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Old 07-17-2017, 12:07 PM   #1
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Default Maxum 2400 5.0L Alpha engine loss of power

Engine under load will run at 2600 rpms then when I increase throttle rpms will drop a few hundred rpms. With no load the engine seems good with high rpms (very brief) out of gear. Boat is 2000yr with 270 hrs. Ran great last Oct. when I bought it. Carb. was just rebuilt, new plugs, wires, dist. cap and rotor.
Gas is good, new gas filter, gas lines from tank good. I found one bad spark plug crusted over and rechecked the plug after running engine as we were on the water adjusting carb. for a few hours. The prop looks good and the bottom has new paint. Outdrive was rebuilt in 2016. During all our testing on the water under load the rpms went up to 3600 then later after stopping engine we could only get 2600rpms. I have not run tests on fuel pump. Has anyone experienced this or have suggestions as to what to do next. I hope all the carb measurements are correct as we took the specks from on line video rebuild by a mechanic in-law of mine. Engine sounds good at idle and under load it still sounds good except for rpm problem. Thanks
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Old 07-17-2017, 01:49 PM   #2
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Engine should run between 4400 - 4800 RPM's. 3600 rpm's is too low. Obviously 2600 is too low as well.

'Crusty' spark plug would indicate water ingestion. Is your mechanic-in-law a marine or car mechanic?
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Old 07-17-2017, 03:50 PM   #3
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What year is the boat/engine? 1998 or newer will have an electric fuel pump.

Perform a compression test and fuel pressure test.

Checking rpms in neutral will tell you nothing.
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Old 07-17-2017, 04:57 PM   #4
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Car mechanic. Can find no water in fuel or oil. Thanks
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Old 07-17-2017, 05:00 PM   #5
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year 2000. Thanks will do those tests.
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Old 07-17-2017, 09:43 PM   #6
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May also be a bad ignition coil heating up and voltage breaking down. HV probe to check voltage or a spark gap tester may show this or just spend the money to find out after ruling out the other things.
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Old 07-18-2017, 01:14 PM   #7
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Water in fuel would be unrelated. Water in oil can only be detected by either pulling a valve cover or doing an oil change. oil doesn't circulate in the dipstick tube. You would need a decent amount of water in the oil to detect in in a oil change.

A small amount of water could be coming in through the exhaust valve, getting vaporized and ejected. It wouldn't be enough to show in the oil, but it would be enough to corrode a plug and possible start impacting a ring. A bad ring would manifest itself with low compression and eventually power loss.
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Old 07-20-2017, 12:53 PM   #8
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After replacing the plugs I checked plug #7 which had been crusted over and found it clean after about 2 hrs. run time. I will do a compression test this weekend. Thanks
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Old 08-01-2017, 11:41 PM   #9
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Update: The #7 cylinder had zero compression while the one next to it read 190 . Not good. I am having boat moved to repair shop hoping it is just a broken valve spring. They will start with a leak test on all cylinder and we will go from there.
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Old 08-02-2017, 12:01 AM   #10
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What I suspected, most likely a tuliped valve(burnt) from water intrusion due to leaky exhaust. Valve seat may be melted as well. Post pictures of what is found.

Look for signs of rust the the cylinder and exhaust port.
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