Stihl ms391 woes

[LAWN BOY]

Here are some piston pics. I pulled the plug which looks new and pulled the saw over a lot. Seemed flooded. Reinstalled plug and gave it a thimble of mix down the carb and problem still remains. Any chance the carburetor is just dumping that much gas into the engine causing it to do this, is that possible?

 

[hacskaroly]

Seemed flooded

If the saw seems flooded, put the lever on run (not choke or fast run) and just pull upwards of 20 times, at some point it will start popping and even run. I had many customers that brought in saws in this condition and it just took pulling it until it fires. I was just working on an 034 and after replacing the piston could not get it to fire. It popped once and I just put it down to come back to it. A couple weeks later went to start it and it fired right up - it shut down a couple of times while adjusting the carb and somehow ended up flooding it. Realizing what I did, gave it about 20 pulls and it fired up.

 

[Wood Doctor]

However, some saw engines will not start after being flooded, regardless of how many non-choked pulls. I usually just let them sit for half an hour and use another back-up saw that runs. They almost start right up after watching another saw take their place. Sometimes chainsaws are half human.

 

[Woodwackr]

I have pulled the plug and flipped the saw upside down and pulled the cord a few time to clear a very flooded saw.

Have you pulled the carb and checked the diaphragm and pump flaps?

 

[LAWN BOY]

I have pulled the plug and flipped the saw upside down and pulled the cord a few time to clear a very flooded saw.

Have you pulled the carb and checked the diaphragm and pump flaps?

I did have it upside down but didn’t pull upside down. Thanks for letting me know.
I haven’t pulled the carb yet. I can do that next I suppose.

 

[Woodwackr]

I did have it upside down but didn’t pull upside down. Thanks for letting me know.
I haven’t pulled the carb yet. I can do that next I suppose.

Carb is the most likely cause.

 

[LAWN BOY]

Carb is the most likely cause.

I hope that’s all it is. I’ll probably work on it this weekend.

 

[LAWN BOY]

Update: the new $30 carburetor arrived from the Dukes today. I installed it. I also held the saw upside down with the spark plug out and pulled it over to get all the gas out of there. Same problem. I noticed the decomp leaks/sprays a bit of fuel, could that be the whole problem? I didn’t adjust the Duke’s carb yet at all… I’ve heard about weird issues with Stihl easy start or elasto start starters, is this one? The pull a handle is just a normal one someone installed after buying it.

 

[Woodwackr]

I've not seen that saw with the easy start recoil. Those things are just plain weird

 

[hacskaroly]

I noticed the decomp leaks/sprays a bit of fuel

Is the decomp in or out when it is leaking or spraying bits of fuel? If it is out (supposedly sealed) then there is an air-leak issue and the decomp should be replaced or plugged.

 

[hacskaroly]

I've not seen that saw with the easy start recoil. Those things are just plain weird

Correct, the largest Stihl with the easy start is the 251. I don't like them, but they work well for people who lack upperbody strength or have had shoulder surgeries where just yanking on a chainsaw pull cord will undo any repair work they had done on their body. For people used to regular pull cords, they can easily break the easy starts. I told a customer about it on a 251 that he bought and the next day he came back with a broken pull cord...he forgot that you don't have to pull like a normal saw.

 

[hacskaroly]

I’ve heard about weird issues with Stihl easy start or elasto start starters, is this one?

I don't think it will have elasto start. If I remember correctly the 391 I worked on had just the standard pull starter.

 

[LAWN BOY]

Is the decomp in or out when it is leaking or spraying bits of fuel? If it is out (supposedly sealed) then there is an air-leak issue and the decomp should be replaced or plugged.

I don’t recall too many gas fumes lately I guess lol. If the decomp is causing an air leak, that wouldn’t cause the main issue currently at hand, would it?

 

[hacskaroly]

If the decomp is causing an air leak, that wouldn’t cause the main issue currently at hand, would it?

If it is not allowing proper compression, then all the saw is doing is just pumping more fuel into the cylinder with each pull.

 

[LAWN BOY]

If it is not allowing proper compression, then all the saw is doing is just pumping more fuel into the cylinder with each pull.

That makes sense. Thanks for letting me know. I might buy this: _skw=stihl+ms391+decompression+valve&itmmeta=01JJZ5TZ266VPPEH4JRJM6VPZA&hash=item54af511f15:g:IxYAAOSwiq1iATL3&itmprp=enc%3AAQAJAAAA4HoV3kP08IDx%2BKZ9MfhVJKmqlBBR1737JspshM0EgZWw5uSij8LLsS96e8jrNG1LBlzabE2foXDBnDiOc0Ejs6ppBAY14LUdfS4uuzoseITueC0OilV6ARBVfFzM0gBdqJHyanf9%2BdHq0LTvv0IXoMch0PyhULZtMY0vS2x%2FJjLGZsPl23n80CR%2F6j%2FcCoXp8RbhuB6Nb81fiCsdi0Zd1537slEdqfmvzN4zw5IFOZ5Aql523N9vzps6bvWD9gmqpPFcsFN5CJjvO0AjcSTgBfPVtxNrmaG%2F9ya4E4BRmOm9%7Ctkp%3ABk9SR6Tx6

 

[hacskaroly]

_skw=stihl+ms391+decompression+valve&itmmeta=01JJZ5TZ266VPPEH4JRJM6VPZA&hash=item54af511f15:g:IxYAAOSwiq1iATL3&itmprp=enc%3AAQAJAAAA4HoV3kP08IDx%2BKZ9MfhVJKmqlBBR1737JspshM0EgZWw5uSij8LLsS96e8jrNG1LBlzabE2foXDBnDiOc0Ejs6ppBAY14LUdfS4uuzoseITueC0OilV6ARBVfFzM0gBdqJHyanf9%2BdHq0LTvv0IXoMch0PyhULZtMY0vS2x%2FJjLGZsPl23n80CR%2F6j%2FcCoXp8RbhuB6Nb81fiCsdi0Zd1537slEdqfmvzN4zw5IFOZ5Aql523N9vzps6bvWD9gmqpPFcsFN5CJjvO0AjcSTgBfPVtxNrmaG%2F9ya4E4BRmOm9%7Ctkp%3ABk9SR6Tx6

In the future when dropping in a URL, it works best to use the link button on the bar above:
A popup will appear:


And you just paste the URL in the URL slot and give the link a title in the text box, like "eBay - Stihl Decompression Valve". This makes it easier for people to see what the link is for before clicking on it and makes it a little cleaner in the forum.

Here is an example: eBay - Stihl OEM Decompression Valve MS 391 311 361 362 441 1135-020-9400 #GM-F1C3

If this is what you were linking you should be good to go. One way to check your decomp is to pull it closed, spray around it with soapy water and then start pulling the cord. If it starts bubbling then air is getting passed. If you don't get bubbles, then it would appear be working as it should.

 

[LAWN BOY]

In the future when dropping in a URL, it works best to use the link button on the bar above: View attachment 448770
A popup will appear:
View attachment 448771

And you just paste the URL in the URL slot and give the link a title in the text box, like "eBay - Stihl Decompression Valve". This makes it easier for people to see what the link is for before clicking on it and makes it a little cleaner in the forum.

Here is an example: eBay - Stihl OEM Decompression Valve MS 391 311 361 362 441 1135-020-9400 #GM-F1C3

If this is what you were linking you should be good to go. One way to check your decomp is to pull it closed, spray around it with soapy water and then start pulling the cord. If it starts bubbling then air is getting passed. If you don't get bubbles, then it would appear be working as it should.

Ok, I never knew how to link like that before, thanks for letting me know.

 

[Wonkydonkey]

I’ve just read this thread from front to back.

There are quite a few vids on youboob about starting flooded saws.

I think it’s something like . Hold throttle fully open and pull 20 or so times.

I’ve done this and Sometimes it will pop doing this .. Personally if it hasn’t popped , Ive then had the spark plug out (because possibly a wet plug ?. It won’t spark wet also it’s under compression, you know spark plugs do different things under compression).
However if I’ve got the plug out I pull it over throttle fully open and watch the vapours come out the spark plug hole ( make sure the saw is in the off position)
I wash the plug with fuel and then burn the fuel on the plug (light it and let it burn out and make sure it’s dry by trying to light it again) now the plug is warmed up a bit.. the saw should fire up when it’s replaced…tmally I pull it over with the throttle fully open on the first few pulls & listening for the pop. If it does pop with the throttle fully open I pull it a few more times. Because I know the case is still rich and the throttle is giving more air to this rich mix..If not I then try starting it this order.
Normal start, just switched on …four or so pulls
Hot start, again four or so pulls
Then on choke , each time listening for a pop. Then back to a few pulls on run again.

Eventually you will get to know what that saw likes and what works etc..

My 660 is pull it, full choke with decomp in it will pop on 5th pull, but if the decom is not in it will rip your fingers off. It will then start on the 3rd pull with the hot start position and decomp out . If decom is pushed again in it won’t start.
Once it’s hot, I just pull it 3 times and it starts. And you may have guess it, if it’s hot and I use the decomp it won’t start.

It’s probably due to the carb tune.. I can’t be bothered to retune and then figure out the starting characteristics again..

I hope this helps cos I’ve typed more than enough

 

[LAWN BOY]

Carb tune is about where it should be very approximately. Installed new OEM decomp. Same problem. Although after adjusting spark plug gap to .020 like google says it should be instead of the .030 I got it with, it pulled over without the locking up problem a few times until it ran for 7 seconds, then died, then problem came back and I didn’t get it started back up… Ideas? I can’t really afford to put more money into it plus I have no good place to do in depth repairs for while even if I wanted to.

 

[FederalQ]

Try a different spark plug if you haven’t already. Either a new one or a compatible one from a running saw.