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MS 391 Starting Issue

Started by Radar67, July 03, 2014, 07:21:25 PM

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Radar67

My MS 391 won't start. It fired and ran for a few minutes, then died as I gave it gas. From that point on, it seems like it will not fire. I changed the plug, but no luck. Any ideas on what to check next? To the best of my knowledge, there is no spark. I tried pulling it with the plug out and could see no sign of spark on the plug. I am using the Stihl oil mix.
"A man's time is the most valuable gift he can give another." TOM

If he can cling to his Blackberry, I can cling to my guns... Me

This will kill you, that will kill you, heck...life will kill you, but you got to live it!

"The man who can comprehend the why, can create the how." SFC J

Ianab

Basic fault finding starts with the 3 things that a 2 stroke engine needs.

Spark, Fuel and Compression.

To test spark you need to ground the plug back to the metal of the cylinder head. Pull the saw over and see if it sparks. If you can't get a spark, then Coil or ignition modules. Spark plugs can fail too, so it's worth putting a spare in anyway.

Fuel? You can squirt a little fuel into the carb and try starting it. If it fires  after that, then you investigate fuel lines, carby etc.

Compression? A proper tester tells you a lot, but failing that, pick up the saw by the starter cord, If it spools out and the saw lowers itself to the ground, then it's got serious internal injuries. Taking the muffler off and looking at the side of the cylinder is another simple check. If it's badly messed up, that's your problem.

After you narrow things down to one of those 3 things, we can dig deeper.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Radar67

Thank you Ianab. It is a fairly new saw, little used, about two years old. It passes the compression test. I will check the spark and fuel later this afternoon and let you know.
"A man's time is the most valuable gift he can give another." TOM

If he can cling to his Blackberry, I can cling to my guns... Me

This will kill you, that will kill you, heck...life will kill you, but you got to live it!

"The man who can comprehend the why, can create the how." SFC J

ladylake

 
Keep in mind that spark is hard to see in full light, if you cant see it in a darkened room then it has no spark, plus you have to have the plug grounded to the engine good.   Steve
u
 
Timberking B20  18000  hours +  Case75xt grapple + forks+8" snow bucket + dirt bucket   770 Oliver   Lots(too many) of chainsaws, Like the Echo saws and the Stihl and Husky     W5  Case loader   1  trailers  Wright sharpener     Suffolk  setter Volvo MCT125c skid loader

Radar67

Problem solved. I checked the spark and found it was good. I pulled the air filter and the plastic cover behind it. The upper butterfly was stuck. After I got it free, the saw fired right up. That was an easy fix. I had not been looking forward to working on this saw, because I am not comfortable with 2 strokes. My mechanical background has always been with V8 Chevy and Ford engines. Glad it was an easy fix, as it gives me more confidence working on the smaller engines. Thanks to both of you for the information.
"A man's time is the most valuable gift he can give another." TOM

If he can cling to his Blackberry, I can cling to my guns... Me

This will kill you, that will kill you, heck...life will kill you, but you got to live it!

"The man who can comprehend the why, can create the how." SFC J

Ianab

Cool. Choke stuck in the closed position. That will stop it for sure :D
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

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