by Mike C. -
Oh yeah, the title says it all. You have tuned and
tuned on the engine, adjusted the carb, until you can’t tune and adjust
anymore. And still you pull the spark plugs out and they are
black as an ace of spades. What gives? Where do I turn now? Well, my
beloved VW folks, I have been given the arduous and somewhat scientific
task of explaining the causes of sooty spark plugs and what to do about
them. I had heard that many of you have had this problem, or are still
having it, and have just hit a dead end on Soot Street. Hopefully, I can
help you get turned around in a solution to the problem…
is defined as a black, somewhat powdery stuff (carbon deposits) that covers
the center and side electrodes and the center porcelain of the spark plug.
If it is in a heavy enough concentration, the soot can cause a loss in power
or misfiring through a weakened spark or a spark that jumps incorrectly.
What I mean by incorrectly is this…sparks will jump through the path of
least resistance. If this least resistance is somewhere other than the
center electrode, the spark will find a place where there is less carbon
buildup and jump there. This causes the flame to spread in a different
manner, not always pushing down on the piston before loosing its punch. In
other words, lost power! I am going to go through the causes one by one.
There may be more, but these are the most common and likely to cause the
carburetor is not correctly adjusted. The Solex carburetors are finicky
animals in that there is a certain way to adjust them. Whatever the manual
says, follow it. If the idle fuel mixture (small screw) is not correct, it
may be allowing more fuel than is required for idling the engine. If the
float level is too high, excess fuel will actually spill internally from
the carburetor, unmetered, into the engine, causing a rich condition that
cannot be corrected with the normal procedure of adjusting mixture screws.
Do try to get the timing correct before doing any carburetor adjustments,
as the timing will affect the final mixture, as I’ll explain later.
A loose carburetor or warped carb flange or intake manifold flanges,
plus bad gaskets will make adjustments useless and your fuel mixture all
out of shape, so get all that straight.
A plugged heat riser passage in stock intake manifolds can cause the
mixture to be out of kilter. The reason? An air and fuel mixture needs to
be heated to be fully vaporized so that it does not puddle on the bottom
of the intake manifold. Once the fuel puddles, it drips on down to the
cylinders, then the spark plugs, then…you get the picture! The heat risers
on old manifolds are most likely plugged due to carbon buildup over the
years. Reconditioned manifolds can be bought and are often the only
solution to this common problem.
The engine may not be reaching the proper operating temperature. I
know many of you out there have taken your thermostat and flaps out of the
shroud on your engines. While this may be okay in the summer if you drive
your VW a lot, the engine will not warm up quickly at all in the winter.
The engine will live longer if allowed to warm up more quickly and fully
during any season. I will confidently tell you that if these thermostats
fail, they fail wide open, so the engine will not overheat. Do yourself a
favor and leave the thermostat system on or put it back on if it is not on
The ignition system may be weak or there is something keeping the full
spark energy from reaching the spark plugs. Take out a spark plug with the
wire still on, touch the plug (holding it with the wire) to the engine or
body, and get a buddy to crank the engine over with the idle solenoid
disconnected. The resulting spark you see should be blue in color, not
white or yellow. Replace the ignition coil if the spark is not a blue
color. If the spark plug wire insulation is old and cracked, spark energy
will find its way out other than through the spark plug. You will usually
feel this as a misfire and can see this at night in the dark. Replace any
wires with the Bosch original plug wires if you can, as these are some of
the best quality replacement wires out there for the air-cooled VW engine.
Make sure the points and condenser are in good shape and the points are
correctly adjusted. If it looks like the points are burned and pitted and
have seen better days, replace them. Electronic ignitions (the ones that
replace points) either work perfectly or they don’t work at all, so this
alone is not usually a problem associated with sooty spark plugs. Again,
make sure the initial timing is set correctly. The more the initial timing
is retarded, the more idle fuel is required, contributing to the sooty
spark plug problem.
Make sure the heat range of the spark plugs you currently have
installed is correct. If it is too "cold" a heat range, the spark plug
will not get hot enough to burn off deposits effectively. I have found the
Bosch W8AC to be a very good heat range for stock to mild street
applications, but experiment around if you have a race engine or hot
street setup to determine the correct heat range. The proper color for the
spark plug porcelain in the center electrode should be very light tan near
the tip, turning white the further inside you look, after operating the
engine normally for a few days or weeks. Check to see that the spark plugs
are fully seated and torqued down properly. I know that sounds elementary,
but it does happen, and can cause other potentially serious problems
besides sooty spark plugs.
There may be an exhaust restriction. While it is rather difficult to
think of a VW exhaust system to be that restrictive, check to see if there
are any tubes bent or crushed beyond the normal design limits. If enough
exhaust gas backs up into the cylinder, it will certainly soot up the
plugs, plus cause a lean condition and lost power.
you run a hot street or race engine with a "big" camshaft, this can very
well be the cause of the sooty spark plugs. A camshaft with too much
overlap (intake and exhaust open at the same time) will cause strange
mixture flows at lower engine speeds, especially if the cam was designed
to be efficient at 5000 or 6000 rpm versus 3000 rpm or so for a good
street setup. It would be very difficult at best to tune the carburetor(s)
to get rid of the soot problem with a hot camshaft that is used on the
street, even though I’m sure it has been done. If you put in a hotter than
stock camshaft, make sure you install it advanced four degrees in relation
to the crankshaft, and be sure it matches the driving you plan to do. It
is always better to get a little less cam than you think you may need
unless you strictly plan to race your VW on a dragstrip only and trailer
Lastly, it may make a slight
difference whether you run regular or premium gasoline. Why? Regular 87
octane gasoline actually has a higher heat content per gallon than premium
93. This means that the 87 will actually burn hotter and possibly a bit
cleaner than the 93. That is, if you can run the 87 without risk of
detonation or pinging. If your engine is well-tuned and you keep the
compression at or near 7.5 to 8:1, you can probably use the 87 octane
fuel. Any higher on the compression, and you’ll more than likely need the
93 octane. Experiment with this and use only what you need. If your engine
seems to run well on the 87, then by all means use it. It is cheaper,
- First and foremost, check the condition of the air cleaner (oil bath
element for models up to mid-’72 and pleated paper element for later
models). If the filter is dirty, it can act like a choke and pull more
fuel since it is restricting air to the carburetor. You won’t get to any
solution of the problem until you eliminate this one first. Besides, when
you unnecessarily limit airflow through the intake system, you lose
- Sticking or poorly-adjusted choke mechanism or choke not pulling off.
A sticking choke will pull more fuel than is needed for cold engine
operations once the engine has been started. This causes excess fuel to
deposit itself on the spark plugs and causes soot to form once the fuel is
burned off. If the choke does not pull off, check the choke pull-off
diaphragm and make sure that it is not perforated or hardened and cracked.
Make sure the choke is adjusted per the VW shop manual. Also replace the
heated choke element if you are not sure how old it is.
- If you just take your VW around the block to the store and back, you
will not warm the engine fully. Also, constantly starting and stopping the
engine just to move the car around will definitely get the spark plugs
fouled. This aggravates the problem by not allowing the choke to fully
release and all excess fuel burned off. Getting the engine fully warmed up
keeps the internals a lot cleaner, in addition to the spark plugs. At
least once a week, get the VW out on an interstate or big highway and run
it out in the wide expanses for half an hour to an hour. Don’t we all love
to cruise in our VW’s anyhow?
I hope all these tips get you closer to getting rid of
the soot on the spark plugs. It is a lot to think about, but once you have
found the solution, it will be easier to stay on top of the situation from
that point. If there is anything else anyone can think of on the subject of
sooty spark plugs, by all means post it on the midsouthvw website Forum or
let me know. All comments and suggestions are certainly welcome.
Your VW Maniac and tech specialist,