I've got an 83 Magnum sport with twin 330's. Last Friday filled her up took off up the river, made it about 1/4 mile and the port engine sputtered and died. Idled back to the dock on the starboard engine. Port engine would run, but sounded like it was on 4 or 5 cylinders. Plugs were OK, swapped the entire ignition system out (module, distributor, coil, cap, rotor) - no change. Pulled the fuel filter and found it about 3/4 full of water and scum. Changed the filter, ran a lot better but still did not want to idle. Pulled the carb (Rochester), dumped out the bowls, cleaned out the screen filter - a few drops of water in it but not bad. Still won't idle like it did, I ran it this evening for about 15 minutes between 3000-3800 rpm and it ran fine. Still have that bad idle and a slight miss up to about 1500 rpm. Any ideas? All gauges read fine. Could my idle circuit have gotton some of the scum in it? I think I'm going to run some fuel thru it and see if it gets any better, but it is not running the way it was. Any suggestions as to next steps? I may swap the carbs to see if the problem follows...

By the way, I think I'm getting the water and scum from my tank vents, they are towards the rear of the boat and the Magnum sits pretty low with the big blocks in it. Thanks for any feedback!

I would just try and change your plugs. THey could have gotten damaged when the water went through there.

Jon

You may hve some passages in the carb blocked with trash.
I'd spray carb cleaner through all available passages and try it again.
Whenever I rebuilt a carb I'd use two cans of carb cleaner after soaking in a bucket of regular carb cleaner. The dirt and trash that would come out was amazing.

Before you pull the carb try this, disconect the throttle linkage and remove the flame arrester,start the engine and goose the throttle before the rpms get to high force the choke plate closed until engine almost stall's then release choke do this a few times and it should clear idle circuit.