I have a Bluewater with 2 Chrysler engines.. One, I can engage transmission at 700 rpm and work like a champ..
But the other need 1000/1200 rpm to engage.. go less and engine quit.. First I tough could be a dirty carburator.. I clean, remove, send to re manufactured.. you named, I did..
Could be transmission?? go in and out marina with 2 engines at 1K rpm..humm.. not a good idea..
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It could honestly be anything from maladjusted carb to ignition issues, vacuum leak, stuck exhaust valve, all kinds of stuff.
Did you adjust the two "idle" screws on the carburetor for highest idle?
Heck it could even be a bad tachometer.
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Might sound crazy, but I wouud suggest suggest new set of spark plugs.
Sounds like you have to bring the rpm up above the idle circuit in the carb for success. So idle mixture, ignition timing, and spark plugs have the greatest effect there. Also a vacuum leak as A64 said.
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If the problem engine idles well and turns up to rated speed in neutral, I too would suspect the driveline, even something as simple as multiple wraps of fishing line between prop and strut or cutlass brg might cause this.... A slight overheating or lack of rpm at speed in the problem engine could also point in the driveline direction.