NGK Vs Bosch plugs

[KE4AVB]

IS the Bosch USR4AC is any better than the NGK CMR6H spark plug? The NGK version is failing at a high rate here.

[bgsengine]

never used bosch here,and oddly enough have not seen any failures on any NGK plugs here.. Normally I would expect a couple a year,but have had none that were due plug issues alone. (so far cause of plug failure had always been found and fixed) - lots of bad Champions,Autolites and off-brand (Torch?) plug failures fixed with NGK replacements, though.. Rarely see much Bosch around here any more,but then I don't do Stihl But the CMR6H specifically , no problems in the Dolmar units I've used them in. (one would assume you're using the correct heat range?)

Might also want to make sure of the source of your plugs - couple years ago a customer question led me to do some research,and amazingly enough NGK is a popular plug to counterfeit... had to read up on how to identify them so I could be sure any returns weren't just a switcheroo.. Since we buy from factory distributor (Briggs distributor) I'm pretty confident of the sourcing of ours..

[KE4AVB]

Per the IPL I ma using the correct heat range for the Stihl HS56. I have gone thru 6 plugs already on the three hedgers this customer has. One in particular has just got it 3 rd this year. This is the one that was dieseling back in the Spring because of a bad plug. I also have replace in the last two years in Husqvarna backpack blowers.

These are NGK and not counterfeited ones as all the markings are correct and they are coming a known good source. These CMR's are the only plugs that I am having problems with and not just the 6H heat range either as the 7Hs are just as bad. I haven't had a call for the cooler 8H which are harder to get.

[bgsengine]

Try a CMR5H NGK Stock number 7599 maybe? NGK book calls for HS86 to use a CMR5H stock number 7599 at a.028" gap Its a hotter plug which seems might be what you need if they are fouling up.

[KE4AVB]

The Stihl IPL and service manual for the HS86 is calling for the Bosch USR7AC set at .020 or .5mm. and the Stihl HS56 a Bosch USR4AC (NGK CMR6H) set at .020 or .5mm.

Not fouling out just loses spark.

[bgsengine]

ever try a de-construction of one of the bad plugs to see why it loses spark? Most of the times when I find a non-fouled plug that won't fire, the insulator is cracked, and that can happen in shipping if they are loose packed, can also happen if plug gets over-torqued (owner doing maintenance with a cordless wrench or driver?) or experiences a shock load (being dropped on the plug boot? - getting banged around in the back of a pickup truck?) - Ive seen all 3 types of failures , grind the crimp off the hex body to pull the insulator out, chances are good it comes out in 2 pieces.. that many failures does not seem like a consistent plug problem but a mechanical issue, one which could be happening to any plug (might even have been why the original plugs failed too?) I'd be curious if you get your hands on some original bosch plugs if they don't continue to experience the same issues..

edit to add - Also the info I posted earlier, comes out of NGK 2018 catalog, and while I would not call it authoritative as far as plug gaps (it doesnt even list a HS56 Stihl model) , I have found that sometimes even the factory manual is wrong (typo in specs?) but .020 does sound about right for current production models.. I would tend to assume they reduced gap to deal with leaner fuel mixtures...

[KE4AVB]

Okay deconstructed. I think found what going on the center electrode insulator is becoming conductive. I getting about 800K if I press hard on the insulator otherwise infinite resistance is measured. Apparently the insulator is porous and allowing carbon tracing to occur causing a short out under high voltage operation. No visible cracks under 10x magnification loupe.

BTW the metal shell had stamp K521 and once I removed the crimp ring the ceramic plug had K5210 imprinted on it where the crimp ring had it covered. The rest of plug easy to remove once I dug out the white chalky stuff.

About the only I can do is to try the Bosch plugs when I place my next Stens order and see if the problem continues or goes away.

[bgsengine]

fascinating.. funny thing seems to be the problem is showing up on the machines owned by a single user, correct? ran a quick search to see if it was a common issue on those but it doesnt seem to be .. Which leads me to wonder, if it might not be something in how the unit is used... Or possibly the choice of 2-stroke oil (might it have molybdenum or some similar additive that can build up?) - might they be running the machines at part throttle most of the time? (not getting hot enough to reach operating temps? Kinda like some trimmers and chainaws we get in now and then with rapid soot build up in spark screen..owners aren't running them at full throttle..) Of course, if the problem goes away with using the specified Bosch plugs, it could just be one of those (like some Fords I used to work on) that only every work properly with the correct exact BRAND of plug (similar to Echo Shindaiwas getting best performance on NGK BPM8Y where they can fall flat and seem weak if you run them on Champions... Some Ford cars just would not run quite right unless you use OE Motorcraft plugs....) However if the issue persists with Bosch, it'd have to be a fairly safe bet that the plug is just a symptom of the problem, cause being located elsewhere (and the biggest challenge to find the cause)

[KE4AVB]

Currently it is but have been others in the past. I think they are using Echo or Stihl fuel oil but I need to find out for sure what they are using. It is not the fuel oil that I sold them in the past. I know it is red dyed where mine is blue dyed. They didn't like my vendor changing over to the foil packs from the miniature oil bottles even though it is a higher grade of synthetic oil.

EDIT: They are using Echo fuel oil so can I get a MSDS on it?

Like you said it is finding true source that can be real pain; so many variables. Now that I know I have a shorting insulator problem I can start by trying another brand plug and work from there. Meanwhile on finding what is in the fuel oil and it maybe even be some fuel additive at the fuel supplier of the gasoline. Years ago I had terrible time with unleaded plus clogging my Malibu carburetor paper fuel filters. It was something that was chalky. The solution then was to fuel grades back to regular unleaded. It was not fun to change out filters on interstate roadsides.

Operator operation is one area that none of us can control much. At least I got them to check the 4 cycles for at start up and during the day for oil and to change air filters when they are dirty. That in itself was a major feat to get done. These operators barely understand English as it is. All I get as a complaint is "It no work".

Well any way I made up a 10mm pressure/vac adapter out of the old plug base for my Mityvac tester this morning. At least one good has came out this so far.

[bgsengine]

Echo Power Blend (blue) or Red Armor (my guess based on your saying color is red) oil? I'll grab the MSDS for both and send em your way