Stihl MS170/180 - Completely Clogged Spark Screen?

[SteveSr]

Hello,

This saw was used intermittently by one of our trail volunteers. Saw was originally completely rebuilt and tested about a year ago. At the time it would run 14-14.5 K at WOT out of the wood. It was definitely NOT running rich, at least at WOT.

Got the saw back about a month ago with the complaint of no start. Checked the plug and it was oil/gas fouled. User said that they has just recently cleaned the air filter so I never got to see how bad it was. Put in a new plug and burned all of the accumulated oil out of it. I was also expecting that this and proper WOT running would clean any crud from the spark screen.

Got the saw back again yesterday. It would start and idle but wouldn't rev past idle. I first checked the carb and found nothing. Then I decided to check the spark screen and found it completely plugged with black carbon!

So now I am trying to figure out how this user managed to completely clog the spark screen. It is not like you could get any amount of cutting done with this small saw at part throttle operation. I am beginning to think that this started with the dirty air filter and went down hill from there. After looking at the muffler design with many SMALL outlet holes I have just about convinced myself that the screen will never get hot enough to burn off any carbon deposits.

Your thoughts?

 

[motorhead99999]

Does the operator mix his own gas or is it provided? Can you not run the screen where you at located

 

[SteveSr]

Does the operator mix his own gas or is it provided?

Mixing his own gas which is going to be one of my first questions - gas/oil type.

Can you not run the screen where you at located

These saws have completely non-adjustable carburetors so require the spark screen restriction to keep them from blowing up.

 

[motorhead99999]

My guess is to much oil or old gas.

 

[SteveSr]

Does the operator mix his own gas or is it provided? Can you not run the screen where you at located

Mixing his own gas which is going to be one of my first questions - gas/oil type.

Can you not run the screen where you at located

These saws have completely non-adjustable carburetors so require the spark screen restriction to keep them from blowing up.

 

[HarleyT]

So, with the spark screen cleaned and the air filter replaced,
how does it run?

 

[RiverLogger]

Replace that carb with the WT215 and you’ll not have any more issues.

 

[Brent Nowell]

The mix shouldn’t really matter imo for this particular issue. I have seen 50:1 saws with oil running down the exhaust muffler. It’s the tuning.
Its running rich, excess oil that wasn’t burned in combustion is clinging to the screen, and solidifying causing the clog.

First thing imo to see if you can reduce the oil coming out of the muffler if there are no adjustable jets, reduce the quantity of gas in the mix.

 

[SteveSr]

So, with the spark screen cleaned and the air filter replaced,
how does it run?

With an ambient temp of around 50 it will run 14K WOT. It is definitely not running rich at WOT. I did put a 0.45 main fixed jet in it to try to slow it down a bit but didn't have much effect.