Elusive Miss-Fire
There are many possible cause for a miss-fire and the most common relate to the plugs, points or HT system although the fuel system and valves can add their share too.
Some time ago I had a very elusive problem with a Riley RME. The miss-fire was quite violent being felt throughout the car and all the usual checks failed to find the problem. Eventually I traced it to an intermittent distributor earth. On Riley RMs there is a manual advance-retard control on the facia; push it to advance the spark and pull it out to retard it. It works by twisting the distributor body in the block on 2� litre models or in timing case on the 1� litre models. The distributor earth relies on a good electrical contact between the distributor and the block or timing case and on my car this contact was being broken intermittently. Each time the contact was broken an ignition spark was generated thus causing the miss-fire. The cure was to fit a separate earth lead to the distributor by clamping one end of a wire under a baseplate screw and earthing the other end to one of the inlet manifold retaining nuts. With the wire in place the miss-fire vanished and has not reappeared.