Stihl ms391, help?

[Chainsaw Collectors]

I recently acquired a stihl ms391 from an elderly man that does small engine repair. He said he installed new piston rings and a new coil, but he’s not sure it was the right coil. He also played with the coil gap. It does currently have spark and fired a couple times. Sometimes the decomp pops up without the saw even firing half way thru the pull and hurts my hand in the process. Also, the fuel he had in it was pretty much clear, isn’t all 2 stroke fuel blueish? Maybe I’m just used to my Lawnboy and Husqvarna fuel mix being tinted, or maybe the old man forgot to add oil… If it has spark and even fired a couple times I’m assuming the coil is right or suitable. Perhaps an issue with the decompression valve? Any ideas? He also said he installed new piston rings.

 

[pioneerguy600]

First thing I do on any saw brought in for work is dump the fuel that is in it, no one knows what ratio or how old it is so it gets dumped. New fresh mix gets added before even a test firing. When someone else has dicked around with it then it is the same run of tests always performed , no matter what the previous operator or owner tells me. If they knew what was wrong with the saw they could fix it, difficult to trust anyone concerning a non running or hard to start saw.

 

[ZeroJunk]

My MS660 kicks the decomp out about every time whether it fires or not.
I start all saws on the ground with my right heel in the far side of the handle.
So, no big deal to push it back in with left index finger, or just not use it.

I would consider it a good sign the saw has compression.

 

[stihltech]

Always start with a fuel check, as stated before. Give it a good lookover. Pulse hose, fuel hose and intake boot connected and not cracked (#2 thing to look at on a 390).
Why a coil? Is the flywheel key OK?

There are lots of small engine repair guys that are great on 4 strokes, not so much on 2 stroke.
Time to diagnose from step one.
It may be simple. Just need to undo some repairs first and verify the other systems. Then you will find it.

 

[MacAttack]

If there's a part number on the coil you should be able to find out if it's the right part.

 

[billyboy]

I start my 036 on the ground, toe under the handle, with my index finger on the decomp. Works most of the time. 175 psi, so decomp pops up most every time.