Rough running on a Qualcast 35S Classic Petrol Mower?

A few weeks ago I inherited one of these mowers. It was running fine, with excellent low-speed tickover and smooth running under load. It is now reluctant to idle, with the governer constantly cutting in and out, resulting in "hunting", and stalling if the throttle is set at the tick-over position. The spark plug is sooting up, indicating rich running, but there is no mixture control on the carb. The fuel has been changed for fresh, but the problem persists. The plug is fairly new and has been re-gapped at approx 15/20/ thou. Various settings have been tried but with no difference.

Any ideas please?