Motor won’t stay running

Hi All, bit of an odd question, but I’m gonna put it out there for thoughts. I recently freshened up a 400 that I have fired up on the floor. ( with a rad, of course.) the purpose of it starting on the floor was to make sure there’s no rear main seal leak before it goes in the car. It’s running on a points distributor. It’s a mild motor with camshaft / steel crank / ported 906s in it. Lots of oil pressure. It starts pretty quick. ( I haven’t timed it yet. ) but it only wants to run for a few seconds and then it shuts off. I’m thinking condenser has gone bad or possibly a ground. Anybody got any other ideas? I don’t believe it’s fuel related as I’m using known good carburetor. The most it’s probably run at this point is 30 seconds. Don’t worry it’s a used cam that’s not an issue.