066 Maintenance surprise

[Jim Spencer]

I cleaned my air filter,
replaced my sprocket drive rim 7T with a new one,
replaced my fuel filter in the gas tank because I read where Joe Indie had said that a plugged fuel filter might starve the engine for fuel and cause the piston to seize.  I appreciate Joes input to this forum he seems to be very competent and constructive.
I also blew all sawdust off this 7-10 year old saw that has milled at least 10,000 bd ft of lumber,  red oak, white oak, and maple, and ash.
I thought that maybe I should check the carburetor settings to ensure I am running on the rich side to provide good lubrication.   Started turning the high speed screw out and tried setting saw so it would burble or flood out at high speed and could not determine when it changed sounds.  Kind of deaf from 75 years of working around noisy machinery.  Anyway tried milling an ash board about 18" wide with a 24" bar on my Logosol.  Saw would cut great for about 10-15 seconds then bog down.  Tried numerous settings of the high speed screw with not much change.  Saw would actually cut better at about 1/3 throttle for about 30 seconds then die.  After setting for 30 seconds or so saw would start right back up and run great until I started back into cutting and same symptoms all over again.  After an hour or 2 of screwing around trying to adjust the high speed needle I decided to take the saw and clean it up to take it to the Stihl dealer for adjustment.   Decided to blow dust off of saw and out of air filter.  Under the airfilter there are (2) 5/16 nuts which I have never had off before.  Removed these nuts and removed filter behind airfilter. Shocked! Packed solid with very fine dust which was plugging air intake.  Found my problem!!!!!!!!

[Jim Spencer]

I guess I used too many characters because my 1st post would not let me write any more.  I never dreamed that I would have to remove these (2) 5/16 nuts and clean behind my air filter.  The manual does not even mention that this can get plugged with dust.  Of course my Logosol generates am very fine dust that is not normal when cutting across the grain of a tree.
Anyway, owners of the 066 should be aware that this can cause their saw to use excessive fuel and lose power due to the air restrictions caused by this buildup of dust in the area behind the airfilter mounting surface.
Hope this helps someone.
              Jim

[joe_indi]

Jim,
That's quite a lot of good things you have said about me.
Thank you.I am happy that my bits of info are actually of use to people.

Jim, when the area below the air filter gets clogged with saw dust, one of Stihl's patented 'gems' come into action.
The air compensator.The saw dust blocks of  the air passages causing a low pressure area below the filter.This causes the metering diaphragm to distend, move away from the needle valve, lower the fuel level and thus cause a leaner fuel flow into the engine.
The theory of all this is that lesser air needs proportionally lesser fuel.So, as the air filter gets clogged, instead of the fuel getting richer, the compensator works to make it leaner.
That's fine in theory but in a 2 stroke engine that is running at high temperatures, lesser fuel also means a lesser amount of lubricating 2 stroke oil.
Not a good thing you would agree.
So, make sure to clean under the filter and behind that black screen(fastened by those 2 nuts), at least once a month.
Its just a few minutes' job. With that screen removed and the choke shutter closed, use a blast of air to get the saw dust out.
I use a very hi-tech equipment to provide the air, my son's foot pump that he uses to blow up his football.  :D :D
In the present situation, if the cleanup does not work, probables are that you have a blocked or partially broken impulse hose.Or that the jet valve in the carburator is not functioning properly.
If the saw does not run properly after the clean up, post the results here and lets see what can be done.
Joe

[Jim Spencer]

Joe;
Your analysis was absolutely right Joe as your intuition comprehended the cleanup did not work.
I guess I assumed the dust was the problem.
  I still have the same problem.
If I try to rev up the saw it will die after about 20 seconds.  Seems to me to be running out of fuel and stalling.  Will start right back up if I choke it but then will run great for about 10-20 seconds and die again.  Yes I value your input on this forum as I have found your suggestions are generally accurate and precise.

I am going to see if I can identify the impulse hose to detect if it is blocked or broken like you stated.  Is the impulse hose the fuel line with the filter on it ? or is it another hose?  I really appreciate the help.
Jim

[Jim Spencer]

After thinking a little more I do realize that the fuel line is not the impulse hose.
Impulse hose would be like a fuel pump actuated by the piston cylinder probably working the diaphram in carburetor.  I have seen this on another saw.
I guess I will check this next.
Jim

[Jim Spencer]

I removed the screws (7) I think it was from the shroud that is behind the carburetor and covers this area.  I did not feel comfortable removing all the lines and mechanical parts that seems to be required to remove this shroud. I believe I would need to remove this shroud to check the impulse hose.
I think I will take it to the Stihl dealer tomorrow and see if they will fix it.  I used to do things like this when I was younger but I will soon be 75 and my memory, recognition, and a lot of other things are no longer working as they used to.
Like I said in my first post I screwed out the high speed jet about 6-8 turns and then turned it back in to the seat and then back 1 turn like the manual states.  After that is when it started running bad.
Previous to screwing out the carb jet  I had removed the (2) 5/16 nuts and blown out the dust. I may have gotten dirt or something under the carburetor mounting surface because I used high air pressure to blow the dust. Is there an "O" ring I may have blown or mislocated???  If I did I don't think that would cause fuel starvation after 20 seconds of cutting.  This will be interesting to solve.

[Jim Spencer]

I wonder if my high pressure air blast damaged the carburetor diaphrams?

[Rocky_J]

No. More likely the carb is out of adjustment.

http://www.madsens1.com/saw%20carb%20tune.htm

[Jim Spencer]

Rocky,
I will be very surprised if it is a carburetor adjustment.  It is starving for fuel and no matter how far out I turn the high speed jet it will still die after 10-20 seconds.
Runs better when its being choked.
Used to flood out when choked but now it will keep running, with no power.
I am thinking maybe the diaphram fuel pump may be damaged but I don't know how that could have happened.
Appreciate the response though.
Jim

[joe_indi]

Jim,
The initial running and subsequent dying out is 90% of the time caused by a damaged or blocked impulse hose.
You will need to remove the carburator to access the impulse hose.
Before you can remove the carb, you need to disconnect the fuel hose and the throttle rod.
The fuel hose is simple enough. For the throttle rod, have a small screw driver and a nose pliers at hand.Depress the throttle trigger fully.You will see the throttle lever on the carb move forward.
Use the screw driver to hold the throttle lever in that forward position.Release the throttle trigger.The throttle rod will be loose now.
Use the pliers to lift the rod out of its groove.
Now, the carburator can be removed.When you re-fit the carb, move the throttle lever forward with the screwdriver and use the pliers to drop the rod into its slot.
I am posting two images here to show you the impulse hose at the carb end


and  the crankcase end.



In case you want to do the complete job by yourself, let me know, I'll lead you through the steps.
Just to confirm its an  impulse problem, here is a very simple test.
Before going any further make sure you have enough fuel in the tank and that the fuel filter (pickup body) is not blocked.
This test tells you if the filter is good:
Lay the saw tank side up.
Open the fuel tank breathe into it and close it.
Move the saw back to its normal position.
If the filter is good fuel should spurt out of the open end of the fuel hose.

Remove the metering chamber cover.This is the top one with a ventilator on top.
Carefully remove the metering diaphragm.
The metering needle is held in place by a spring loaded lever.This is very much like the float chamber of a regular carburator viewed bottom side up.
If you press that lever the needle valve opens and  when you release it... etc, etc.

Now mount the open ended carburator back in its position with the fuel hose & impulse hose connected.
Remove the spark plug.
Control switch should be in 'Off'.
Use a finger to press down on the needle valve lever (to keep the valve open)
Now pull on the starter a couple of times.
After a few pulls, fuel should shoot up through the needle valve.
If it does not happen, maybe you need to 'prime ' the fuel pump.
Do the 'breathing into the tank' bit once more with the fuel hose connected to the carb.
Now when you press on that lever the fuel will shoot out even if you dont crank the engine.Wait till the fuel stops shooting out.
The fuel pump is primed now.When you crank the engine, fuel should come out in little spurts now.
If that does not happen, you have a fuel pumping problem.
Open the bottom cover of the carb.The one with the connector for the impulse hose.
The tan colored sheet is the pump diaphragm.
Lift it out, you will see a small circular piece of mesh in a recess.
Make sure that this mesh is clean.
Blow through the connector on the connector on the cover, it should be clear.Any brown gummy residue in the cover should be removed.
Use oil to coat the bottom face of the carb and the gasket of the bottom cover.Put the diaphragm in its correct position on the cover and drop the cover in position onto the carb.Tighten it down .
Try and send a couple of drops down the impulse hose. When you pull the started the oil should shoot out. If not, the hose is damaged or blocked.
If the oil shoots out, fit the carb back into its position and crank the engine once more to see if the pump works.
I think all that should keep you busy for some time.
Do post the results so that I can give you suggestions on how to proceed next.
And have a look in my new picture folder https://forestryforum.com/gallery/thumbnails.php?album=2580.
The Valve jet could also be the possible cause.

Joe

[Dalden]

Noting your in souteast MI too -  I have the Stihl service manual for this saw if you need to reference something and you live near Pinckney?

[Jim Spencer]

Dalden;
I have the owners manual and I have went through everything that Joe Indie mentioned on the forum.
The impulse hose was like new and not the problem.  I did find a lot of dirt under the diaphram in the carburetor.
After putting everything back toqether I started saw and thought it is running great now I need to put blade on and mill a log.
I have a Logosol chainsaw mill.
I started saw a couple of times and was certain it would be o.k.
The last time I tried to start saw before installing blade The engine fired before top dead center and jerked the rope so hard I thought it had pulled my arm out of socket.
I tried to start saw numerous times again and each time when it fired it jerked the stating rope out of my hand.
For some reason it is firing too soon and kicks back.
Don't know why this would happen now because this has not been a problem before.
Maybe the key for the flywheel magneto has sheared and maybe that was my problem all the time.  This is really getting confusing. 
Thanks for any help,
Jim

[Jim Spencer]

I went back and tried to start saw again and it started right up and I think it ran good.
Today I did everything Joe Indie mentioned.  I removed carburetor, checked under metering chamber cover which seemed clean, pressed the needle valve to actuate, seemed O.K. reassembled.  Opened bottom cover, cleaned lots of fine dirt from under pump diaphragm.  The small circular mesh cavity was full of fine gummy residue, removed residue and cleaned mesh. Removed carburetor mounting shroud and also shroud over top, removed and blew thru impulse hose, (this was O.K.).
I really believe now that the problem may have been the fine gummy residue in the mesh cavity of the carburetor.  The worst trouble I had was getting up enough confidence to do the job of course it helped when the Stihl Dealer told me that it would be, 5 days before he could look at it. The hardest was reassembling the throttle trigger mechanism.  Dark and <0 deg. centigrade so did not cut anything yet to certify operation.  Will let you know tomorrow after testing.  Thanks to Joe and everyone else who gave me courage,confidence,and ideas.
Started saw about 5 times and let it run at high speed for a few minutes and it now seems O.K.
Jim

[joe_indi]

Jim,
I am really glad that the DIY on you saw came out fine.
The first time is difficult, but now you know how, try and clean the carb and the surroundings once in a way.
The backfiring that you mentioned would not be from a non-aligned flywheel.When the flywheel moves out of alignment, it usually retards the timing.
Yours sounds more like deposit build up in the cylinder and piston.
In the course of the repair, these deposits soak up fuel, along with the spark plug getting wet but the cylinder is dry otherwise.
So, when you try to start normally, the increased compression and extra fuel in liquid form cause a pre-ignition.
That is what I suspect. It happens quite often to me.What I do in such cases is to kind of semi flood the engine a bit.
That is pull the starter once or twice more than necessary.Then start the engine as usual.No misfires then.
The gummy stuff in the bottom cover is from saw dust sucked into the crankcase and which was then sent through the impulse hose into that cover.
To reduce this from happening, you have 2 options, that is if you are using the regular wire mesh airfilter.
Buy a Heavy Duty(HD) filter. This has a removable felt pre filter.The filter itself is of a stronger mesh and it also has a paper element.This filter will prevent fine saw dust and resin from entering the engine.
Or, as a partial cure, buy the pre-filter of the HD filter.This is a felt band with elastic that goes over the mesh area of the air filter.

A third but I am not sure whether its available there, is the best option.This is an optional  filter system, which I believe was developed for Australian users.
In this system the saw runs without a filter cover.
The regular filter is fitted with a very thick fine pre-filter sponge.The filter is fitted onto the saw with a plastic plate.
Here are the pictures of the pre-filter band and the 'Aussie' filter



Joe

[Jim Spencer]

Yesterday I milled 2 logs to try out my saw after cleaning carburetor, checking impulse hose, cleaning sawdust from behind the filter mounting surface.
I an happy to say that my saw ran perfect.
It runs just as I remember when it was new.
Stihl should add more information to their operators manual telling customers to service the area behind this filter mounting surface.
I will be cleaning the sawdust from this area at least a couple times a year and maybe more frequently as it will depend on how much i use the saw.
Thanks everyone for the help.
Thanks Joe Indie for the advise and encouragement.
Jim Spencer