Q: Richard in Western Australia asks about misfiring , loss of power and a breakdown resulting from sooty fouled spark plugs. What’s causing the plugs to foul?
A: There are many possible answers to this question starting with the fuel. If the carbon on the plug is dull black (as above) it’s likely due to a rich fuel air mixture. Alternatively the problem could be oil burning if the carbon looks glossy. Usually these problems clear up to a degree if the engine is running at higher speeds – try checking your plugs after a high speed run to see the difference.
The fuel we get now is formulated for fuel injected cars with EFI computers which doesn't help things however many people find they can get their engines to run OK with it. If I have a choice between 91, 95 or 98 octane ( Australian fuels) I go for the 95.Possible causes:
- Overly rich idle mixture
- Oil burning
- Spark plugs too cold – incorrect heat range
- Excessive use of chokes
- Weak ignition or coils
- Excessive idling and low speed running