OK, so this is getting frustrating for all of us. Everything we suggest results in an "Already been there, done that". So let's sit back and do some out of the box thinking.
1. I seriously doubt the lower carbon seal is making it run on one cylinder. I've seen way too many of those engines run to destruction due to water intrusion through the lower bearing when the seal was broken or missing. The point being, they ran very well with no seal, at least well enough that nobody detected it until the whole thing turned to trash.
2. I've never gone to the trouble to test the seal with air pressure as you have, so have no idea what to expect.
3. The carbon has to slide up and down on the crankshaft to maintain contact with the bearing as the shaft moves up and down in its end-play limitation. Actually that is said wrong. The shaft slides up and down in the seal quad ring. So, the question is - is the seal maintaining contact? Quad rings are chosen over o-rings because they are better suited for use on sliding situations. Sliding or not, see #1
4. As a BTW, the carbon seal is an adaptation from shaft seals used on millions of well-water pumps. A sound and proven design.
5. I had a thought in mind when I asked whether it has spark on the lower cylinder while it is running. OK, the thought was possibly the vacuum cut-out switch has a broken spring inside, or maybe a petrified diaphragm. Could result in spark while cranking, but lose it when actually running?
So, those are my thoughts. Money refunded if they don't work.
