Yamaha 90 HP outboard (2003)

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Old 05-31-20, 07:48 AM
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Question Yamaha 90 HP outboard (2003)

Just bought a Scout 17.5 boat with Yamaha 90 HP outboard. Cylinder #4 has no compression - engine will start but has obvious tapping noise when running. We knew this when the boat was purchased & understand it has to be repaired. My question is, does anyone have any ideas what the possible problem(s) could be. I understand this is a difficult question to ask but I am just trying to get a "ballpark idea" on the possible problems/remedies. Any suggestions would be greatly appreciated.
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Lonelyringer
 
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Old 05-31-20, 08:23 AM
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I would first check check cylinder 4 valve to rocker arm clearence by removing rocker arm cover and measuring the gap. This requires knowing how to get cylinder 4 piston at top dead center on its firing stroke and using feeler gauges. Could be burnt valves or worn compression rings on cylinder 4. If you have knowledge and tools, this is a DIY project, otherwise take it to a marine engine service shop
 
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Old 05-31-20, 09:09 AM
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A knocking sound could be almost anything; bad head gasket, valve problem, worn bearing(s)... You'll just have to get into the engine and find out. Prepare yourself mentally for the worst and you won't be taken by surprise when you get the bill.
 
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Old 06-01-20, 04:47 AM
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Thank you for the quick reply and suggestions.
Lonelyringer
 
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Old 06-01-20, 04:53 AM
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Thanks for the quick reply. Do you know if there are any good Youtube channels to view about Yamaha engines?
Lonelyringer
 
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Old 06-01-20, 05:01 AM
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There are many YouTube's you can watch. You don't need to limit yourself to Yamaha's though as most outboards are similar.
 
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