273 won't start regularly

Have you checked the timing with a timing light? I've never used the method you mention for checking the timing on an engine. I wonder if it matters. Points or electronic ignition?

I recently had a similar problem with my 273. Car would not start regularly, ended up having to replace the fuel pump.

I did have a problem with my carburetor about 5 years ago. Car would start fine, but after driving it to the store and parking it, it would refuse to start. The problem ended up being too much fuel in the carburetor due to a bad float (hole in one of them).