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Old 21 April 2008, 21:01   #1
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Country: USA
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Make: avon
Length: 6m +
Engine: 90hp merc
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compression of 4hp mercury

I was trying to check the compresion on a 2000 mercury 4hp 4stroke. I believe the motor has a compression release feature to help for easy starting. I tried to get a reading but could only get just over 50lbs of compression. Dont know if the motor is no good or if I cant get an accurate reading with this feature. Any help would be great, thanks
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Old 21 April 2008, 21:18   #2
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Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
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If it were a 2 stroke, should have 115/125 lbs after several rope start pulls. I guess must have the same readings.
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Old 21 April 2008, 21:42   #3
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I know, thats what Im looking for but wonder if it is not giving full compression until it starts. I am haveing problems starting it. so I believe it is bad compression. I believe this motor has a compression release feature that makes it easier to pull start but dont have any more than 50psi. Hopeing someone who knows these enginescan help me
thanks loco
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Old 24 April 2008, 05:06   #4
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Country: UK - England
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Make: metzeler merkant rh
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mergury

i have just had the same problem with a 2 stroke 4 hp and the compression was 115 on both cylinders but after investigating found that the reeds where open behind the carb cleaned them and all was fine i cant believe that they reduce the compression to start when you need it to most try and squirt some engine oil in the cylinders and do another compression test if it increases it is your rings if not probably valve trouble hope this helps
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