1983 Mercruiser 120 Engine Surge

MMCS/SS

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Jul 28, 2011
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My son just bought a boat after it had been sitting out of the water for a couple of years. The engine starts up fine, but appears to idle a little low (just under 600 and sounds a little rough even after warmed up). In the water it does fine at WOT running at about 4000 rpm for about 30-45 mins then the engine starts to run rough and starts to surge. Rpms drop off to anywhere between 3000 and 3500 and continues to hunt. Today the engine actually drop off to 2000 rpm. I then reduced throttle to minimum. The engine stalled, but started right back up and I was able to immediately return to 4000 rpm for about a minute when it starting surging again and returned to the 3000 range.

No maintenance has been done on the engine since my son bought it. We have looked at the inline fuel filter but not the fuel pump filter. We have cleaned the flame arrester which was real dirty. I was thinking of looking at points, plugs, timing etc, but wasn't sure if any of that would provide a smoking gun. I can fix things, but I couldn't diagnose a dripping brown paper bag. Any assistance would be appreciated.
 
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