You don't say how old it is, but if it's the Honda engine with coil inside the dizzy, then check the connections onto the coil itself. If they're okay the coil is breaking down under load, capable of producing a spark but it looses its power travelling through the leads to the plugs.
(a quick visual check for the ignition amplifier, whilst turning the ignition on, watch the rev counter, it should flick once) :D
If it's the Rover engine, with coil down the back of the engine, check the king lead first, they go hard and waste most of the spark passing through the lead. If that's ok it needs a new coil.