Perhaps that carburetor had been serviced the way we are told to do it in the carburetor manuals, Rob: remove the choke and throttle butterfly plates, and pull out the shafts they mount on. I literally never do it unless the screws are already loose. As a lad many, many years ago I took some car carburetors apart, and was never happy with the security of those screws afterward. Because the shaft is normally brass, I ended up soldering the screws in, getting them hot enough so that the solder penetrated the threaded section. None of them ever came out after that, but I stopped taking the screws out anyway.
I've never had an engine eat a butterfly valve plate screw, but once again way back, I drove an old 800 cc OHV Morris Minor. One night I was putting along through the suburbs when suddenly the engine rattled and lost a cylinder. I think I had the ignition off in considerably less than a second, and caught a tram home to pick up the spare car and drive back, with a rope and a co-driver, to tow it home. Next morning I pulled off the head and found I'd been very lucky. A disused weld-nut had fallen off the inside of the giant oil-bath air cleaner, and slid down the intake manifold straight into the cylinder. The loose nut in the cylinder had immediately hammered the spark plug gap closed, and hadn't done any other damage in the brief time it had been running. So, it was going again in about half an hour at no cost. It could have been a great deal worse with another few seconds of running: a broken piston crown at least.
