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Engine RPM surges between 1500-2500 but then levels out at 3000 1994 5.0 L

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bhw23

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Its a 1994 22' Larson with a 5.0L V8 mercruiser. Changed main fuel filter, screen in fuel tank, checked anti-syphon valve on tank, and changed the filter in the line just before the carb. The engine idles fine, and revs normally when not in gear. Once in gear, still idles fine, but once it reaches between 1500 and 2000 RPM's the engine either sounds like it's about to stall, or it continues to surge up and down between about 400 and 500 RPMs. Once past about 2500, it smooths out and runs beautifully. But about an hour into running, the RPM's slowly drop off and even at full throttle, is bellow 3000 RPMs. If I slow it down, it's almost impossible to get it back up to any speed and easily stalls. Had to limp back to the ramp 2 times now. I am thinking it has to do with the carb. or possibly the timing. Any ideas?
 
Re: Engine RPM surges between 1500-2500 but then levels out at 3000

Loosing fuel pressure? Put a gauge off/a "T" in-line; needs at least 4 PSI. Does the vent hose drop below the tank? It could have fuel it the low point preventing the pump from siphoning fuel out of the tank. Check the vent for spider nests.
 
Re: Engine RPM surges between 1500-2500 but then levels out at 3000

Check that your carburetor and intake manifolds are properly torqued, not over tight. Air leaking by can give you a surge. Also check that your fuel line has no cracks in it.
If all that is good there is occasionally a report of Thunderbolt IV ignition amplifiers "breaking up" when hot. Does yours have the insulators to help isolate it from engine heat? Is your engine compartment pretty hot underway?
 
Re: Engine RPM surges between 1500-2500 but then levels out at 3000

Check the fuel lines and vent lines themselves to make sure they are not soft and closing in on themselves when you give it the gas. That would cause a delayed reaction like you're describing.
 
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