OK. So I got my problem resolved and though I detest when its my fault, nobody dislikes a thread that simply ends like I do. Here's what the trouble was.
I opened the VST and everything looked fine. Cleaned the big screen on the electric fuel pump, checked the diaphragm. There was a bit of fine dirt on the screen but nothing that made me think I found the issue. So I reassembled it, fighting the good fight with that top seal by tiring of the game to get it to fit (they seem to expand with age) and using 3M weatherstrip sealant to hold it in place. Worked perfectly. Seal looked perfect, resilient, free of any hint of aging.
Next, I bought a new distributor cap, rotor and correct NGK plugs, figuring I'd go back to basics even though I keep all my machines in fine tune. Cough. Um - not so much. Old plugs looked fine and normal but I numbered each one with a sharpie as I always do for closer scrutiny later. Distributor cap - uh what's this? Each contact had a literal beard of corrosion and electrolytic crud on it. Worst I have ever personally seen. Rotor was worn looking and stuck on so hard with age I broke it removing it. I replaced both parts and then sat there staring at the engine when suddenly the ol' hard drive came up with this gem. Years ago I had replaced the plugs and did not have the extra time to replace the cap and rotor. Figured I'd get to it. Over the season that memory became "I remember replacing the plugs, so I'm sure I did the cap and rotor." Nope.
So a couple hours ago I backed it into the lake for a test run. It took about 20 seconds of cranking in 5 second intervals to fill the VST so it could pump fuel to the injectors. Then the engine caught instantly and burbled happily. Cast off and let it warm up, but could already tell the engine was running smoother and back to its old self. A few minutes of running on plane uneventfully and I moved up and down the RPM range - something guaranteed to make it stumble and surge badly last week. Nothing but the deep note of a big block back on song. We hung out in a bay swimming for a while, then down the length of the lake again looking for any hint of running issues.
So thank you for the tips and advice - particularly Thumpar's offer to come over and help me wrestle with the VST. There ought to be a thread reminding us all to check the simple things first, and this one should be front and center. I'm frankly amazed the engine ran all, and with 40 years of maintaining engines the symptoms did not seem to match what I eventually found. Just another reminder to check the basics first.
DougM
