Let's play diagnose the farm boss chainsaw.

[fearofpavement]

Ok, after a couple different sessions of trying to figure out this saw and pulling out the 8 remaining strands of hair, I have finally found a solution. It was a long long road. Tell me what you think is wrong with the saw. I'll start by describing the symptoms and we'll go from there.

The Stihl MS390 saw came in a multi saw lot of non working saws. They were from a tree service I believe. The saw was missing the muffler. (I may have pulled it off after I bought it, I don't remember)
I tried starting it with a bit of mix in the carb. Fired and revved up. Really loud of course but I thought maybe not much wrong with this saw? I located and modded a muffler and installed it. Fueled the saw, put some oil in the tank and started it up. Oiled ok so I put a bar and chain on it. The saw would rev but was really slow to come off idle, had to milk it until it got up to rpm. I took it out to my test log and tried cutting. No deal. Saw could rev but no power at all. Didn't seem to be starving for fuel but I got out the carb screwdriver and started messing with the screws (limiters previously trimmed by others). I just could not get this saw to make power. I had already looked at the piston/cylinder and it looked clean enough and compression felt ok while pulling it over. So I take it back to the shop and start in on it. Two days later, it's running. What was it?

 

[Philbert]

Ice in the fuel lines that melted while in your warm shop?

Philbert

 

[fearofpavement]

Nope, no ice. I emptied the tank and refilled it.

 

[old-cat]

Weak spark.

 

[MnSam]

Bad fuel line.

 

[Wood Doctor]

Fear said,
"So I take it back to the shop and start in on it. Two days later, it's running. What was it?"
---------------------------
Well if you do not tell us what you did when you started in on it, how are we supposed to answer the question? It looks like you solved the problem. Water in the fuel system could have caused it as Philbert said.

My follow-up advice is to never run a saw without a muffler. That will kill it but not quite as fast as straight gassing it will.

 

[cus_deluxe]

air filter

 

[cobey]

diphram was dry... when it set it loosened up, or crud desolved in the fuel system

 

[fearofpavement]

I'll throw in some diagnosis I did...
Checked the fuel line by pressure and vacuum testing it while wiggling it around. Checked ok.
I cleaned the fuel filter and it's fine.
I pulled the carb and cleaned it.
Then I pressure checked the carb and it passed.

 

[kyle.kipple]

Clogged Air filter?

Collapsed Fuel line?

 

[fearofpavement]

I checked the diaphragm in the carb and it seemed flexible enough. checked the carb screen, checked the pump side, pulled the carb screws out and inspected them and blew carb cleaner through them, Carb seemed ok. Installed it and same symptoms

 

[fearofpavement]

I checked the air filter. Washed it out with solvent and blew it dry. Looked new when finished. Fuel line in good order and no leaks. I disconnected the vent line and checked it with vacuum and pressure, it was working properly.

 

[fearofpavement]

Finally decided it must be the carb and tried a different one. Didn't make a difference. So I installed the original carb on a running saw and it worked fine so I have eliminated the carb.

I tried a different spark plug (known good one) No difference

 

[cus_deluxe]

you said compression felt ok pulling it over. did it turn out to be not ok? sounds like you have the fuel figured out. so its either questionable compression or spark it would seem

 

[fearofpavement]

Decided I better check the compression even though it seemed good. It read 135 on my gauge. So I checked a good running MS390 and it read 135 so not a compression issue.

 

[1Alpha1]

Bad flux capacitor. opcorn:

 

[fearofpavement]

How the saw felt when running is like the muffler was plugged. I knew it wasn't as I had checked that. It just wouldn't put out significant power. Idled unstable and was real slow coming to rpm.

 

[fearofpavement]

Bad flux capacitor. opcorn:

I figured this saw didn't need one as it's not a pro model.

 

[rich450es]

that flipping ignition switch spring was bent

 

[fearofpavement]

So by this time I'm really puzzled. Got out the gear and pressure tested the saw. (passed)
Vacuum tested the saw. (passed)