Stihl ms391 woes

[LAWN BOY]

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, I don’t think that’s normal, it also hurts my hand when it does that. 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?

 

[Woodwackr]

There are all sorts of different colors of mix. Mine is red
Sounds like a timing issue. But, do a vac/pressure test first to make sure he resealed that thing correctly.

 

[redneckhillbilly]

if the fuel is questionable, dump it out and fill with fresh known mixed fuel.

try to figure out what the coil is exactly.
try to start without using the decomp.

 

[LAWN BOY]

if the fuel is questionable, dump it out and fill with fresh known mixed fuel.

try to figure out what the coil is exactly.
try to start without using the decomp.

I dumped the old gas right away and replaced with my mix. With the decomp out I can pull it over a few times good but occasionally it would jerk and kinda stop like I mentioned before still.

 

[FederalQ]

Check the coil gap to make sure it’s .010”.

 

[hacskaroly]

Sometimes the decomp pops up without the saw even firing half way thru the pull, I don’t think that’s normal, it also hurts my hand when it does that.

Decomps can pop up on an initial pull without firing. Every once in a while I get the same thing, when pulling and it jerks the handle out of my hand. That usually happens when my hand gets tired. I don't like starting a saw with the decomp depressed, I like having some compression. If the compression is too much, I will depress it and then pop it back up again. My 272 doesn't have a decomp on it and with new bearings and a new piston, it has some really good compression. Sometimes you just have to pull through it.

 

[hacskaroly]

He said he installed new piston rings

But, do a vac/pressure test first to make sure he resealed that thing correctly.

Yes, good to check the vac/pressure. This is a clamshell cylinder and if you don't get the dirko on there correctly , its easy to have air coming in around the bearings. I found that out working on my Poulan. When I worked on a 391, I made sure to get that stuff on there good and follow the directions on using it and you should be good to go.

 

[redneckhillbilly]

i've never used the decomp on any of my saws, im paranoid that by using it, it will get stuck or something and give me problems.

 

[hacskaroly]

i've never used the decomp on any of my saws, im paranoid that by using it, it will get stuck or something and give me problems.

Someone once said that the decomp is a factory installed air leak. When possible I pull them out and put a plug in.

 

[PA Dan]

I use them on 80cc and up ported saws. Never saw why they have them on 50 and 60cc saws.

 

[hacskaroly]

I use them on 80cc and up ported saws. Never saw why they have them on 50 and 60cc saws.

My 2100 doesn't have a decomp...we shall see how that goes when my last few parts arrive.

 

[PA Dan]

My 2100 doesn't have a decomp...we shall see how that goes when my last few parts arrive.

Neither does my ported 064. Not easy sometimes!

 

[redline4]

Neither does my ported 064. Not easy sometimes!

Elasto-start helps. I have one on my Stump Shot 064.

 

[PA Dan]

Elasto-start helps. I have one on my Stump Shot 064.

Same here!

 

[joseph24]

On our STIHL ms390(that has no decomp) my siblings were always shearing off the plastic dog/Pawl, when I put on an Elasto-start that seemed to fix the problem.

 

[Brad Button]

Pull the muffler and look at the piston before you go any further.

 

[SCHallenger]

I use them on 80cc and up ported saws. Never saw why they have them on 50 and 60cc saws.

I didn't like the idea of my 261 having it until I had it ported & tried to give it a "he-man" pull after it had been sitting for a few months. The saw said NO, & I ended up in PT for 6 months with a torn rotator cuff & torn biceps tendon. The compression on that saw read 205. My ported 241 & 462 are considerably higher in compression! I have learned how to start them, but when they get resistant,& I tire myself enough, I have to resort to a kick staarter. When you develop enough arthritis, you will appreciate a decomp!

 

[Mad Professor]

Pull the muffler and look at the piston before you go any further.

^^^^This.

Then if P/C looks OK, pull out the plug and pull it over a few times to be sure the bottom end is not flooded with mix.

Clean or replace the plug, and give a thimble full of mix in the carb. See if it fires and/or turns over easier.

 

[Wood Doctor]

"

I didn't like the idea of my 261 having it until I had it ported & tried to give it a "he-man" pull after it had been sitting for a few months. The saw said NO, & I ended up in PT for 6 months with a torn rotator cuff & torn biceps tendon. The compression on that saw read 205. My ported 241 & 462 are considerably higher in compression! I have learned how to start them, but when they get resistant,& I tire myself enough, I have to resort to a kick starter. When you develop enough arthritis, you will appreciate a decomp!

Or when you try to start a big Husqvarna 2100 that has its typical compression. You need help from another Swede such as Dolph Lundgren.

 

[hacskaroly]

Or when you try to start a big Husqvarna 2100 that has its typical compression. You need help from another Swede such as Dolph Lundgren.

I have a big D-ring handle on mine, so I can use both hands to pull