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QI’m not quite sure, but I think my engine is misfiring. Can you help me identify and solve this?

James Phillips, via email

Ed says

AThere are three basic types of misfire: a regular, rhythmic miss under any conditions or specifically under load; fluffs, hesitations or sputtering, usually under load; and an irregular fluff or miss at idle, or conditions of light or no load.

The rhythmic miss is often a single spark plug or plug lead that’s failing. It can be identified by removing individual HT leads (with insulated pliers) while the engine idles. Each should make the engine idle much worse. If you remove one and the engine note barely alters, that’s your culprit. Try swapping the plug and lead, in turn, for one you know is good. Alternatively this may be a burned valve, or one with no clearance set. Sometimes it’s simply caused by a rich mixture (stuck-on choke; flooding carb etc) – look for sooted-up plugs.

The fluff, hesitation or sputter is perhaps a component (common to all cylinders) that is failing – coil, rotor, distributor cap, king lead, etc. On the ignition side, it might be a distributor cap that is damaged or whose central (usually carbon) contact is missing or not springing out to touch the rotor.

The rotor itself may be

burning through (usually from a rivet down to the central shaft); or the coil or king HT lead may be failing. Dirt or damp on any of these parts may be an issue, as may a weak connection in the 12V ignition circuit – or a rogue condenser. Alternatively it may be an air leak from a perished vacuum hose or servo; a misplaced vacuum hose; stuck-open EGR valve; blocked accelerator pump jet; carb with a loose or distorted mounting flange at the inlet manifold; low carburettor float level or blocked carb jet.

The miss at idle or light load is easier to track down: the ignition timing is too far advanced for some reason, or the mixture (usually idle mixture) is too weak due to bad adjustment or a partly-blocked jet or an air leak (or valve clearances too small).