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Starboard Crusader 270 loss of power at 2000 rpm

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BoatAlaska

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My starboard Crusader 270 with less than 300 hours since rebuild will not go past 2000 rpm. Found the distributor (Prestolite) mechanical advance was locked up. Pulled, cleaned and replaced all equipment (points etc.). Same issue after repairing distributor. Swapped Carbs, swapped coils, timing and dwell set. Still the same problem. Pulled and replaced both fuel filters. Starts and idles very nicely.

How would you tell if the exhaust is partially plugged?

What's next?
 
Use a manometer and make an exhaust backpressure measurement.

Ensure you have an adequate fuel supply - make a vacuum measurement on the suction side of the fuel pump. List of other potentials: anti-syphon valve, fuel pump, carb inlet filter (under the 1" hex nut fitting where the fuel line connects), vent line obstructed, shut off valve not fully opened, fuel line old and cracked (leaking air).
 
You might also check the advance at speed (where have I heard this before). About 10 at idle, about 20 at 2000, about 28 or 30 at 3200. Can do this in neutral. I'd think that exhaust clogs are pretty rare on boats, although I had a catalytic converter do this in a car and it did result in the equivalent of a 10 hp motor. I suppose a fiberglass muffler COULD break an internal baffle and do this.
Does the motor come up ok and then go back down to 2000? or, never reaches 3000?
 
Motor never reaches 3000 rpm. It will tend to backfire through the carb. as you add throttle. I'll check the advance (mechanical in my case) as you suggested.

I will figure out a way to get fuel pressure readings at the carb. and a vacuum reading before the pump. All filters are new.

I'm going to check the compression first though to make certain I haven't burned a valve.

Thanks very much for the suggestions.
 
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