Well I started this thread a month ago and in the meantime took it out to the field. My instructor told me that there was no problem with the engine dying when vertical at idle. He adjusted the needle valve a bit, and we flew twice that day with no problems. It still does have the somewhat rough but reliable idle.

But as long as I have you're here: Yesterday I was working with a Magnum .46 that I bought used on a used trainer. The guy I got it from said it was a trusty good engine. This weekend was the first starting it since at least last year. I got the main needle adjusted to where it wouldn't die at full throttle when vertical, but at that setting, when I let it idle for about a minute and then punched it, it died immediately. Does your same advice apply that it's still too lean? This is a different engine mind you than the one we were talking about. Does it have an air-bleed carb? I haven't even touched the low speed idle adjustment yet.