speed was lean. It would start out with power for a few seconds, then would slowly loose torque. I tweaked the metering lever, problem solved. 40min including a couple test cuts, adjust the idle, refueled and repeated because it ran out of gas, also checked the plug and fuel filter, and cleaned the bar studs so the nuts thread easily.

What did I save them? Buying a new saw? Buying a new carb? Paying whatever a Steal dealer wanted to charge? It was a super easy fix with no parts cost, so basically they are paying for my knowledge perhaps more than time spent.