Testing Stihl chainsaw coil

[miket]

Stihl MS271. Ran perfectly for years, then suddenly the rpms went up and down like it was running out of gas. Let off throttle and ran just fine. Throttle up and died. Did this a couple of times then died for good. Sounded like fuel issue initially, so made sure the fuel line in tank wasnt broken, filter in tank not clogged etc. Pulled plug, no spark.

Bought a spark plug and installed it. No go. Pulled it and held it to the head to ground it as usual. No spark. Opened up the side of the saw and disconnected the kill switch wire to make sure it wasnt bad and shorting in. No spark.

Do some research on resistance numbers should be when testing the coil and found only one reference to 2200-3000 ohms, all others say you cant test with ohm meter.

I get 2000 ohms. Apparently you cant order stihl parts online anywhere. Must go to authorized dealer. So went to one in Magnolia to buy a coil. First thing their mechanic says is "how do YOU know the coil is bad?" Heavily emphasized "you". I told him what I did and he said "no, that doesnt work, and checking it the normal way of pulling the plug and touching it to the head doesnt work because its aluminum magnesium alloy and its a poor conductor". Apparently I have to use a spark tester which I have never owned because I have never run across a head that wasnt made of some conductive material...
He said the coils "do not go bad" on these saws. Says it is NOT the coil. Very insistent that it is not the coil.

Anyway, I bought a tester and still no spark.

Checked gap between flywheel and coil, wires etc. Dont know what else to say other than the MF coil is bad!

And ideas? Something Im missing?

[Brazos Hunter]

The control module went out on mine and fortunately it was under warranty. It was doing the same thing as yours.


Michael

[Walker]

By a cheap coil from Amazon. If that works you can by a oem coil.

[miket]

The control module went out on mine and fortunately it was under warranty. It was doing the same thing as yours.


Michael

I had to look up whether control module and coil are the same on this saw, and it is. Since the module is not just a coil, that may be why I cant just check the resistance like you can with just a regular coil.

The mechanic said his 30yrs experience says they dont go bad. My 49yrs on this planet say everything electronic goes bad!

[miket]

Anybody else with some wisdom/experience?

[SRK14]

Initially it sounds like you had an issue with the high speed jet in the carburetor if it bogs under throttle. I’m no expert by any means but I can’t say that I’ve seen a coil go out on one. I would peel back the rubber spark plug cap and look at the clip. On many saws it has a piece of spring steel bent at a 90 degree angle with a needle point that penetrates the actual wire that conducts your spark. Goofy to explain but You’ll see what I mean if you look. I’d wiggle that around while testing. Also don’t overlook the possibility of dirty flywheel magnets/magneto if the saw could use a good cleaning.

[JhuntsAlot]

By a cheap coil from Amazon. If that works you can by a oem coil.

Yup, this.

Buy a cheap coil and carb on Amazon. Probably last you for years if you only use it a few times a year. Maybe longer.



J

[miket]

Initially it sounds like you had an issue with the high speed jet in the carburetor if it bogs under throttle. I’m no expert by any means but I can’t say that I’ve seen a coil go out on one. I would peel back the rubber spark plug cap and look at the clip. On many saws it has a piece of spring steel bent at a 90 degree angle with a needle point that penetrates the actual wire that conducts your spark. Goofy to explain but You’ll see what I mean if you look. I’d wiggle that around while testing. Also don’t overlook the possibility of dirty flywheel magnets/magneto if the saw could use a good cleaning.

Carb was the first thing I thought, but it felt different. At higher RPM it didnt feel weak like it was running lean, but more like missing out. But would idle fine. Then died and would not start at all. In past experience, even if high speed jets are clogged they usually idle, but just bog when the rpm changes from using the low speed/idle circuit to the high speed jets. Still not ruling it out, especially if I was getting some kind of spark, especially with a tester

I am getting continuity from coil to boot so I think the circuit to the plug is good ( I know what you are talking about with the spring, I have had to assemble wires and boots before )

[miket]

Also want to add, I had used this saw a couple weeks before with the same tank of gas. I guess maybe the diaphragm in the carb could have crapped out, gradually then completely, but why no spark?

[miket]

Yup, this.

Buy a cheap coil and carb on Amazon. Probably last you for years if you only use it a few times a year. Maybe longer.



J

I may have to do this, but hate putting chinese junk on a good saw

[JhuntsAlot]

I may have to do this, but hate putting chinese junk on a good saw

I hear ya. I am the same.

Have a MS260 and had issues with the carb also. Bought one on Amazon for a cheap test, to make sure my diagnoses was right and I wasn't crazy.

Been on it for the last few years.



J

[Froggy]

My Stihl blower was giving me the same problem. I found a video on Youtube that was an easy fix. I pulled the spark arrestor out of the muffler and cleaned it out. Reinstalled and it ran fine. This may not be your issue since you have no spark.

https://www.youtube.com/watch?v=Ia65uDBQOVU

[Chase4556]

I too found out the hard way you can’t buy parts online when I had to put a new cylinder head on my fs90 trimmer(stripped spark plug hole). I was however able to find exploded parts diagrams off a European site I believe it was. Allowed me to compile what I needed and bring to the dealer(D-S Lawn in Tomball) to put on order.

If you can find one, I too second ordering a cheap aftermarket coil online. If it fixes your problem, you can always try to return it and get an OEM to actually use on the saw. As you know, no spark won’t be a carb issue.

[Gerald G]

I talked to my buddy that has a repair shop, he used to be a stihl dealer. He said its most likely the coil.

[andre3k]

I may have to do this, but hate putting chinese junk on a good saw

It already comes with a Chinese carb from the factory. You're not hurting it.