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2001 Mercury 200 redlining at startup

2

216 Bay Reef

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I have a 2001 Mercury 200xl carbureted engine that has been sitting for a couple months while I replaced my boat's fuel tank. Upon startup after the new fuel tank was in, the engine wouldn't start. After going through the carbs, I found 2 of the 3 carb float needles stuck closed. I cleaned the carbs while I was in there and reassembled. Boat now starts right up after using the choke at the key, but the engine immediately revs to 6000 rpms and stays there. Since then, I've taken the carbs off two more times to double check everything, and did a smoke test for leaks with nothing coming up. Throttle cables are not bound up, and butterflies are closed on the carbs at startup. I'm having a tough time diagnosing this one as I can't let the engine run at these RPMS.
 
Looking down the throats of each carburetor, did you confirm that the throttle butterflies are closed with the throttle at idle?
 
This may either be a large unmetered air source or timing stuck at full advance. Start by verifying the spark advance linkage and trigger arm—if timing is locked at wide-open, the motor will race even with the throttle closed. Next, recheck carb sync and pickup cam settings, and make sure throttle plates truly seal. If linkage checks out, suspect an air leak: carb base gaskets mis-seated, a reed block leak or broken reeds, or possibly a failed crankshaft seal. Since the motor sat, seals and reeds are high on the list. In short: confirm timing first, then carb sync, then look for intake or seal leaks if the problem persists.
 
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