MS 390 will not idle

[Ward Barnes]

Howdy:

I have a MS 390 purchased in 2007 that in the last few days quit idling.  The saw will start and run a full speed and cut with what seems like full power, however, once I let the RPMs slow it will idle a minute or so and then the engine quits and will not restart while the engine is hot.  The air filter is clean as is the spark arresting screen. I re-adjusted the H and L and adjusted the LA to max idle with the same result.  Any ideas?

God Bless, Ward.

[JohnG28]

Check the muffler scren to see it, might be clogged.

[JohnW]

One idea -- the fuel line.  I don't work on engines, but the only problem I ever had with my 310 was the fuel line, and it acted a little like that.

[T Welsh]

It does sound like a cracked fuel line or something has clogged to low speed passage in the carb. pull the cover off it and take the air horn off and fire it up and watch the fuel line,and twist it and see if its wet. if thats not the case take the carb apart and clean it and reinstall and see what happens,also check the impluse line while your at it. this ethanol gas we have been getting is a killer on small engines. Tim

[Ward Barnes]

Howdy:

Like Robin said to Batman...    Read the instructions -  what is written - rather than what you thank is written and ZOWIE it works!  The original problem I had mid-last week was the saw would start and run fine at high speed but would not idle so I adjusted the H and L per the book (first time that had been done since the saw as new - if it is not broke don't fix it) and what I thought the book said about the LA.  After re-reading the book (again) I had the idle set backwards.  Works fine now.  Thanks to all who gave advise to this new-bee.  Now to set the the top end speed down just a tad.  It sounds to my un-trained ear to be a bit fast.  I wonder if there is an egg-on-the-face smilie? 

God Bless, and thanks again.  Ward.

[T Welsh]

Ward Barnes,Great fix ,dont be hard on your self. thats the way I learn things Look at the good side of life,it did not cost a dime to fix it. Tim

[Ward Barnes]

Thanks Tim:

One real good thing with the problem with the saw was it brought me here when I was searching for an answer.  What a great group of folks on the forum.

I just love the quote I read from a early post that said, "You don't sharpen a chain after it gets dull, you sharpen a chain to keep it from getting dull".  To which I may add, "A sharp cutting instrument can prevent accidents.  A dull one causes to much force to be applied trying to get the same result".

God Bless, Ward.

[T Welsh]

Ward, Lots and Lots of wisdom and common sense on this forum. the people on this forum truly enjoy helping people out, makes us feel good because we have been there and done it . Tim

[Ward Barnes]

Howdy:

The saw would not idle again this morning so I bought a carb re-build kit ($18.00 for the kit at the local Stihl dealer, vs $86.00 for a new carb) and tonight I rebuilt my first ever carburetor.  Good feeling, but, now the saw will not run at all.  Ah shucks.

Tomorrow is another day and I'm too tired to try and figure it out tonight.

God Bless, and thanks to all who have tried to help.  Ward.

[T Welsh]

Ward, Watch this it will help,its not your carb, but will give you a general idea of what to do. good for you for trying,thats how you learn. Tim

!

[terrifictimbersllc]

Nice video, thanks.

[Ward Barnes]

Howdy Folks:

Well I put the saw aside until after work today.  Reset the H and L and HL settings and the saw started and ran at high speed.  BUT...  Still the idle problem is there.  Runs good at high speed will not idle.  Humm...... I am learning more about see-saws in the last few days then in the last 60 years.  Now to get the rest working.  It could still be a cracked line or something.  Wish I had purchased a new fuel tank line and replaced it either at the same time I rebuilt the carb or replaced it first.  If I had done that I would not have learned how to rebuild a carb though.  Any other thoughts folks?  BTW the video was a good one.  Thanks.

God Bless, Ward.

[joe_indi]

Ward,

There are two types of no idle situations
#1. Leaning out due to low to no fuel supply at idle.Spark plug will be dry without carbon. Engine will sound as if its running out of fuel.Engine will sound too light with irregular beats.Even the slightest  turning in of the LA screw will have the engine in high revs. Engine will restart probably with choke.

#2. Too much fuel or gradual flooding.Plug will be wet and dark. Engine will have strong pulses which slow down and stop abruptly.The engine might run a bit longer if LA screw is turned in more. Engine will only start with partial or full throttle.

So, which of these is the condition of your saw?

Joe

[Al_Smith]

All good advice .One more thing that happens rarely but it does happen .

Depending on the carb some have a very tiny narrow slit on the butterfly or throttle plate  that must align with the low speed or idle jet .Some times for what ever reason the butterfly becomes out of alignment by turning on the throttle shaft .

Fact I had a similar situation that about drove me nuts which really didn't take much being about there any way .On the advice of Rocky J .I found what I just stated to be the problem .

[Ward Barnes]

Joe:

Number one sounds the best.  The plug is dry.  The engine now starts well in the 1/2 choke position after starting in full choke.  I think it is running a bit fast.  Even when warm/hot the engine dies as soon as the trigger is released with no attempt to idle.

The engine has a new plug.  The air filter is clean and the spark arrestor screen has been removed.  I rebuilt the carb with a carb repair kit from the Sthil dealer.  From what I have seen on the web and here it appears that I rebuilt the carb correctly.  It was my first rebuild so I may not have it right.  One thing that confuses me is the directions on adjusting the High and Low and the LA so I may have them way off.

God Bless, Ward.

[joe_indi]

Ward,
L and H screws turned clockwise the fuel(& air) mixture leans out.
L and H screws turned anti clockwise fuel mixture becomes richer.
LA screw turned clockwise beyond a point, with L turned too much in clockwise direction can cause a further leaning out of the fuel mixture. This happens from idle to mid range speed.
So to start with, you need to bring all screws to Standard setting.
If your carb has limiter caps all you have to do is turn the L and H screw Anti clockwise to their stops.
If your carb is without limiter caps, turn the L and H screws Clockwise to their stops. Then back them both Anti clockwise one full turn.
#Hold the carb up to the light and look through the inlet end at the butterfly valve of the throttle. If you see any background light at the top and bottom turn the LA screw Anticlockwise until light is visible only at that small semi circle on the edge of the butterfly.
Once you have done this, turn the LA screw Clockwise till you see a tiny wedge of light at the bottom.
The carb is now in Standard setting.If everything else is fine with the carb and the saw, in this setting the saw should start and run a bit rich in idle which can be adjusted by turning the L screw about quarter turn Clockwise.
However, Al has raised a very (very) important point there.
So, to confirm that is not the issue with your carb, make sure the butterfly closes completely when you adjust the LA screw Anticlockwise as mentioned in #
Why dont you do all the above and lets see what happens

Added  nearly 10 hours later (Thought of it during an exceptionally long sermon in church):
Another possibility is a damaged or blocked Impulse hose.
This is a picture of the end of the impulse hose that connects to the carburetor.
 

 

The carb needs the air impulses for the fuel pump to work.
To check if the impulses are coming through remove the spark plug and carburetor.
Drop some oil through the open end of the impulse hose. Now crank the engine.
The oil should shoot out.If it does not shoot out the impulse hose is blocked or broken.


Joe

[Ward Barnes]

Joe:  Thank you for your continued interest.  This morning I set the carb to what I thought was Standard setting and started the saw.  The saw would run at high speed and then again shut off when the trigger was released.  Restarting was one pull in the half choke position.  Tonight after work I pulled the carb and found that I had missed the Standard setting by a little so I set the H and L per your directions.  I also set the LA by holding the carb up to the light.  I checked the impulse inlet tube and it may be clogged/broken as I did not get any oil through it.  If that is the problem it looks to be a real bugger to replace.  I have reassembled the saw and will test it tomorrow morning after the neighbors have gone to work (I am off tomorrow) and give the saw a good work out on some oak.  If the problem is still there I may go ahead and tear down the carb again and ensure all is right and re-clean all the ports before even thinking about the impulse tube again.  I will keep you posted as to the progress with the saw.

God Bless, Ward
P.S.  What was the Pastor's sermon on this morning?

[joe_indi]

P.S.  What was the Pastor's sermon on this morning?

He started with Mark Chapter 4 . But along the way he took a wide deviation, faith moving mountains etc. After 15 minutes with no horizon in sight, I switched off and tuned onto your carb problem.I had another 30 minutes to mull over your problem before I heard the words "Let us pray'

Joe

[lumberjack48]

I would get a carb that i knew was good and try it, so i knew what i was dealing with.

[Ward Barnes]

Howdy Folks:

  My friend "who knows what he is doing" came to the house and picked up my saw.  Worked on it a bit and it now purrs like a kitten.  Now to get him to teach me what he did.   

Thanks to this experience and the help I received from this board and it's members I now know more about saws then I ever thought I would.

God Bless and great cutting.  Ward.