Ask The Mechanic – Checking spark plugs

An article from Chairman Malcolm Marchbank

Spring has sprung and those classics will be
starting to come out of hibernation. After months
in the garage with the occasional start up to
keep it ticking over, your engine can suffer. I have
personally experienced this after months of an
engine sitting during restoration work and being
moved from one side of a workshop to another.
Once the work was complete, trying to drive
away from the workshop, my T2 Bay Campervan
wouldn’t accelerate down the road. Reason –
fouled spark plugs.

I have also had a spark plug with a closed gap
(don’t even ask how that happened, but it
involved losing part of the carburettor through
the engine… lucky it didn’t do any other damage!)
The condition of your spark plugs can make a
massive difference to the running of your engine,
so it’s worth checking them every so often,
especially after a period of time unused.
Hopefully the following information will help to
make you a spark plug expert.
Before starting work on checking your plugs, it is
helpful to have the right tools to hand; accessing
the rear two spark plugs at cylinders 1 and 3
can be a real fiddle, especially on later twin-port
engines where access is further compromised
by the inlet manifolds. A short 21mm socket and
universal joint may give you a bit more flexibility.
When checking the plugs, it can help to remove
each lead and plug individually so that you don’t
get them mixed up. This will cause an incorrect
firing order and your engine will not run.
When removing the ignition lead from the plug,
be sure to pull it off by the connector, not the lead
itself, as you’ll run the risk of pulling the lead off
the connector (trust me!)
If you notice any damage to a connector or if a
lead is a lose fit, it is best to go out and buy a new
HT lead set.
Make sure you have the socket on the plug
properly when you’re undoing them and it’s also
best to do all this while the engine is cold to avoid
burning yourself!
Once the plug is out, take a good look. Is it brown,
grey, sooty or oily? If the engine is running right,
it should be light brown or grey. If it is sooty but
dry, your engine is running rich and not burning
all the fuel. If the insulator is white and flaky then
your engine is running too lean. Either way, you’ll
need to tune your carb to adjust the fuel/air
mixture.
If the plug is wet and oily, there are a couple of
possibilities. The first is that you’re not getting
a spark, in which case you may have noticed
a misfire. If this is the case, check the HT lead
connection at the plug and also where it pushes
into the top of the distributor cap.
A worse scenario is that your engine has worn
piston rings and/or valve guides, which means a
rebuild is on the cards. If there is serious carbon
build up on the plug, or what looks like molten
bits of metal, chances are your ignition timing is
out.
Whatever their condition, while the plugs are out
of the engine they will benefit from a good clean
up using a brass wire brush. While you are at it,
check the spark plug gaps using a feeler gauge.
For most air cooled engines the gap should be
0.024” or 0.6mm, however check your workshop
manual because the gap will be different on
some engines. If the gap is correct, the gauge
should slip in and out without much resistance.
If it is too loose, you can adjust it with a gentle
squeeze in a vice to close it slightly, or if the gap is
too tight, carefully prise open the contact with a
flat bladed screwdriver.
Spark plugs should be checked every 3000 miles
and replaced every 10,000 miles as part of your
service routine. If you suspect a poor running
engine there is no harm fitting new ones sooner,
they are relatively cheap for a set.
When refitting, always start screwing the plug
back in by hand, only using the socket for the
final tightening, otherwise you risk forcing a cross
thread. If you feel any resistance early on, unscrew
and carefully try again