The valves on Isetta - possibly more so on British, take advice - featured some that were made of a head and a stem welded together. These suffer fatigue to the rear of the weld as the metal is stressed. A build up of crude on the valve stem can be a cause, over revving also will promote fracture, but often times the valve actually fails at low revs from metal fatigue. The head simply parts from the stem and drops onto the piston. Normally this cause quite catastrophic damage to the top end which can be cleaned up and piston replaced. It has been known to end with the spontaneous random dismantling of a previously good engine for no apparent reason with cylinder in half, crank bugnered, head chewed and so on making the majority of the unit scrap. It has been, I believe, identified which valves were the main problem and it is good sense to replace them with a better manufactured item either spun welded or one piece, I assume. I do not think these are expensive. Certainly for the price a worthwhile investment if there is any doubt about Isetta valve condition.
Trienkels are far less prone to dropping valves but it is not unknown. I think the frequency is within acceptable bounds and in fact I have seen more failure of the ali loops that hold the valve rocker bearing shafts in than actual dropped valves. The Heinkel engine runs twin springs to the Isetta's, I believe, single so if a spring should fail - not heard of one that has - the second will salvage the situation as the engine will see a drop in performance leading most folk to find out what happened before an expensive failure. Again take advice as to Isetta valve spring failure or not from someone who knows more than me. Again crud can get deposited on the valve stem and it is advisable to clean it off before it interferes with the valve stem. This was offered as a principle reason for valve failure back in the day on any of these types of engine. Heinkel engines actually like revs, within reason, and despite the racket that sounds like the unit trying to get through the bulk head and sit next to you it is actually better to use the revs than hang on the superb torque such a small unit can give. Not least because as a splash fed crank it needs revs to get oil and cooling air about. Slow running means poor oil feed and early failure from heat and faster wear from poor lubrication. A situation not aided by many owners insisting on running the incorrect oil grades! The Isetta has an oil pump and is perhaps driven with less revs relying on its larger cc to make progress though clearly some enthusiasm is needed to get the air blowing round the cylinder. On running at high revs the problem with the Heinkel engine is the long ali pushrods. They can deform(?) or otherwise jump out of their cups at each end, possible due to valve bounce. At the bottom this can lead to the cam follower/rocker arm snapping in half, it is not very strong across its shaft because it does not have to be when all is in place. At the top the shaft can be jammed into the rocker arm. Either way your not going anywhere. It is possible to get home on a bust ali loop though. I have done so by inserting a large screwdriver through the oil filler (big one in this instance) and levered the rocker back into place. As long as tension was kept on the lever the car drove as normal.
Trust this is of help, interested to hear others suggestions/factoids/critic as I am not an engineer, this is roadside wisdom, its less accurate cousin.
