[imported_stump jumper]

Need some help from the crew. I have a '91 200 HP Mariner Offshore. The motor was underwater, but pickled and was running great last week. Now I have developed a problem where themotor will not start unless in fast start. It runs fine in fast idle, but cuts off as soon as I bring to Neutral. I pulled 1 carbuerator off and it looked fine. I am getting spark at all 6 plugs. I am running off an external fuel tank to eliminate the possibility of contaminated fuel in the inboard tank. Any ideas where to look next?

[JUSTINTIME]

what do the plugs look like