Ms391 issues

[MorganE95]

So I posted a little while back about a stihl ms391 that wasn't getting spark. Well it turned out to be the ignition coil cause I managed to get the thing to finally fire up.

Now my newest problem is that I can get the thing to idle but it doesn't have a very consistent and confident startup. Like when I go thru the whole choke, partial kick over on the 3rd pull (or so), then half choke, then it may fire up on the 3rd or 4th and stay idling but I've been adjusting the carb to try and get it to start up properly, then idle properly and then upon giving it gas there's a small boggy delay but within about 2 or 3 seconds it will get going pretty fast. I'm not exactly how fast but what I thought sounded about right, once I put it onto some wood to start a cut. It's like it becomes a gutless wonder, like nearly any pressure into some wood makes the thing bog to a complete stop within about 3 or 4 seconds (varying with the amount of pressure applied to the saw).

I feel like it's maybe not getting enough gas or too much air or maybe both. Now I must say. The fuel line is good and doesn't have any leaks, the fuel filter is brand new. The carb is brand new, the fuel mixture is a perfect premixed 50:1, air filter is brand new. Chain is wicked sharp. Clean bar with proper chain tension.

And with all of that I consult the ones who have more of an idea of what is going on with this damn thing. Please help. This saw is a long time in the making and I just want to finally have this not so little project finally crossed off my list. Lol.

 

[Mike Kunte]

Great that you got it running! With a new carb on, I'd be looking for other sources or air leak, for example in the intake boot or the crank seals. Also, the tank vent may be blocked.

Mike

 

[irhunter]

My 2 step for poorly running saws.

1. Replace spark plug and set carb to factory setting. If it still runs poorly:

2. Run pressure and vac test of case and carb.

 

[Mad Professor]

Bogging off idle is usually (barring other problems) too lean on LO carb setting.

Is new carb OEM or an Ebay Chi-Com wonder?

What is HI set at? Saw should burble/4-stroke at WOT (rich) and clean up in the cut. Better too rich than lean when going WOT. Try fattening up HI.

Pres/vac test would make sure no leaks.

 

[stihltech]

Without a pressure and vacuum test first you are wasting your time.

 

[derekjccdist]

So I posted a little while back about a stihl ms391 that wasn't getting spark. Well it turned out to be the ignition coil cause I managed to get the thing to finally fire up.

Now my newest problem is that I can get the thing to idle but it doesn't have a very consistent and confident startup. Like when I go thru the whole choke, partial kick over on the 3rd pull (or so), then half choke, then it may fire up on the 3rd or 4th and stay idling but I've been adjusting the carb to try and get it to start up properly, then idle properly and then upon giving it gas there's a small boggy delay but within about 2 or 3 seconds it will get going pretty fast. I'm not exactly how fast but what I thought sounded about right, once I put it onto some wood to start a cut. It's like it becomes a gutless wonder, like nearly any pressure into some wood makes the thing bog to a complete stop within about 3 or 4 seconds (varying with the amount of pressure applied to the saw).

I feel like it's maybe not getting enough gas or too much air or maybe both. Now I must say. The fuel line is good and doesn't have any leaks, the fuel filter is brand new. The carb is brand new, the fuel mixture is a perfect premixed 50:1, air filter is brand new. Chain is wicked sharp. Clean bar with proper chain tension.

And with all of that I consult the ones who have more of an idea of what is going on with this damn thing. Please help. This saw is a long time in the making and I just want to finally have this not so little project finally crossed off my list. Lol.

 

[derekjccdist]

Check your fuel lines and carb with a vacuum gauge