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  1. #1
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    50hp Johnson late 70's or early 80's

    Helping a friend work on a 1970's Johnson 50hp. When I checked compression it had 150lbs on all cylinders is this normal for the older motors. Thing turns over good until the plugs go in then it seem that the battery is weak for the compression showing. Just wondering if the compression should be that high and if so do you need to run 93 octane fuel?

  2. #2
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    150 PSI is borderline on the premium fuel but acceptable. The battery is suspect as it sounds like it won't hold a load. Typical of a marine used battery. Had one in my 25 hp fishing boat that checked out fine til you put a load on it.
    www.eagleoneperformance.com


  3. #3
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    Were these motors built with this type of compression or does it sound like someone shaved the head? Never seen compression like this from the factory on a motor, but the motor looks to be as old as me.

  4. #4
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    Not sure but It seems a tad high for a stock engine?? Maybe, maybe not, just not that familiar with that particular engine. I'd think some of the guys who race in SST-45 could tell you.. Is this a 2 cylinder model??
    www.eagleoneperformance.com


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    might have had oversize pistons in it and in that era would push the comp up. The combustion chamber is radically different shape than a later engine and a very small amount of shaving bumps the comp up a lot.
    Therefore you need to run 93 to be safe as that is what it was designed for.
    Be careful if it has 70's rods and pistons they dont like running over 5500 rpm yet the engine will happily make power to 6000
    Late rods and pistons are fine as in the 80's as per sst 45 are the same as Eagle One points out.


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    Last edited by powerabout; 05-08-2013 at 06:49 AM.

  6. #6
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    That's on the high side of normal compression for those old OMCs, your gauge could be a bit happy also. If it was mine, I'd run premium fuel and be careful with the timing.
    > Stainless steel Merc cowling plates - $110 shipped TYD - LINK <

    1979 16' Action Marine/2.5L Merc S3000 - Metalflake Maniac
    1984 18' Contender Tunnel/2.4 Merc Bridgeport

    "Where does the love of God go, when the waves turn the minutes to hours?"

  7. #7
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    Quote Originally Posted by 1BadAction View Post
    That's on the high side of normal compression for those old OMCs, your gauge could be a bit happy also. If it was mine, I'd run premium fuel and be careful with the timing.
    good advice

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    What is full advance suppose to be at WOT? Is the timing set at 5500 like the 150hp Johnson? This is a two cylinder engine.

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    Quote Originally Posted by rickriddle View Post
    What is full advance suppose to be at WOT? Is the timing set at 5500 like the 150hp Johnson? This is a two cylinder engine.
    I can't recall that, I've had more experience with the mid-size twin (18 to 35), maybe PM instigator, I know he had one for quite awhile.
    > Stainless steel Merc cowling plates - $110 shipped TYD - LINK <

    1979 16' Action Marine/2.5L Merc S3000 - Metalflake Maniac
    1984 18' Contender Tunnel/2.4 Merc Bridgeport

    "Where does the love of God go, when the waves turn the minutes to hours?"

  10. #10
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    While you're trying a new/different battery, make sure all the connections are good and that the cables are good also. I've seen them corrode from the inside. I have a '72 50 and run nothing but premium and good oil in it. I don't use it much, but it's never been apart.

  11. #11
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    It was the battery. Put the new one in and it spins over good now. Seems like it not getting fuel though. I'll check the voltages with the DVA just to be sure.

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    You can also pull the starter armature and hit it with scotchbrite to help it hit harder cranking. Has the motor been sitting? If so, maybe swuirt some pb blaster in the bores to help lube it some. Lasltly, i agree that your gauge might be a bit ambitious. 150 seems high. I love those old motors!
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

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