Weird running MS390

[Engineer]

I want to get my Stihl MS390 in good running condition, before I either sell it or decide it's useful.  Either way - I sharpened it and filled it up with fuel yesterday, tried to start it.  It would start OK, but then within a second or two, stall and die.  Finally got it to where it would roughly idle.  Tried using it, the only way it would run at full throttle was if the bar was pointing straight up at the sky.  The instant I brought the bar horizontal to cut, even under full throttle, it would lose most of its' power and the chain would be still turning but slow enough so that I could see the individual teeth go by.  If I let off the throttle, it died after about five seconds. 

I think this was why I put the thing away a year ago, not because the gas tank leaks (that's another problem entirely) but because I couldn't get it to run properly.  It's a powerful saw but not at the moment.   What could be causing it to run in one position but not in another?

[beenthere]

Sounds fuel related, and by the position you describe when it runs...I'd give a close check to the fuel pickup in the tank.. and figure out the gas leak....might solve the run problem.   Good luck.

[ladylake]

Check the fuel line and impulse hose for cracks and the connections first. The holding it up in the air and running better could be the rubber boot between the carb and cylinder torn or cracked (caused by yanking on the saw when pinched, we all do that).  Holding in the air might close the crack and other positions let it open. Also it wouldn't hurt to take the carb apart and clean all the passages. most are simple to do.   Steve

[joe_indi]

Simple :
There are 4 screws holding the engine block onto the body.These screws have the habit of becoming loose.
Once this happens the 2 halves of the engine block have the tendency to open up a bit when the saw is held in different positions.When this happens the engine runs leaner, accelerates and dies off.If you do any adjustments of the carb screws, at times the engine runs super rich (when the two halves come together) ,stalls or slows down, and super lean (when the halves separate), revs up and stalls.
You will need to check these screws before doing anything else.

Serious:
You might have a stuck or leaking diaphragm in the slow speed circuit of the carb.
When this is the cause the symptoms are:
Engine runs smoothly for some time, leans out and dies as if running out of fuel when the saw is held with starter side towards the ground or when the guidebar is  pointing skywards.
Engine runs smoothly for some time then starts to slow down, floods and stops when the clutch side is towards the ground or when the guidebar is pointing downwards.
If this is the case the only option is to replace the carb.

[Engineer]

I got the saw back from the dealer yesterday.  It was a simple problem and a simple fix.  Gas line was about 3/4 of the way broken, so that it only drew gas when it was vertical.  A new gas line and we were back in business.  He also sharpened it, cleaned the carb and air filter, replaced the gas, topped off the bar oil, and gave it a good thorough cleaning and degreasing.  All for $50.

I ran two tanks of gas through it yesterday and was very happy with it again.  I guess I'll be keepin' it and not trading it in on something else.

[woodhick]

That's why I stick with Stihl, a DEALER. ;)

[RSteiner]

I have also experienced a similar problem with the fuel filter getting almost saturated with water so it would not pass enough fuel to run above an idle.

Randy