Spark-Plugs Inspection & Replacement

Warning! Do not remove or install spark plugs while engine is hot. Do not overtighten them or you risk damaging engine's threads.

If you just want to check them only, remove the one that's easier to locate (per cylinder). Gap must be between 0.8-0.9mm (0.031-0.035in).

Now look at the images below, to get an idea of what a used spark plug might look alike. By examining every plug, you may derive useful information about the respective cylinder and carburetor. (Photos taken from "HONDA C50-90 Owners Workshop Manual", Haynes).

Fig 1. A brown, grey or tan firing end indicates engine conditions are correct.
Fig 2. White deposits due to excessive amount of oil or low quality oil in the combustion chamber.
Fig 3. Black sooty deposits indicate an over-rich fuel/air mixture or a malfunctioning ignition system or incorrect (colder than appropriate) heat rating plug.

Fig 4. Badly worn engine or a malfunctionig engine might produce such a "short-circuited" plug.
Fig 5. Blistered white or melted electrode. Indicates overheating due to over-advanced ignition or heat cooling malfunction or wrong (hotter) heat rating plug.
Fig 6. Worn spark plug. Indicates that it's time to replace it ! Otherwise, increased fuel consumption, ignition system overloading and increased pollutants emmission occurs.

Replacement of the four spark plugs is a tough job for a "TA service rookie", especially after understanding where they are located ! One has to remove both front fairing side parts, side covers and saddle and to loosen gas tank and left radiator (phew!). Tools required:
Philips screwdriver for the fairing side parts.
Screwdriver and 10mm nut for the side covers and the radiator.
12mm nut or wrench for using the spark plug wrench.
HONDA spark plug wrench (it is of low quality but, nevertheless, it's the most appropriate for this job).
0.8-0.9mm (0.031-0.035in) filler gauge
If you are afraid of tightening the plugs, a torque wrench for 16 Nm torque.

What spark plug type to use? NGK DPR8EA-9 for normal operating conditions. DP8EA-9 is a similar type without the interference blocking resistor. My opinion is to always use the "R" type in order to avoid problems on your (or on the others) radio, intercomm, alarm and other sensitive electronic apparatus.
Alternatively you may use DPR7EA-9 for ambient temperatures below 5^oC/41^oF and DPR9EA-9 for extended high speed driving.
Avoid exotic racing types, they do not worth their money in a twin spark ignition system.

You can use just the tools provided from motorcycle manufacturer, this job require patience instead of muscle power. First try the two easier to locate spark plugs. However, now the really bad news are coming. Dismount the two fairing side parts and the back side-covers (first try to understand how they are fitted, otherwise you may damage them). Remove the left radiator plastic cover by unscrewing the 2 bolts. Loosen the radiator itself by unscrewing its 3 bolts but do not loose any hose or holder. Thus you create room for removing the left front spark plug. Be careful not to bend radiator cells. After replacing that plug, proceed for the trickiest one, i.e. the one on the right of the rear cylinder. You must remove saddle by unscrewing it's 2 bolts and loosen gas tank by unscrewing the 12mm bolt. Do not loosen any hose or holder. Pull tank to the back in order to see the plug. Jesus ! Just look where it is fitted ! Replace it and ...

Congratulations !