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Do Not Rotate Engine While Distributor is Out - Merc Manual

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  • Do Not Rotate Engine While Distributor is Out - Merc Manual

    1998 Merc Gen VI 7.4

    I have my engine half disassembled and I would like to do a compression test on it before totally re-assembling. But the Merc Manual says I shouldn't rotate the engine w. the distributor out. Is this solely because it doubles as an oil pump? What if I used one of those oil priming tools with a drill? If I use the tool, can I disregard that warning?

  • #2
    you won't have a problem other than resetting your timing when your done.
    1999 Powerquest 260 legend
    502 mpi
    B1

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    • #3
      Cold engines usually don't yield good compression test results.
      xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
      The world takes on a whole new perspective when viewed from 100’ below.
      1972 Bertram ‘Bahia Mar’ 20
      2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
      (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


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      • #4
        Yes distributor drives oil pump so no oil with out dizzy in place. What’s her purpose of the torn apart compression test?

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        • #5
          Originally posted by Scott06 View Post
          Yes distributor drives oil pump so no oil with out dizzy in place. What’s her purpose of the torn apart compression test?
          Thanks for everyone's replies.

          So I had not started the engine on this boat for two years (due to a bad gas tank that had to be removed. That was the real project). When I finally got to the point of running her, I decided to do a compression test first. The results were pretty bad, but one cylinder had zero compression. Was a seized valve. So off came the heads and to the machine shop. Now I'm putting things back together and I want to do a compression test before putting everything back together. I understand that it's not ideal to do so with a cold engine, but I imagine it's better than putting an engine back together and then finding out you have a problem that needs the engine to come apart again. That's just my conclusion. I'm not a very experienced mechanic though...

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          • #6
            Originally posted by scoflaw View Post
            you won't have a problem other than resetting your timing when your done.
            OK thanks!

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            • #7
              Sorry, but I have had major experiences with salt water cooled iron blocks and if this is a saltwater cooled 98 block, ditch it and find you a good core at a junk yard. Maybe Craigslist? Marine it! If not saltwater block, then take other head and have it tested and valve job, if necessary. I suspect it condition is near like the other one. Having dist out is not a nightmare to retime. Just bring #1 cyk. up on compression stroke and timing tab "0" matches harmonic balancer's "0" mark. Drop in dist. (easier said than done too) and make sure rotor points to #1 plug. Aligning cam teeth to dist's and the female slot on oil pump can be a bear but not impossible. Unfortunately, the only way that I know how to do a complete compression test is to have heads on. But, honestly a 20 year old motor that is mostly broken down and NOT a salt water motor, I'd do a total rebuild and have peace of mind. Good luck!
              Last edited by GA_Boater; December 1st, 2019, 09:04 PM.

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              • #8
                Originally posted by tank1949 View Post
                Sorry, but I have had major experiences with salt water cooled iron blocks and if this is a saltwater cooled 98 block, ditch it ...
                Seems like you've had a bad experience, and I'm sorry to hear that. But that doesn't mean every salt water engine out there is in the same condition as the one you have. 20 years on an engine is not a great deal in a boat. That will only be around 1,000 hours (on average), and a half reasonable SBC engine will still be in excellent condition, given that standard maintenance has been done (including changing elbows and manifolds)

                Best not tar all engines with the same brush as yours.

                Chris......
                xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                The world takes on a whole new perspective when viewed from 100’ below.
                1972 Bertram ‘Bahia Mar’ 20
                2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
                (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


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                • #9
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                  Originally posted by achris View Post

                  Seems like you've had a bad experience, and I'm sorry to hear that. But that doesn't mean every salt water engine out there is in the same condition as the one you have. 20 years on an engine is not a great deal in a boat. That will only be around 1,000 hours (on average), and a half reasonable SBC engine will still be in excellent condition, given that standard maintenance has been done (including changing elbows and manifolds)

                  Best not tar all engines with the same brush as yours.

                  Chris......
                  Back in the day before 4 stroke OBs ruled, most people that I fished with in NW Florida just figured 7-9 years on salt water cooled Chevy v8 block and that was if it was always flushed with fresh water after each trip. Ones that stayed salt wet were eaten out sooner. The risers went first and then manifolds clogged up followed by heads (I experienced all) . Then, heat exchangers became very popular. Now, people throw 60 K into a couple of HPowered 4 stork Yammies. And, the same people have heart attaches if they blow a power head. Not me!!! However, I learned real soon in life to never store an OD wet in salt water.

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