The starter drive on the larger Continental engines is available in two variations. They are both similar in operation, but with unique characteristics. The older original design, if operating correctly, will allow the propeller to be gently turned backward. The newer design will not. Either is okay, if operating correctly, but both are a great risk to the engine on the whole if operated after they begin to fail.
In either case, failure is indicated to an alert pilot by prolonged slipping during the start. They all have a brief slip, as an aspect of normal operation, but if that slip continues more than a second or so, take the aircraft for maintenance at the next possible opportunity. The prolonged slipping is causing the steel clutch spring inside the starter adapter to dramatically overheat, and begin to weld itself to the shaft drum on which it operates. In extreme cases, and depending upon which type of starter motor is fitted, this could cause the engine to drive the starter motor at high speed for a period, which could cause a bizarre electrical problem.
Equally serious, is a starter motor which is too stiff (needs rebuild) or continues to be powered after the pilot releases the start [key]. This situation will equally cause an expensive failure of the drive. The UK CAA got it exactly right requiring an indicator to the pilot that the starter motor was running after release of the [key]. If this is the case, stop the engine, and seek maintenance.
The problem is in either case, eager pilot gets the engine running, and goes flying, forgetting that getting the engine running (including by hand propping) was abnormal. This goes on for many flights, and the failure worsens. Pilot information on this important topic is few and far between, but this is a very expensive failure mode of these engines, and pilots need to know.
The people at Canadian Aero Manufacturing are expert at these starter drives:
Canadian Aero Manufacturing
The ICA found here, is a worthwhile read for maintainers and pilots:
http://www.canadianaeromanufacturing...ringoldica.pdf