Hello FlyBoyJ again !
Problem still not solved. He did a flight recently, did not notice any change in behavior when he pulls back quickly or not the throttle, the engine always rattles a bit but no different as before. Like said, this is only happening at idle, engine hot. He is now in the process of looking at the two scavenge pumps, the one in front is fine and he will be looking at the one on the back side of the engine, fact is when he activates the clean kit electric pump after a flight he pumps a lot of oil back in the oil tank, which would suggest that the mechanical pumps do not scavenge well the oil during or just after the flight.
Compressions are all OK. Also, the harness was changed after the problem appeared, to no avail.
I have also suggested to look at the surface of various cylinders after a flight, by taking the plugs off, the two down cylinders but also others, for comparison, if the surface is oily then that should indicate a worn scraper ring, shouldn't it ? (I'm no wizard in radial engines).
I've also heard, on a Franklin engine (flat-six), of a cracked boss in the cylinder where the valve guide is inserted, the crack was minor and only visible with a magnifying glass, but obviously oil could then flow by. But that required taking the jugs off, this is only for the next step of problem solving.
This problem is actually quite puzzling.
Many thanks for your help,
Henri.