Stihl 039 uncontrollably racing

[Catfeesh]

I've got a Stihl 039 which after some use, will run at full throttle even when I let go of the trigger. I have to throw the chain brake in order to stop the beast. Before the racing started, it was running poorly and stalling at idle even after carb adjustments.
I installed an NWP cylinder into it last fall, after which it had been running very well.
The trigger linkage / carburetor action looks fine, but admittedly I'm an amateur.
I've been told that it could be the oil seals, impulse line, or fuel line, but am hesitant to do a full tear down.

Any advice would be appreciated.

 

[HarleyT]

Why wouldn't you just shut it off, instead of using the chain brake?

Just curious....

 

[ANewSawyer]

Almost definitely air leak. If you keep running it like that, you will ruin your saw. Less likely contengince is that the throttle plate isn't closing properly.

 

[HarleyT]

Look at the cracks in the fuel line first... If they are there, then the carb is full of crud too....

 

[Catfeesh]

Why wouldn't you just shut it off, instead of using the chain brake?

Just curious....

For whatever reason, I couldn't move the control switch to the off position. I haven't had the fuel line out yet, I'll check it and the throttle plate tomorrow.

Maybe it's unrelated, but all of my issues started when I couldn't restart the saw after it had warmed up unless I opened and closed the fuel tank cap. I figured that a vacuum was forming in the tank. I replaced the tank vent with some tubing I had, maybe there's a problem there?

 

[TBS]

Look at everything attached to the carburetor. If you had a hard time moving the lever to off something may be binding up or dislocated. Make sure the carburetor is not misaligned.

 

[67L36Driver]

Look at everything attached to the carburetor. If you had a hard time moving the lever to off something may be binding up or dislocated. Make sure the carburetor is not misaligned.

And, look inside the handle at the presence lever & spring. Could be hanging up the throttle rod.

The Stihl 'one lever does all' is problematic.

 

[HarleyT]

For whatever reason, I couldn't move the control switch to the off position. I haven't had the fuel line out yet, I'll check it and the throttle plate tomorrow.

Maybe it's unrelated, but all of my issues started when I couldn't restart the saw after it had warmed up unless I opened and closed the fuel tank cap. I figured that a vacuum was forming in the tank. I replaced the tank vent with some tubing I had, maybe there's a problem there?

Put up a pic of the throttle control, it may be on the wrong side of the spring.

 

[Catfeesh]

Put up a pic of the throttle control, it may be on the wrong side of the spring.

 

[HarleyT]

I mean this view. The contact spring. Yours should look like this...

 

[Catfeesh]

The fuel and impulse lines, trigger mechanism, and carb all look good to me. Dealer will have to do a pressure test for the crank seals. Seals were new last fall, would it be unusual for them to fail so soon?

 

[HarleyT]

Have you gone through the carb yet?

 

[Catfeesh]

I replaced the carb gaskets last year but that's where my knowledge ends. I'll have to look into that so I don't mess it up.

 

[HarleyT]

Take the two covers off, take pics as you go, then remove the lever/spring/needle. Look through the passage under the needle, see if it is choked up with crud.

 

[Catfeesh]

Take the two covers off, take pics as you go, then remove the lever/spring/needle. Look through the passage under the needle, see if it is choked up with crud.

Looks clean. I don't have carb cleaner, but Im soaking the carb parts minus the gaskets in some fresh gas. Thanks for all the advice by the way.

 

[ANewSawyer]

soak it in this: http://www.autozone.com/fuel-and-en...-dip-carburetor-and-parts-cleaner/139313_0_0/

 

[a. palmer jr.]

After I soak them I rinse the parts in hot water then use compressed air to dry them off. The Autozone cleaner is good stuff but don't stick your hands in it and wear safety glasses.

 

[Catfeesh]

As a follow-up, I cleaned the carb by boiling it in distilled water, and let it air dry. It didn't help with the over-revving. I took the saw into the dealership. They pressure tested it and found an air leak in the seal between the crankcase and the cylinder. The warranty recently ended, but I'll mention it to Bailey's anyways.
Thanks again.

 

[a. palmer jr.]

As a follow-up, I cleaned the carb by boiling it in distilled water, and let it air dry. It didn't help with the over-revving. I took the saw into the dealership. They pressure tested it and found an air leak in the seal between the crankcase and the cylinder. The warranty recently ended, but I'll mention it to Bailey's anyways.
Thanks again.

When they rev up like that it's usually an air leak somewhere or a stuck throttle..

 

[giver]

Check for cracks in the intake boot between the carb and cylinder, and check the cylinder bolts sometimes they come loose