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I've had this FS 85 a good while, but has limited use & (was) starting very fine. Now, not so much. Fresh gas w/ stabilizer.
It could be the plug. I checked resistance from tip to center electrode. Nearly 9000 ohms. Bosch wsr 6F is in it. It'll also take NGK BPMR 7A.
A new NGK ( for a different small eng) had a few 100 ohms from tip to center electrode. I shined spots on both plugs to put meter leads, to get good readings.

It has a spark - bluish, not yellow. I wouldn't call it really strong (but consistent). I'm not sure how strong the (blue) spark needs to be on small, 2-cycle engines like this.

Other possibly more important issue: I noticed inside the plug wire boot, the coiled, shiny wire "connector" that fits on the plug tip, was turned partially sideways, after I gently worked the boot off.
I don't know if that style plug wire is toast if the coiled connector ever moves from being perfectly aligned or comes up from the bottom of the boot?
Even if you can push it back into place.

I got the coiled connector back in there & it was quite snug when I pushed it into place. I don't know if the coiled connector is supposed to be fused to something down in the boot.
Or if the coiled connector just depends on a tight friction fit to what appears to be a smooth, metal sleeve - down in the boot?
Thanks.

Coiled wire inside the plug boot will move around and should be no problem as long as you are getting spark. Blue spark is hotter than an orange or yellow spark which is good. Checking resistance of a spark plug is useless, don’t waste your time.

Here is something that many of you may not know. The color of the spark is not always bright blue. Sometimes it can be bright orange of another color. It is affected by the composition of the air around it. In extreme salty air it be orange. Ever seen those sodium vapor lamps? Here the state of Hawaii usually sees orange sparks.

As for wire coil moving it is fine as long as the end that pierces the high tension lead hasn't broken off the internal wire. It has has then you can have lost of spark as the lead moves around. Otherwords you can have spark in one position and none in another. When in doubt here I replace the lead.

One of these little fellas'.


slomo

And a magneto converts rotational energy into electrical energy
SO the more rotational energy you have ( ie faster speed ) the more electrical energy ( bigger spark ) you get.
There is a limit to how much energy you can harvest with a given coil windings ( number of turns & thickness of the wires ) .
Most are designed to be most efficient at the operating speeds ( 6,000 to 12000 rpm ) so at cranking speeds you are working at the extreme lowest end of it's operation

Thanks guys. No, I never heard that ocean air can cause the spark to change color.
(possible update?): I already knew the manual for Stihl FS 85 brush cutter, spark plug gap is only .020 in (for NGK & Bosch they listed).
When I looked on NGK's site (couldn't reach anyone by phone), they show the exact plug number(s) for this brush cutter as the original manual shows.

If interested in looking, at link above, click any category except Cars / Trucks. Then for trimmers, blowers, etc., click Miscellaneous. That should open a search form
On NGK's site, they list TWO plugs fitting the Stihl FS 85 brush cutter. Both spec sheets show Resistance 10,000 ohms. They show the gap = .030 (on one page) & .028 on another. I assume they're not gapped to a particular engine's specs.

One is "BPMR7A SOLID", stock # 6703. I assume "solid" means solid core electrode.

The other is "BPMR7A BLYB", Stock #6761. I don't know what "BLYB" means.

Here's what I figured out. So called "resistor spark plugs" have a, well, resistor in them, to somehow eliminate interference with other close by electronic equipment. Most people that bought spark plugs knew that. Back in the day, resistor plugs cost a fair % more than non-resistor. The auto parts store employees, who knew most customers' names, would ask if you wanted resistor or non-resistor plugs.
So when poor people drove down the street or planes w/o resistor plugs flew over, often the TV or radio would have interference.

I quickly looked at a few "non-resistor" plugs on NGK's site. If they mention the resistance at all for non-resistor plugs (on a spec type page), it is very low. That's what I saw the other day when I also checked a new NGK plug, I said had very low resistance. Turns out, it was a non-resistor plug (no "R" in the part #). So it's resistance would be low. That one showed only 1 or 2 ohms.

What can we do w/ this info? If you check any resistor spark plug & it shows very low ohms, at best it means the resistor is shot. Will that prevent it from sparking as designed? Without looking at a diagram of internal construction, I don't know (for now).
If the resistance of a resistor or non-resistor plug goes way beyond 10K - 15K ohms, something's wrong. If the internal electrode is damaged , the resistance may be headed toward infinity.
If you get a NO ohms reading on either type plug (make sure the points touched w/ the multi meter leads are clean), it may mean the electrode inside is broken.

@ Rivets - why do you think checking spark plug resistance is useless (similar models or even same brand / part #)? If it's an open circuit, there'd be no spark, so recycle it (may have platinum or iridium).

For some reason, a lot of self claimed (pro) mechanics talk about measuring spark plug resistance.

@ slomo - what is that? A bulb planter, a cigar trimmer? PLease explain.

"@ slomo - what is that? A bulb planter, a cigar trimmer? PLease explain."

It's a spark plug TERMINAL. It's the little fella inside your spark plug boot. This is what the whole post is about.

Resistor plugs should measure between 4-6k Ohms. Non resistor plugs should measure like a dead short or 0 Ohms. These all would be clean plugs. Any carbon can skew your readings with a multi-meter.

Long story short, run non resistor plugs for better sparks. Resistor plugs eliminate FM radio interference in car stereos and such.

slomo

Electricity has no colour is is a type of energy and thus has no measurable mass ( understanding of physics changes on a daily basis ).
Sparks follow the path of least resistance so actually jump from one atom / molecule to the next .
As the energy passes through an atom it causes the electrons to jump up or down a shell level or two where they are stable for the length of time the atom has the extra energy of the spark.
Once the electricity has left, the electron fall back into their normal shell and when doing this emitt light of a specific colour , most of which humans can not see.
But the bit we can see is what we call a spark and in most cases is not where then energy is now but the trace of where it has been.
The caveat to this is very high voltages which will cause the electrons to shift shells so fast you actually see 2 sparks, the energising & the discharge .
But again too quick for the human eye to process .

bertsmobile1 said:

Electricity has no colour is is a type of energy and thus has no measurable mass ( understanding of physics changes on a daily basis ).
Sparks follow the path of least resistance so actually jump from one atom / molecule to the next .
As the energy passes through an atom it causes the electrons to jump up or down a shell level or two where they are stable for the length of time the atom has the extra energy of the spark.
Once the electricity has left, the electron fall back into their normal shell and when doing this emitt light of a specific colour , most of which humans can not see.
But the bit we can see is what we call a spark and in most cases is not where then energy is now but the trace of where it has been.
The caveat to this is very high voltages which will cause the electrons to shift shells so fast you actually see 2 sparks, the energising & the discharge .
But again too quick for the human eye to process .

Click to expand...

Just goes to show how STUPID I am...... WOW. Nice write up Bert.

slomo

Not sure if that is High school physics or undergraduate physics been a very long time since that was hammered through my thick head.
No one is stupid if they don't know some thing unless they refuse the opertunity to learn.

This change in energy levels is the basis of spectrochemistry , starting with flame chromotography which was the bread & butter for oil analysis.
Next step up was atomic adsorbtion spectrophotometery
Followed by emission spectrometry where a sample is sparked

What does measuring resistance of a plug tell you, some resistance, open or solid? A little carbon cani distort the readings and some plugs use a small gap internally to act as resistance. When you are charging a customer by the hour why do it? Replace and recycle, go for it, but I still say testing it is a waste of time, unless you’re that curious.

Stihl terminals look like this.
The wire has a hole pierced into the center of the wire @ 3/4 inch down from the end, and the 90 degree prong pierces the wire to the center core, and all is shoved into the tight boot, so the boot could rotate and cause the plug connection to rotate.

Rivets said:

What does measuring resistance of a plug tell you, some resistance, open or solid? A little carbon cani distort the readings and some plugs use a small gap internally to act as resistance. When you are charging a customer by the hour why do it? Replace and recycle, go for it, but I still say testing it is a waste of time, unless you’re that curious.

Click to expand...

Most plugs are 4-6K Ohms. Some are 10K Ohms. Same as a resistor on a circuit board. If it doesn't measure to rated spec, toss it and replace.

Engine carbon WILL skew your readings buy insulating your multi-meter probes. It will read open with carbon = no connection. Clean the tip to fresh metal might get you back to 4k Ohms.

Testing saves your customers money. Firing the parts canon replacing everything under the hood and it still doesn't start, isn't wise. We all read this same recipe on here. Guys changed everything THEY new to change, mower still won't run.

Far as your business model, you charge for your time and materials. Not the entire Briggs line of replacement parts. You use some carbutrator spray, charge for a new can. Shop rags, check. Thread locker, yup. We just got charged $40 for shop supplies at a Chevy dealer for a WIPER MOTOR replacement. They are doing it, you do the same. Everyone is price gouging.

And to add YOU are one of the guys I would pay for your service. If it costs what you said it did, I would pay it. Slap the dead parts on the counter. Test them in front of them. Show the customer you know your stuff. They will come back. Let your wisdom and confidence shine.

slomo