Apologies that this is an old thread - I've just had exactly the same symptoms and the 'springbar' in the (metal) female part on the cap was gone on both.

New leads from RO and it now feels like a different beast.

Did you measure the resistance of the leads and plug cap? I am sure you will find the reason for the performance improvement is the lead and plug cap with the correct resistance and not a functional retaining spring. Resistive plug cap fail with time and eventually go open circuit. The NC30 has two resistive plug caps and eventually the ignition system has trouble developing sufficient voltage to jump the dud plug caps when the engine is running at high rpm.

The plug caps should measure 5000 ohms each and the ignition coil secondary secondary approx. 12,000 ohms, total cap to cap 22,000 ohms. Any more and one or both of the plug caps are suspect.

Right, I get zero resistance between the end of longer of the two leads for the front head and the cap. This means that the cylinder 4 was likely to have been the one misfiring?

You mean zero, or infinite resistance from cap to the lead end?

Going back to the screw on caps on the plugs.... I did my plugs a while back, and I'm sure the ones that came out didn't have them on, so I unscrewed them and fitted the new ones without. I seem to recall that the plugs didn't fit in the Honda plug spanner that I bought with them on so I figured that they were there just to protect the plug. The caps seemed to press on pretty firmly when reconnected. If they are *supposed* to be on there and are a really, really tight fit, firstly is there a potential to be losing some power of spark as a result, and secondly, is it not a possibility that being almost 20 years old, the cap might break if I try again with the things attached?

The spark plugs do not use the screw tops, the plug caps have a spring clip that slips over the exposed thread on the spark plug and helps holds the plug cap in place.

So mine is right then. Sorry, sounded from a previous post like the things were supposed to stay on!