Don't forget to check for loose spark plugs

Ax-Man

Don't make me chop you
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It has been awhile since I started a thread so with it being a dull rainy Saturday night I figured I might as pass this little bit of info along.

Ever run a Stihl saw and for no apparent reason it just starts to run a little squirrely. You start to ask yourself what the heck is causing this problem, is it a carb problem? crank seal going bad ? or does it have an ignition problem. More than once it turns out to be a simple fix. Darn spark plug is loose in the cylinder head. It is easy to overlook because the saw acts like it could be a different problem. It really doesn't back out enough to really cause a compression problem but enough to make a saw not idle quite right or an acceleration problem or if you have an ear for ignition it can make it sound or feel like it has a miss in it.

This doesn't happen often and doesn't seem to be model specific but has happened often enough to be one of the first things to check on a saw. I am not a big believer in changing sparks because a bad plug is rare and very seldom the problem in a bad running saw. Spark plugs are good to give you clues as to what might be wrong with a saw like it is running to rich or lean. I will pull one out just to see how it looks that is about as far as I go these days . If it looks good and the tang and insulator base are in good shape it stays in the saw.

I know some of you are saying " Ax-man why don't you just tighten that plug down more to fix that problem or you have a thread problem in the head" To answer that one. I always start my plugs by hand to avoid cross threading and continue to thread them till I can't turn the plug anymore by hand then tighten with scrench. I do it this way to be able to feel how well and how far the plug will thread into the head. I am not a big fan of over tightening a plug but I do snug them down enough to feel that enough torque has been applied to keep the plug from backing out. Even when everything feels good I have a plug that works out a little to cause a running problem once in a blue moon. About the only thing I can say it is just a vibration problem from the saw when it is run.

One model in particular that has this little problem is my aging ms 200's . They get the heck run out of them and also get the most sprocket changing because you have to remove the plug, jam the piston to take off the clutch to replace the sprocket. So there might be some wear in the cylinder threads but even a new plug doesn't solve the problem. I am always thankful a loose plug is the problem instead of a carb replacement. I have also had this little problem on a 66 and most recently a 361 which prompted this thread .

Just thought I would pass this tidbit of info along with checking those other easy little things before assuming the worse if your saw doesn't seem to run quite right.
 
I have a saw that runs perfect for an hour or so, then it starts to act like it's running outta gas when there's plenty of gas.

Any ideas on what's up wid dat?
 
If it's that 200T most likely the bearings are shot which heats the seals up causing leakage .If that's the case it will get progressively worse .

The damned things use caged roller bearings with a hard fiber thrust washer .After a period of time it gets side slop .It not a big deal to rebearing and reseal them .About 60 in parts OEM from a dealer and an hour and a half at least for me to change them .I've done several .
 
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  • #6
Al, I agree with your statement on a 200 but wouldn't that problem show up sooner that an hour of run time??
 
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  • #8
It could be multitude of problems with those 200's .

Butch when you say an hour. Is that straight running or is that alot of stop and start type cutting. My experience with vent problems is usually about mid tank after fillup. Sometimes the saw won't run or is very hard to restart. Vent problems really aren't that common but can drive you nuts. Without the aid of a vac tester it is almost impossible to tell if you have a vent or vapor locking problem where the fuel won't flow the way it should to the carb. Vent problems on a 200 are kind of rare but does happen. On the other hand if the saw gets run to the point it gets good and hot then I would lean a little more to what Al is saying. Bad seals, possible bad bearings causing an air leak due to heat expansion. If you have an air leak it takes forever for the saw to idle down properly or won't idle at all after it gets warmed up.

Most of the running problems I have had with those 200's is the carb . We don't want to go down that road as it has been heavily traveled already.
 
Larry, my 020 has a tendency to loosen the plug when running. It took me awhile to understand whet the odd popping sound is, which turned out to be the loosened plug. Twice the saw caught on fire as a result of that happening. Whether it was the spark contacting residual slopped gas after a fill up or the unburnt gas igniting in the exhaust (the plug on those is right next to the exhaust opening), possibly a combination, I'm not really sure. I sure don't like when that happens, though it can get a laugh from the crew. It can make you uptight so that you forget to shut off the engine, which will basically stop the fire. if there is enough unburnt gas in exhaust to catch on fire, there is a whole heck of a lot being spewed into the environment.
 
The 441 is notorious for spitting out plugs for some reason.
 
I had one of the 066s go limp on me one day... No compression.. Just like now..
So I shelved it once I got back to the shop swearing I would dig into it sooner than later.
Some time went by.. I go to dig in... Loose plug.. I had to just about slap myself.
Of course I let it set too long and now have to replace hoses and carb kit :lol:
 
Ha.Well Tom thought he blowed a hole in the piston ,never checked it just bought another one .Brought it out to me a week later and I found it in about 2 minutes and gave it back to him the next day .He needed another saw any way .
 
I had a 200 tank vent block. Saw would run fine, then begin to hesitate, stop, and be next to impossible to start.
The vent was quite cruddy, fairly new saw too.
 
Sparkplugs not sitting on a cone has a compressing washer that is compressed once. To get right torq you seat it then 1/4 turn and you have it sitting correct.
I rarely re use plugs for a many reasons, this is one of them.

MB
How long you run saw on a tank of fuel?
 
The sparkplug.
Isn't that what we are talking about?
 
How long you run saw on a tank of fuel?

So many variables on that, I can't give you a real answer. At the least - 30 minutes, at the most - 1 or 2 days. It all depends on what I'm doing.
 
Some spark plugs have tapered seats to match the opening. Wonder why the do it that way? Another good reason to follow the manufacturer's specs on which number to use.
 
So many variables on that, I can't give you a real answer. At the least - 30 minutes, at the most - 1 or 2 days. It all depends on what I'm doing.
So when You say it ran "a hour or so" what does that mean? Two tanks+ or one tank with stop(s)?

Tank vent issues is usually noticed as fuel is going out, no air coming in and a negative pressure is built up instead of Neutral or positive.

It could be a carb leak if yours has that accelerator pump.
 
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