It's driving me nuts...... what I thought was an air leak originally was a very, very slight one around the flywheel side seal. Replaced both and retested and the hold pressure and vac forever. I have ultrasoniced the carb twice, kited it,adjusted, sealed. Tryed new carb and it still was the same.
Didn't seem like ignition but I was outta things to try so I put a new coil on, ( I did use the old wire though) still the same.
I have never and 0 mean never seen anything like this.