Carbon on Spark Plugs

[Richard Madison]

Hi All:

I pulled a couple of plugs from the 1971 Brown Beauty and found both plugs had hardened black fuzzy stuff all over the top. Not real thick, just a thin layer of carbon.

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The Bosch web site for Reading Spark Plugs, says
 

quote:Sooted — Carbon-fouled
Insulator nose, electrodes and spark plug
shell covered with velvet-like dull black soot
deposits.
Cause: Incorrect mixture setting (carburetor
fuel injection); mixture too rich, air filter very
dirty, automatic choke not in order or manual
choke pulled too long, mainly short-distance
driving, spark plug too cold, heat-range code
number too low.
Effects: Misfiring, difficult cold-starting.
Remedy: Adjust A/F mixture and choke
device, check air filter.

My engine seems to be running OK, getting 15 mpg, no real smoke problem, starts OK cold or warm, never overheats.

I guess the fouled plugs mean the fuel/air mixture should be adusted.
This looks like a complicated job...trial and error with the screw at the rear of the fuel injector?

Do the plugs mean a Fuel/Air adjust is really needed or can I ignore the symptom and just keep driving?

Richard M

[Benz Dr.]

Try a hotter coil and low impedance plug wires. Minor rich running can be improved somewhat by changing these things.

Dan c

Daniel G Caron

[A Dalton]

First check is to see if it has incorrectly installed Resistor plugs.

[ja17]

Hello Richard,
Looks a little rich to me also. These cars were designed to use non resistor plugs and solid spark plug wires with resistor ends. Non resistor plugs are getting a little hard to find these days, but they are out there. Check the cheap things first, air cleaner, correct plugs and gap. Try looking at the plugs after the engine is hot preferably immediately after a test drive. Do not allow the car to idle too long before removing the plugs. My idea of a "fouled Plug" is one which has begun misfire or ceased to function because of unburnt fuel shorting out the plug. These plugs will often times be wet with unburnt gasoline when removed. Over time, this can cause cylinder wear since the unburnt fuel washes the oil off the walls of the cylinder.  Running a little rich will not cause problems unless the spark plugs begin to foul out. In any event you would notice the misfire and lack of power from fouled plugs.  As Dan mentioned, increasing spark intensity also helps combustion. If its running good you probably have nothing to worry about.
Joe Alexander
Blacklick, Ohio

[A Dalton]

Other common rich possibles are leaking cold start valve,
 and excessive slop in the linkage between the pump and air throttle plate... Worn ball ends and worn cross rod bushings add up at each joint and results in the air plate not opening to correct angle to be in sink with the pump settings, so.. even though the pump is set correctly, the air intake angle will be less than required by the pumps settings. Mechanical FI needs tight linkage between the two
before any adjustments are made...
 Remember , the Thumb Index wheel adjuster on the back of the pump and the air bleed screw are idle adjustments only and the linkage does not come into play til throttle is applied. { in other words, don't expect proper load air/fuel mix just cuz one has set the idle CO correctly..

[Malc]

Before ripping the car apart...
How old are your plugs??

If you are only doing short journeys the car never really warms up, yes the temp gauge will be at normal, but the rest is surprisingly cold for a long time.
Check your plugs after going for a long hard thrash of at least 50 miles/ 80Kms, then you will get a better indication of what's going on by the colour of the plugs.
If you do alot of short journeys you may want to concider going to "hotter" plugs, and thus they will be less tendancy to foul up. Finally remember you are driving an old car and even if it's a MB the manufacturing tolerances, wear and tear will mean alittle oil is getting past the rings and burning too, plus the engine recirc system
etc. causing hard black deposits.
I personally think its nothing to worry about.... only if it starts building up at an alarming rate, plus blue smoke on start up or trailing throttle would I start to worry about valve guide seals and piston rings

Finally remember Bosch are trying to sell you more plugs!

Malc
Scotland