A plug running at the correct temperature in a healthy engine will be indicated by
blackish or greyish-tan deposits on the end of the plug body and earth electrode. The
firing end of the insulator will vary from a very light tan to a darkish brown.
Fig.
4.
The sparking plug
A sooty deposit on the firing end of the plus is generally an indication of an over-
rich mixture, whereas a light grey deposit on the end of the plus and a white-ish
appearance on the firing end of the insulator may indicate a lean mixture. If the
carburation appears correct, but the plug insulator at the firing end is black with
carbon or oily deposit, this is generally an indication that the grade of plug is too
cold.
The plug should be cleaned on an abrasive blast machine and afterwards the
sparking surfaces of the electrodes on standard type plugs should be dressed with a
fine file in order to restore clean, flat, parallel faces. It is most important that the gap
should be re-set to specification, ·025 ins. Gap adjustment should be made by
bending the side wire. The threads of the plug, and the gasket seat, should be wiped
clean before refitting the plug to the engine. The upper portion of the insulator
should also be wiped clean after fitting, and prior to connecting the H.T. cable. It is
good practice to wipe periodically the top of the insulator with a clean cloth in order
to remove any accumulation of grime or dust etc., as such deposits can be conductive,
and result in poor plug performance.
The special non-detachable steel gaskets fitted to Champion standard sparking
plugs are designed to last the life of the plug.
When refitting the plug, screw in as far as possible by hand, and then use a box
spanner for final tightening, to avoid possibility of damage to the insulator. An
adjustable spanner should not be used.
The carburetter is of simple and robust construction and the only attention that may
be required is adjustment of the pilot jet and throttle stop.
(
Above
)
—Fouled
with burnt oil or soot
(
Below
)
—Correct
