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Old 11-07-2011, 08:08 PM   #1
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Causes of low compression

Hi, I've recently bought mercury 175 v6 which is a non runner, was told "It simply needs the carbs cleaning" yeah right, I'm the dumb one for believing it ! anyway, I've done a few checks, it has a good spark and getting fuel ok but not even a chance of firing, even sprayed a bit of fuel down throats and still nowt, have just done a compression check and theres nothing on all 6 pots, the engine tutrns over by han easily also, the previous owner has also bypassed the thermostats which may tell of an overheating problem, can any one confirm that if the engine has overhetatede and ran for a while would it have burnt the rings and or ruined the bores ? thank you for taking time to read this
Davey
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Old 11-07-2011, 08:26 PM   #2
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Hi Yea anything is possible from stuck rings to wrecked bores. Pop the heads off and have a look! Although for all 6 to have no compression is pretty rare.
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Old 11-07-2011, 08:33 PM   #3
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All 6? You sure it's actually got pistons in it?
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Old 11-07-2011, 08:53 PM   #4
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Check your compression tester !! Stick a finger over plug hole !! If its got pistons it will have compression !!
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Old 11-07-2011, 09:21 PM   #5
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Hi, if I put fingers over they are pushed off when cranking, I also have a good running evinrude 135 v4 which is much harder to turn by hand and is noticably harder to crank due to compression, The mercury seems to spin over much easier on the starter, I assumed that if there was a spark and fuel it should at least try and start so when I saw the thermostats had been removed I started to think something else was up, any suggestions ?
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Old 11-07-2011, 10:04 PM   #6
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You got plugs in


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Old 12-07-2011, 08:35 AM   #7
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Check compression on the good engine with the same tester or get someone to have a look at it. You would have to be very unlucky to have low enough compression on 6 cylinders for it not to fire !!
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Old 14-07-2011, 09:17 PM   #8
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Cheers I thought that but also thought if it had severely overheated it could have caused all sorts of internal damage
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