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Loss of speed over 3800 rpm

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Old 08-06-2011, 10:56 AM
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My Boats: 1987 Checkmate Convincor 251, 1987 Chris Craft Scorpion
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Question Loss of speed over 3800 rpm

Hi guys I am having a problem with my 1987 chris craft scorpion 230 that I just picked up. The boat has a merc 260 with a alpha one. The problem happens at 3800 rpm. The boat runs great up to that point and is cruising at about 38 mph at WOT. At that point if I trim it up the motor will jump to 4300- 4400 and the motor sounds great good oil press, water temp is at 160 and the secondaries are opening (Q jet carb). But the boat loses speed even though rpm has gone up. I lose about 3 mph. It has had a complete tune, filters, good compression (155), carb rebuild. I just dont understand why with an increase in rpm results in a decrease in speed when the engine is running so well. Any help would be greatly appreciated.
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Old 08-06-2011, 11:17 AM
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Cavitating?
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Old 08-06-2011, 12:55 PM
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sounds like youre trimming too high.
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Old 08-06-2011, 01:26 PM
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Sounds like you're over propped. You should hit 4600 with just a bit of trim.
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Old 08-06-2011, 03:41 PM
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Coupler going ?
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Old 08-07-2011, 08:32 AM
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Thanks everyone for the responses. I took the boat out yesterday with the same results. So I am going to pull the prop and see what it is. With four adults and half tank of gas on the boat it was hard to get the boat up on plain ( about 3000 rpm) and then would only run up to 4100-4200 trimmedbut still felt like I was losing speed. I know the motor is running good and I don't believe the coupler is bad. But anything is possible. I will start with the prop and see where I am at.
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