Hi,
I assume this is a problem once the engine is fully warmed up. If it ticks over once the engine is warm but is bad as it warms up, then its the auto choke which may need a good clean or at least a good squirt of wd40.
However, if this is a problem once the engine is warm then check the plug gaps are correct, the points (if you have them - I have them on mine and it's an '86) are are in good nick and correctly gapped and the timing is correct. Check all the vacuum pipes for cracks or old age and so leaking air and finally (Sumo!) check the float level with the engine running.
Did this creep up on you - the idle got steadily worse over a period of days/weeks until it stopped ticking over - or did it happen one day? The latter suggests something has broken - vac pipes for example - the latter that something has gone out of adjustment like the points, plugs timing.
Hope there's something useful there
Tricky
