Reading spark plugs - what's the story here?

[Jerome74911S]

The photos below show a stock Bosch WR5DC+ plug that I pulled from my '75 911S engine.

The plug appears to me to be running too hot. Am I reading it properly? What are the grainy deposits on the plug? Is the timing retarded too much?

If I should run a colder plug, which one?

[rwilner]

I am no guru, but that plug looks perfect to me.

[SteamWolf]

nah looks spot on to me.
The deposits may be from crappy fuel quality, or the plugs not being changed often enough.

[schumicat]

Agree it looks normal.

[afterburn 549]

Reading plugs anymore is snake oil and Voodoo
In the Old days we could. !......
Now with weird gas and a host of other things it changes too fast.
To do a plug ck (if you could)
You have to run the engine @ load and RPM you want to dial for. Shut eng of @ that RPM and Load like coitus interrupts ..
Like dump the clutch and turn the ignition off so as to freeze the plug reading in "time"

[db_cooper]

Heat range is probably fine. The ground electrode strap is dark towards the base and has burned carbon deposits off the upper 1/2. The spark plug was not run very hard or it idled/slow speed operation for a while..the ceramic insulator is partially black..meaning the spark plug was not at full operating temperature, prior to the engine being shut down and spark plugs being removed.

The grainy deposits are "fuel salts"..the cold running rich fuel/additive material that is left behind when the black soot deposits from running are starting to get cooked off the plugs once the spark plug is warming up. A blast at full engine operating temperature down the road for a few miles will get rid of "fuel salts" and brighten/clean the insulator too.

Less than fresh used spark plugs in an air cooled engine are hard to read..yours look fine though.

Take your 911 for a good drive at a fast pace for an hour or more..do a clean engine shut down with no idling..have a cup of coffee letting the engine cool off..remove one on more plugs and look again. The spark plugs should look very clean.

[dshepp806]

@afterburn & db: that's exactly what my Wrench stated! I've never tried that but will do so.

I keep my plugs changed out quite often,..so I get to see them often....

BEST!

Doyle

[James Brown]

aircraft mechanics still read plugs, i pull them every 100 hours, most of the time there is lead all over them, not so much oil. but the best part is i blast them in a small sand blaster and they look new, you can get one from aircraft spruce for around $20 and clean them up yourself (with a air compressor) keep several sets around ready to go and change them 6 months or so. keep a photo log to track the condition over the life of the engine. re set the gap and your done. Also, Champion makes a sparkplug anti seize that you paint on the threads (after cleaning). Thats it

[manbridge 74]

We had one of those plug blasters at work. Some jackhole blew the bag off and replaced the sand material with sand of a different size which proceeded to lodge some grit in the plugs cleaned next. Make sure no sand is left in plugs!

On those tiny speckles:
I agree that it is probably fuel additives or dirt sneaking by air filter. But be aware that detonation can produce similar specks which can be aluminum from your pistons caused by detonation. You'd need at least a magnifying glass to tell the difference.

[afterburn 549]

Air Crap recips are still readable........Leaded gas,and VERY old Technology (Not their Fault, Govt Red Tape)
To compare a aircraft engine to a modern car engine even a car engine 40 years old is not a doable thing.

[T77911S]

Originally Posted by db_cooper

Heat range is probably fine. The ground electrode strap is dark towards the base and has burned carbon deposits off the upper 1/2. The spark plug was not run very hard or it idled/slow speed operation for a while..the ceramic insulator is partially black..meaning the spark plug was not at full operating temperature, prior to the engine being shut down and spark plugs being removed.

The grainy deposits are "fuel salts"..the cold running rich fuel/additive material that is left behind when the black soot deposits from running are starting to get cooked off the plugs once the spark plug is warming up. A blast at full engine operating temperature down the road for a few miles will get rid of "fuel salts" and brighten/clean the insulator too.

Less than fresh used spark plugs in an air cooled engine are hard to read..yours look fine though.

Take your 911 for a good drive at a fast pace for an hour or more..do a clean engine shut down with no idling..have a cup of coffee letting the engine cool off..remove one on more plugs and look again. The spark plugs should look very clean.

i think this is about as close to right as your will get from a pic.

i researched this years ago. i found a lot from drag racing sites.
seems there are some inconsistencies in reading plugs. some say the mixture is read down inside the plug with a magnifing glass, or you have to cut them open to look in there. others say the ceramic around the electrode tells the mixture.

the ground strap tells the timing. the aneiling(sp) of the metal shuold be around the bend of the strap, like yours looks to be.

there is more but i cant remember off the top of my head.

it was very interesting reading even if i will never use it or fully understand it, due to inconsistencies.

but they all did agree, the engine has to be shut down at high RPM to get the proper reading. and you cant use plugs with lots of miles on them.


i also have one of those plug blasters. got it when i had webers and was fouling plugs. great to have when tuning carbs. like someone said, be carefull of the sand that can get stuck down in the plug. if that came lose in the engine it could make a mess out of a cylinder wall.

[Jerome74911S]

Thanks everyone. Yikes, I just discovered more about reading plugs than I ever knew.

I used to have a plug sandblaster gizmo years ago when I drove a 2-cycle Saab that fouled its plugs every time I drove it for three days. The sandblasted plugs would foul up again faster than fresh plugs; I guess the blasting left a small texture for burned oil, etc. to adhere to.

In any event, I am more relaxed about the general health of things in my 911 engine. The plugs were not removed after hard running, but after idling along in first gear for the one mile of dirt road to my house.

I pulled the plugs for replacement @ ~9,500 miles. Soon enough?

[afterburn 549]

Never fix what is not broken for it leads to broken parts.....................and grief