My 066 eats spark plugs

[Scott03]

Lately, my old Stihl 066 has been acting up.  After checking fuel-related stuff - filter, carb settings, etc. - to no avail, I tried changing the plug.  It started right up and ran fine.  But only for just a few tanks of gas:  It quit again, and a new plug fixed it again.

Yesterday, with the new plug installed, the saw ran about 2/3 of the way through the tank before quitting once again.  With still another new plug, it ran again and I finished the day's work, but by the end of that same tank of gas the saw was idling bad and acting like it wanted to quit again.

Each plug that fails has minor black deposits, so I guess that means it's running rich, but they're not fouled and the gaps look normal.  They look pretty much like what they are:  new plugs.  Except they're dead.

Anybody have any ideas?  I'd like to fix it myself, if I can.

[joe_indi]

Try running it with the switch wire (thin black)disconnected from the ignition coil.
Also check if the spring in the spark plug boot is worn out (loose contact)

[Scott03]

Thanks, will try the first one.  I did think of checking the spring in the boot, and it seemed OK, but will check it again.

[ladylake]

Could be a weak coil.  Steve

[Scott03]

I thought of that, too, but doesn't it seem more logical that a plug would fail from too much voltage?  It does seem like it would get weaker as it fails, though.  But whether too weak or too strong, I don't know how to test the coil anyway, or the plugs for that matter.  So, it's either swap in a new coil and see what happens, or take it to the shop and let them sort it out.

Thanks! 

[HuckFin]

Try checking the sparkplug for spark when cold, then rechecking when hot.

[Scott03]

OK.  What I'd like to understand is what is causing so many brand-new plugs to fail, or become intermittent, so quickly.

Thanks.

[Al_Smith]

You must have gremlins or something . I've never had a spark plug actually fail in a chainsaw .

[thecfarm]

I can remember putting about 2 plugs into one of my brothers junk mowers. I went down to help him on firewood and had 2 spare plugs with me. Forgot how long it ran each time,maybe 15 minutes,and would not start again. This mower was so old,I think you had to have oil in the gas.  :D

[ladylake]

I thought of that, too, but doesn't it seem more logical that a plug would fail from too much voltage?  It does seem like it would get weaker as it fails, though.  But whether too weak or too strong, I don't know how to test the coil anyway, or the plugs for that matter.  So, it's either swap in a new coil and see what happens, or take it to the shop and let them sort it out.

Thanks!

  Coils often fail after they get hot so by time you replace the plug they cool off some and run for a while again.   Steve

[ZeroJunk]

I think the voltage of a coil is determined by the number of turns and value of the capacitance. Don't think there is anything in the realm of physics that could make it generate more voltage. Plus, automobile spark can be 40 KV or better, more than you will get out of a small engine coil.

[sandsawmill14]

i agree with steve  next time you change the plug put one of the old plugs back in and see if it fires

[HolmenTree]

There's a intermittent voltage break some where or a bad coil . Never saw a saw burn out new plugs one after the other

What exact brand of plug and model number are you trying?

[Scott03]

It's a Bosch WSR 6 F, pre-gapped at 0.5mm, right from the saw shop. 

When it was new I measured 3.82k ohms on my meter, after it went dead it measured 4.82k ohms, if that means anything.  The continuity tester didn't work, even on a new plug, I assume because it's a resistor type.

I went out just now and started up the saw with the latest, still-working plug, and it ran fine.  Then I swapped in the "new" plug that went dead, and it's still dead.  Cranked it until I smelled gas, and no go.  Then put the good one back in, and away it went, no problem.

[Scott03]

sandsawmill14 and huckfin, you can see from my reply to HolmenTree that I just tried that with no luck.  Maybe a sacrifice to the gods of Stihl is in order!  Or a new coil.

[Scott03]

Zerojunk, yes that's the way it seems to me, too.  But I'm at a loss to think up things that would break a plug.  Another thing on my mind is if the plugs are overheating from a leaned-out mixture or something like that.  Something is killing the plugs, because a new plug will work, and when I swap in a new-but-dead one it doesn't work.  That is, the dead plugs seem to be really dead, they don't connect up again internally after they cool down.

[Scott03]

You must have gremlins or something . I've never had a spark plug actually fail in a chainsaw .

Yes, I think the answer is going to be a sacrifice to the gods of Stihl!  Or a trip to the shop, which is pretty much the same thing.

[Al_Smith]

How in the world does a person check a spark plug with an ohm meter?

[Texas-Jim]

You can check from where the wire goes on to the electrode, it will give you the ohms of the resistor. Should be around 4 or so. But it really doesn't tell you very much. I agree with the others, its highly unlikely the plugs are failing, id look at something else as in coil. Plugs in cars can go well over a 100,000 miles and even though a saw is dirty compared to a car it shouldn't fail that fast.

[Al_Smith]

Okay,never heard of that .However he said 3.8 k ohms to 4.82 k ohms,K meaning thousand .

[Texas-Jim]

I just didnt put the k.

[CTYank]

I have seen plugs that worked poorly, making hot restarts very difficult. Not just going bad and refusing to work again. They were "Torch" plugs- should have been branded "Toss". Which is what I did. Replacement NGKs still working fine, thanks.

[joe_indi]

Lately, my old Stihl 066 has been acting up.

Is it a 'Red Eye'. Though I have not seen one I have read that their coils used to play up
Does it have an aluminum or polymer flywheel? I have seen several polymer flywheels that had their centers twist a couple of millimeters, enough to retard the ignition timing, which could cause spark plug fouling.
Before you start wasting money on spark plugs, try this on the fouled ones. Chances are you might be able to revive them.
My smart phone was a bit out of focus so the differences between the 'Before' (pic 01) and the 'After' (pic 05) may not be quite noticeable unless you look carefully.

[Scott03]

Al, I just clipped one probe to the end where the high-tension wire goes, held the other probe on the center electrode, and tried different ohm ranges until I got a reading, yes "k" in thousands.  I don't really know what it means, either, but the continuity tester I tried first didn't work, I'm assuming because it's a resistor plug.  I was trying to make sure that the new plug was actually good before I installed it.  Just trying to be thorough.

[sandsawmill14]

i have had this to happen to spark plugs was on an old ac tractor that had a magneto on it it would do exactly as your saw.  we finally changed the mag out to dist and solved the problem  ??? the thing is we still have 3 other tractors with with mags on them and have never had any trouble with any of them
the old honda 3 wheelers used to do it too unless you used they ngk plugs in them :-\
but anyway good luck with it

[Scott03]

Is it a 'Red Eye'. Though I have not seen one I have read that their coils used to play up
Does it have an aluminum or polymer flywheel? I have seen several polymer flywheels that had their centers twist a couple of millimeters, enough to retard the ignition timing, which could cause spark plug fouling.
Before you start wasting money on spark plugs, try this on the fouled ones. Chances are you might be able to revive them.
My smart phone was a bit out of focus so the differences between the 'Before' (pic 01) and the 'After' (pic 05) may not be quite noticeable unless you look carefully.



joe_indi, it's an aluminum flywheel.  What's a Red Eye?

None of the dead plugs are fouled, if I can figure out how to put up a photo you can see for yourself (what looks like carbon across the gap is just the angle of the shot - the gap is clean).  I don't think they are defective either, something is killing them.

[Scott03]

A couple of comments on the photo above:  This plug was installed brand new and died after only 2/3 tank of gas, it doesn't work hot or cold.  The colors of the insulator are not due to shadows or angle of photo.  The light gray part must be a hot spot, and the dark part is carbon. 

[ZeroJunk]

Maybe somebody else has already asked, but do you have another piece of equipment you can put the plugs in that quit working in the 066 to see if they will work there?

The Red Eye will have an extra wire coming from the coil to a red light on the side of the carb box. It was used as a tuning feature. Wasn't reliable because the RPM that lit the red light changed over time.

If it goes bad you will have to find an aluminum flywheel that has an X and Y slot if it didn't come with one so you can use a standard ignition module.

[Scott03]

ZeroJunk, that was good advice!  I have an MM55 tiller, swapped the "dead" plug into it, and it worked!  But it still won't work in the saw, so there really must be gremlins in there.  I'm going to take it in and let the pros look at it, and pay up for their expertise.

The saw has an outline on the housing for where the red eye would go, and there were instructions for it in the manual, but mine doesn't have it.  I bought it mid-90's, maybe they decided to get rid of it by then.

[Ada Shaker]

Interesting to see something is causing your spark plugs to increase its internal resistance by 25%. The only thing I can think of that will do this is a higher than usual kv from the coil (which is generally unlikely unless you've changed the flywheel with increased magnetism) or heat is killing off your spark plugs (+ve temp coefficient of resistance), the higher the heat, the higher the resistance. Would be interesting to see the difference between a hot spark plug that has just failed and a cold one. Check to make sure your saw isn't running e excessively hot first.

[Al_Smith]

Well I'll me a monkys uncle.I checked it out and sure enough you can check a plug with an ohm meter .Never knew that,never had a plug fail completely .I've had them foul on old oil burning engines though .So I guess an old dog can learn a new trick,arf.

I've got an old Champion spark plug sand blaster that's probably about as old as I am .It will clean a fouled plug slick as a whistle.

[Ada Shaker]

Just bear in mind that not all spark plugs are created equall. Not all have a high internal resistance.

[ZeroJunk]

Not sure why they put a resistor plug in a chainsaw to start with.

[Scott03]

Interesting to see something is causing your spark plugs to increase its internal resistance by 25%. The only thing I can think of that will do this is a higher than usual kv from the coil (which is generally unlikely unless you've changed the flywheel with increased magnetism) or heat is killing off your spark plugs (+ve temp coefficient of resistance), the higher the heat, the higher the resistance. Would be interesting to see the difference between a hot spark plug that has just failed and a cold one. Check to make sure your saw isn't running e excessively hot first.

Ada, now I'm wondering if it is indeed running hot, and hurting but not killing the plugs, and also has a weak coil that won't fire a damaged plug with higher resistance, but will fire a new one, at least until that one gets overheated, too.  Since the supposedly dead plug works in my tiller, maybe it means the tiller's coil is good and has enough power to fire even the higher-resistance plug.  Anyway, it's in the shop now and they have the whole story, so we'll see what they can find.  I'll let you all know what happens, it's supposed to be ready Monday.

[HolmenTree]

When I bought my 066 Arctic red lite around 1992 when it was first introduced, about a month later my Stihl rep Dave Gordon got me the upgrade module to disable the red lite as they had problems with that new digital technology.
I can't remember if I changed the aluminum flywheel.
Don't remember what the outcome would have been if the upgrade wasn't made.
Bosch plugs are as good as a plug you can buy.


  

 

[Ada Shaker]

Interesting to see something is causing your spark plugs to increase its internal resistance by 25%. The only thing I can think of that will do this is a higher than usual kv from the coil (which is generally unlikely unless you've changed the flywheel with increased magnetism) or heat is killing off your spark plugs (+ve temp coefficient of resistance), the higher the heat, the higher the resistance. Would be interesting to see the difference between a hot spark plug that has just failed and a cold one. Check to make sure your saw isn't running e excessively hot first.

Ada, now I'm wondering if it is indeed running hot, and hurting but not killing the plugs, and also has a weak coil that won't fire a damaged plug with higher resistance, but will fire a new one, at least until that one gets overheated, too.  Since the supposedly dead plug works in my tiller, maybe it means the tiller's coil is good and has enough power to fire even the higher-resistance plug.  Anyway, it's in the shop now and they have the whole story, so we'll see what they can find.  I'll let you all know what happens, it's supposed to be ready Monday.

Sounds feasible,  be interesting to find out what the problem was and what caused it. Do keep us in the loop.

[Ada Shaker]

When I bought my 066 Arctic red lite around 1992 when it was first introduced, about a month later my Stihl rep Dave Gordon got me the upgrade module to disable the red lite as they had problems with that new digital technology.
I can't remember if I changed the aluminum flywheel.
Don't remember what the outcome would have been if the upgrade wasn't made.
Bosch plugs are as good as a plug you can buy.


  

 

Probably a few blacken eyed stihl reps walking around the place most likely. :D

[Pine Ridge]

Try a different brand spark plug and see what happens, I had a similar problem a few years ago , same brand plugs that you mentioned.

[Scott03]

Try a different brand spark plug and see what happens, I had a similar problem a few years ago , same brand plugs that you mentioned.

Not a bad idea.  HolmenTree says Bosch is among the best, and they do seem to make good stuff in general.  But even the best can come up short once in awhile.  Like the RedEye thing with Stihl.  Also, I had a silly problem with the 066 a few years back.  It's a magnum, which means they hot-rodded it to bump up the power-to-weight ratio.  But someone forgot to beef up the crankshaft to handle that extra power, so one day I was running it hard, ripping with an Alaskan Mill, and it broke.  The shop guys knew all about it and were surprised mine hadn't broken sooner.  I guess I hadn't run mine as hard as the loggers around here (I'm in Oregon) so it lasted a bit longer.  They fixed it free, which was nice, but it was a dumb thing for Stihl to have overlooked in the first place.

We'll see what the mechanic says when I get the saw back, and go from there. 

[Scott03]

When I bought my 066 Arctic red lite around 1992 when it was first introduced, about a month later my Stihl rep Dave Gordon got me the upgrade module to disable the red lite as they had problems with that new digital technology.
I can't remember if I changed the aluminum flywheel.
Don't remember what the outcome would have been if the upgrade wasn't made.
Bosch plugs are as good as a plug you can buy.


  

 

That thing has seen some action!  Looks like yours has a metal air filter housing, mine is plastic.

[Al_Smith]

I wonder why they would use resister plugs on a chainsaw? The intent when they first came out was electronic noise suppression .Prior to ii caused it caused peoples televisions to go bonkers .Ha I had a 1959 Pontiac that drove my then girl friends father nuts .

[Ada Shaker]

I wonder why they would use resister plugs on a chainsaw? The intent when they first came out was electronic noise suppression .Prior to ii caused it caused peoples televisions to go bonkers .Ha I had a 1959 Pontiac that drove my then girl friends father nuts .

The carbon ignition leads worked under the same principle. I also thought that was what the ignition filter capacitor was for, to help filter out noise produced by arching, which has a pretty wide band width. Now we have capacitors,  carbon leads and resistive spark plugs working in unison with an inductive coil. I sometimes wonder about the LCR circuit and if it has something to do with resonant frequencies as th3 chainsaws run wot.

[ZeroJunk]

I knew they would wreak  havoc on an AM radio in particular, but didn't realize the range of it would be enough for a saw to interfere with anything nearby.

I don't suppose they hurt anything, he has something else going on, just not sure what.

Could be the compression has gotten on the low end and only a fresh plug will keep it going. Or, could be the condenser part of the coil has opened.

A 000 400 1300 can be bought really cheap off eBay. I would have tried that first.

[Hilltop366]

Its unlikely in your case but often over looked problem with small engines that are finicky is the flywheel/coil gap, a little extra gap can weaken the spark making it hard to start.

[HolmenTree]

When I bought my 066 Arctic red lite around 1992 when it was first introduced, about a month later my Stihl rep Dave Gordon got me the upgrade module to disable the red lite as they had problems with that new digital technology.
I can't remember if I changed the aluminum flywheel.
Don't remember what the outcome would have been if the upgrade wasn't made.
Bosch plugs are as good as a plug you can buy.


  

 

That thing has seen some action!  Looks like yours has a metal air filter housing, mine is plastic.

Yep metal cover. My saw is the very first run of the 066's onto the market. The crankshaft never failed and the cylinder has never been removed and it's over 24 years old.
I always religiously used 40:1 Stihl Premium mix (Castrol Super 2 Stroke) with premium gas. It has bucked and noodle split tons of cords of firewood. Milled thousands of feet of lumber with the original 28" bar.

[ZeroJunk]

I think it was from sticking the nose in to something running wide open and slamming the brake shut on the clutch with the flywheel saying wait a minute. Among other things I am sure.  Something an experienced user is less likely to have problems with I suspect.

[Ada Shaker]

I knew they would wreak  havoc on an AM radio in particular, but didn't realize the range of it would be enough for a saw to interfere with anything nearby.

I don't suppose they hurt anything, he has something else going on, just not sure what.

Could be the compression has gotten on the low end and only a fresh plug will keep it going. Or, could be the condenser part of the coil has opened.

A 000 400 1300 can be bought really cheap off eBay. I would have tried that first.

It has always been known that an uncontrollabe electrical arc of any nature produces RF with a highly variable bandwidth. Electrical motors are quite often the worst culprits. Keeping in mind that the minute signals are picket up by the antenna and amplified through the devices amplifiers stages. A combustion engine may not be so bad as the arc is completely surrounded by metal. As of late these issues have become less of a problem due to good filtration at both the source of thr arc and at the receiving end, service stations still request that you turn off your mobile phone when refuelling though.

[ZeroJunk]

I have been a licensed amateur radio operator for about 35 years, so I understand the concept. But, I was thinking the resistor plugs were designed to limit interference to AM radios in cars where the wanted signal and unwanted interference are amplified equally. In todays technology it may interfere with the onboard computers etc.  FM and digital are not effected to the same extent at all. Plus antennas are a two way street, so I can't see a chainsaw bothering anything ever even in the AM days for more than a few yards maybe.

But, it could be that all they use are resistor plugs anymore for simplicity or other reasons that don't occur to me. May interfere with the electronics in the ignition module itself.

The reason is not RFI from chainsaws.

[bill m]

Almost all areas of the USA have regulations on radio interference, including from internal combustion engines. Hence the reason for resistor plugs being required.

[ZeroJunk]

That explains it. Sort of get grouped in although by nature they are seldom likely to be in close proximity to much they could bother.

[Ada Shaker]

I have been a licensed amateur radio operator for about 35 years, so I understand the concept. But, I was thinking the resistor plugs were designed to limit interference to AM radios in cars where the wanted signal and unwanted interference are amplified equally. In todays technology it may interfere with the onboard computers etc.  FM and digital are not effected to the same extent at all. Plus antennas are a two way street, so I can't see a chainsaw bothering anything ever even in the AM days for more than a few yards maybe.

But, it could be that all they make are resistor plugs anymore for simplicity or other reasons that don't occur to me. May interfere with the electronics in the ignition module itself.

The reason is not RFI from chainsaws.

Chainsaws and other combustion engines are unlikely to produce much RF as stated earlier as the spark plug is encased in a closed engine. Or better said, the RF from the spark would have little interference if any due to it being encased/shielded in a metal enclosure. You'd probably get more RF from an old chainsaw that used the points system that had little shielding, but most saws of those days were pretty much all metallic anyway. Tx Rx antennas are 2 way but tv and receivers are 1 way. I would imagine Rf would have some effect on the Rx circuit in some way, perhaps at some frequencies more than others.

As far as the resistor spark plugs go, they may form part of an LCR circuit within the ignition circuit itself. The R portion limiting current flow when the circuit hits resonant frequency (only taking a guess here) when XL=Xc when the chainsaw hits full speed for cutting. Like i said I'm only taking a stab in the dark, as there isn't much information out there on the design of chainsaws and its all kept fairly secret. Your Tx Rx radio circuits also use resonant frequencies to tune into a particular channel sliding along the frequency scale, its that resonant frequency that determines what channel your on.

I really couldn't think of any other reason for having resistive spark plugs other than for RFI suppression and/or being part of an LCR circuit where it interacts electronically with the inductance of the coil and the capacitance of the condenser/capacitor.

My background is that of an electrical fitter, electrical mechanic, have some studies in the realm of electrical/electronic engineering, and recently added to this, a diploma of visual arts (majoring in photography) which has nothing to do with chainsaws but i can take some pretty pictures of one if you like. :D :D :D

[ZeroJunk]

Just reading around there seems to be some thinking that a non resistor plug can fry an electronic ignition. Resistor buffers the discharge. I can see some logic in that.

[Ada Shaker]

Just reading around there seems to be some thinking that a non resistor plug can fry an electronic ignition. Resistor buffers the discharge. I can see some logic in that.

Yep, the old points ignition had a straight out spark plug with steel ignition lead. The new leads are so floppy these days, id say they're some form of silicon/carbon or some composite material. Vehicle ones are very similar these days in flexibility and also have some resistance to the leads. Adding to this its quite possible that the coil has some resistance in series with its lead. I can't really say as I've yet to measure coil dc resistance but i would imagine it would be fairly low ohms, the lead could possibly bring it up some what. Some one might be able to compare a points coil resistance with the resistance of an electronic ignition module to verify if this is correct.

[Al_Smith]

A lot of the old plug wires were braided copper with non shielded insulation .Then they went to carbonized conductors with semi conductor shielding etc good for around 100 thousand volts .Prevented flash overs,cross fire between the plugs etc .The plugs last almost forever any more .

The noise on radios TV etc was picked up by the intermediate frequency transformer and amplified to the speakers .Often the video picked it up and the TV acted like an osciliscope which drove people nuts like a pass play,fourth down 20 seconds to go on a football game .All they heard was buzzzz.

I'm not sure any of the above has anything to do with a chainsaw ignition system .

[Al_Smith]

Now here's what might be happening .Some Stihls,042 and 048 for example with SEM ignitions had self advancing coils .If for some reason the advance solid state circuitry failed it might just be cutting off .There no repairing just replacing .I have no info on the saw ignition in question so it's a stab in the dark .However I've had advancing coil types either fail on full advance or fail to advance and either condition  causes problems .

[ZeroJunk]

Now here's what might be happening .Some Stihls,042 and 048 for example with SEM ignitions had self advancing coils .If for some reason the advance solid state circuitry failed it might just be cutting off .There no repairing just replacing .I have no info on the saw ignition in question so it's a stab in the dark .However I've had advancing coil types either fail on full advance or fail to advance and either condition  causes problems .

Not a terribly uncommon problem on newer Stihl blowers and trimmers.  I have replaced, rebuilt carbs because the unit just won't take the throttle only to find out the ignition was bad. Idles fine, so I am assuming it lost it's advance.

[Canadiana]

In the dark if you connect your spark plug and ground it to the cylinder, then pull starter you will see spark. Alter the new one with a dead one. Watch closely for a small difference in spark. This may give you another clue but this issue is beyond my skills n experience. Very interested to hear how you solve this. Best of luck

[joe_indi]

If you are using a NGK or Bosch plug an your ignition is weak, the spark might be quite weak. But, if you can lay your hands on one of the cheap plugs that come from China, chances are they are unsupressed and their spark would even put Darth Vader's stun gun to shame  :D. And on a weak ignition coil they work quite well. I posted quite some time ago about even using them to 'force start' engines with temporarily stuck rings.

[Scott03]

Just got my saw back from the shop, and it works like it's supposed to.  So far.  But...

Nobody really knows why, for sure.  The mechanics didn't know what to make of my story:  Brand new plug quits, replace it with another and it works, until it quits, too, after just a few tanks of gas.  Rinse and repeat.  Then put the "dead" plug in my MM55 tiller, and it works, but put it back into the 066 and no go.

They actually asked me:  "What do you want us to do?"  To myself I thought "You guys are supposed to tell me!"  Out loud I said "How about giving it a good tuneup, and check the ignition system and coil?"  That made sense, so off they went.

New air filter, fuel filter, and plug.  Ignition system and coil checked out as normal.  The kill switch didn't stop the engine, once, so they fixed that.  The carb was running a bit rich so they adjusted that and put a tach on it to get the high end right.  New clutch springs.  Then they cut with it for 20 minutes and proclaimed it good to go.  I worked it another 20 min. or so when I got home.

There you go, guys. I wish they'd found something specific, so there was a better sense of finality about solving the problem.   

[Ada Shaker]

Just got my saw back from the shop, and it works like it's supposed to.  So far.  But...

Nobody really knows why, for sure.  The mechanics didn't know what to make of my story:  Brand new plug quits, replace it with another and it works, until it quits, too, after just a few tanks of gas.  Rinse and repeat.  Then put the "dead" plug in my MM55 tiller, and it works, but put it back into the 066 and no go.

They actually asked me:  "What do you want us to do?"  To myself I thought "You guys are supposed to tell me!"  Out loud I said "How about giving it a good tuneup, and check the ignition system and coil?"  That made sense, so off they went.

New air filter, fuel filter, and plug.  Ignition system and coil checked out as normal.  The kill switch didn't stop the engine, once, so they fixed that.  The carb was running a bit rich so they adjusted that and put a tach on it to get the high end right.  New clutch springs.  Then they cut with it for 20 minutes and proclaimed it good to go.  I worked it another 20 min. or so when I got home.

There you go, guys. I wish they'd found something specific, so there was a better sense of finality about solving the problem.   

Good to hear you got your saw back and there was nothing seriously wrong with it. In your opinion, do you think the saw may have been running a little hot?. I've known engines to run hot when the air filter needed replacing so maybe this coupled with a fine tune will hopefully solve your problem. Just out of curosity, has your new spark plug changed resistance since running the saw?. This may give some indication that the problem may have been an overheating issue to begin with.

[Scott03]

[/quote]

Good to hear you got your saw back and there was nothing seriously wrong with it. In your opinion, do you think the saw may have been running a little hot?. I've known engines to run hot when the air filter needed replacing so maybe this coupled with a fine tune will hopefully solve your problem. Just out of curosity, has your new spark plug changed resistance since running the saw?. This may give some indication that the problem may have been an overheating issue to begin with.
[/quote]

I don't think so, even though I was kind of hoping it was, because that might have explained a few things.  The "dead" plugs I would change out all had carbon deposits on them, like it was on the rich side, and the shop guys said that, too.  Isn't it when the carb is too lean that you get too much heat?  Also, the mechanic didn't think the filter was particularly dirty, he just thought it would be on the safe side to replace it.  I still have it, it's the washable type so I'll keep it on the shelf as a spare.  Good point about the resistance, I didn't think to measure it.  Will do that before I run it again. 

[CTYank]

Those carbon deposits on the insulator can have the spark current bypass the spark gap.
A useful tool in cases like this is an inline spark checker. You connect one side to the cable, the other to the plug. The transparent center part has a gap where you can watch sparking. Soooo, while running, you could see the normal spark, and then watch it when/if the engine acts up to see if the spark dies first.

[Mountain_d]

I agree. I would bet on a weak ignition coil. Mountain.

[joe_indi]

It would be advisable to examine the ignition lead (for frayed insulation) ditton the short circuit wire, and the torsion spring in the spark plug boot. It is notorious for wearing out from vibration, just enough to prevent constant current.
All of these or one of them would cause erratic spark at the plug, which in turn could cause carbon buildup or an oily plug

[beauj5]

Did you ever resolve the problem with the 066 going through plugs?

[Al_Smith]

This is a mystery .As per that pic for some reason the electrode insulator is becoming carboned up for some reason .It probably creates a short circuit from the electrode to ground providing a lower resistance to ground than jumping the gap.Obviously the spark will follow the path  of least resistance.

Now the big question is what is causing the carbon? Could is possibly be the fuel?

[DelawhereJoe]

What mix ratio do you use in this saw and what oil in the gas ? I'll also assume you use the same mix in other saws with no trouble.

[Al_Smith]

It's confusing to me because even running a ratio of 16 to 1 on old outboard boat motors  it didn't foul the plugs .About the only thing that would is running at trolling speeds for an extended period of time .Then all it was was to lean out the high speed a tad and run the snot out of it  then get back to trolling .

I'm not certain if some of those gas additives would cause this or not or even if they were used .If so that may explain a few things .

I have some saws I've owned for over 30 years that have the original  plugs .Now I might have wire brushed them a time or two while doing routine maintenance which isn't all that often but they've never been replaced .

[Mountain_d]

I was having a similar problem with a snowmobile just last week. I read on a snowmobile forum someone was claiming they solved this by replacing the cylinder base gasket. I not sure the logic behind it but something to consider. Mountain.