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Bought another project boat to replace my old project boat with the 170 merc-gctid402155

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    #1

    Bought another project boat to replace my old project boat with the 170 merc-gctid402155

    Its 105 degrees outside so I thought I would do a little troubleshooting from the computer, I just purchased a 1993 mariah 20' withwith a mercruiser 5.0 (why dont they just use a damn 5.7l). boat is immaculate and had a new motor in 04. boat runs perfect out of the water but the PO said the boat only gets to 3000 rpm in the water under load. he said he had a compression test done and had low compression in 1 cylinder, couldnt remember what cylinder but he said he thought it was on the starboard side and couldnt remember the reading. as soon as it cools down a bit I am going to do a compression test to get an idea of what I am in for. My question is what would your guess be based on the symptoms, cylinder or valve train? like I said engine runs perfect on muffs.

    I have to admit I am hoping he was wrong about the compression issue and its just a fuel water seperator or timing issue, I got a screaming deal on the boat even with putting an engine in it
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    #2
    If the boat hasn't been run much lately the compression might actually improve after some run in.

    Get it tuned right and run it for a while. Then check again.

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      #3
      trophyboat wrote:
      If the boat hasn't been run much lately the compression might actually improve after some run in.

      Get it tuned right and run it for a while. Then check again.
      I dont think 20 psi in the #3 cylinder is something I can run out, I just did a compression check and all cylinders but #3 was 145-150, put a lot of oil in the cylinder and it didnt add any compression, I know that is not a surefire way to check but if its just the rings it should have increased it a bit, if its a big old hole in the piston I probably wouldnt get any compression, I am praying its just a burnt or bent valve at this point

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        #4
        Or a sticky valve

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          #5
          Pulled the valve cover and found a rocker that I can spin side to side and pull the push rod out, basically the valve isnt opening, if I remember correctly the valve order is EIIEEIIE on both sides, so that would make it the intake on the #3 cylinder the low compression one, the spring doesnt look broken, I am not sure what to make of it, how would a valve staying closed cause low compression? since I am not sucking any air or fuel mixture in would I have nothing to compress and therefore no compression? does that make any sense at all

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            #6
            well apparently my theory was correct about the valve being closed and not allowing air/fuel in therefore nothing to compress, I tightened the rocker nut down a bit just estimating the pressure and redid my compression test, 170 psi which is higher than the others but I figure its because of the oil I shot down the cylinder, I will do a proper valve adjustment tomorrow

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