Hi Alan, While I don't have a Leader now, I have had the pleasure and know they are very simple. My dad had the adage for two strokes of compression / spark / fuel - must be OK..
The coil explanation is a good one - and your symptoms (bar the smoke) are exactly what occurs in this situation. Look for electrical component failure. Perhaps your battery is dying or something in the charging system failing.
Flooding would cause the smoking, but you would expect the most smoke when it starts later..
Have you checked that the breather in the tank cap is clear and that air can easily make its way into the tank?
Two strokes draw a fuel air mix through the crankcases. The pistons act as a vacuum pump to suck the fuel in. The oil you put in the fuel lubricates the crankshaft on the way through. This relies on good sealing in the motor. The crankshaft is an obvious weak link in this and if air gets past it, the motor will suck air from the primary rather than the carby (or from wherever is easiest). This is a possibility and will cause the smoking you are talking about if there is oil to be drawn in. The manual gives a pretty good explanation of how the seals fit. http://www.arielownersmcc.co.uk/members ... 043-62.pdf
.The centre seal separates what is effectively two pumps.
Over time, the two stroke emailers have drifted away from the four. John and Dave have set this group up with a clear intention of providing a worthwhile forum for everyone. It will take a while for those loyal to the original email groups to get the feel of how the forum works and move across. As much as we need their ideas, I expect we will have to be patient.
I hope all this helps.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.