Both engines running rough and backfire thru exhaust

[menos22]

Good morning experts, 1994 35 express bridge, twin 7.4l 454ci bluewaters, weber carbs.
boat ran fine on initial spring run except for bottom being fouled. Put some sea foam in both fuel tanks prior to getting it hauled and power washed. When I started it up both engines died when put in gear. Had to rev it up to 1100 rpm to keep it running and it sounded awful. Made it over to haul out but couldn't get it over 2000 rpm, the throttles would go forward but no response from engines. When in neutral is revs just fine and sounds ok. Under load it backfires through the exhaust and no power. Changed bulkhead fuel filters and engine fuel filters, they looked clean and no water. Could my plugs be fouled from the seafoam?? Any suggestions are more than welcome. THanks

 

[northshore]

You have done everything that I did when my boat did something similar when I first bought it.
It sounds as if you may have ingested 'phase separated' fuel in to the carbs.
I ended up having to rebuild the Rochesters. No problems since.