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  1. #1

    Default 25hp EFI merc, No compression?

    I have a 2006 merc 25hp 4/stoke EFI ( OR107138 )

    This engine will not run or even pop. i checked compression with throttle wide open and can only get about 10psi on all 3 cylinders. what could be causing this? i know this engine has decompression valves built on the cam but i should get more compression than 10psi right?

    I did put new rings thinking maybe it was that. (this engine does have a lot of time on it) but the compression did not change. im stumped, and dont know what to try next.. any help would be greatly appreciated.

  2. #2

    Default Re: 25hp EFI merc, No compression?

    did you check the head? make sure the valves are seating, cam gear is aligned properly

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,554

    Default Re: 25hp EFI merc, No compression?

    That's my guess as well. A valve must be open at the wrong time for it to do that.

    Jeff

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    7,163

    Default Re: 25hp EFI merc, No compression?

    Or the seats have eroded due to "(this engine does have a lot of time on it)".....

  5. #5
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    14,944

    Default Re: 25hp EFI merc, No compression?

    Have you ever had this motor running properly ??-----Bought it to fix it up ?--=---Checked for bent valves ?

  6. #6

    Default Re: 25hp EFI merc, No compression?

    Sorry i should have said that i did have the head and valves check by a machine shop, and everything checked out good. I double and triple checked the timing marks on the crank and timing gear. i thought timing must be off too. but the marks where right. the flywheel key and cam gear key was good. thats why this makes no sense..

    I did a leak down test and each cylinder holds pressure, but at about 40-45 degrees BTDC the exhaust valve opens and releases it. I know it seams the valves are opening way to early (like the timing is off) but i timed this as the book says. aligned the mark on the crank to the mark on the block, #1 at TDC and then the #1 mark on the cam gear to the the mark on the block.

    Am i doing something wrong?
    Could the camshaft be twisted?
    When and how does the compression relief system work?

  7. #7
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    14,944

    Default Re: 25hp EFI merc, No compression?

    There is no way that the engine ran till it " wore out " to having 10 psi compression !-----So something happened to it to change timing I would say.------Sounds like you bought someone else's problem.

  8. #8
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,554

    Default Re: 25hp EFI merc, No compression?

    Okay. Here's what I'm seeing: The timing marks (or how you're looking at them) is off.

    Here's what to do; You need to find what's called the 'split overlap' position of the valves. Put # 1 piston at TDC and look at the valves. If you're on the right stroke, BOTH intake and exhaust valves should be just a tiny bit open. (If one of them is way open, rotate one revolution and try again.)

    When you're on the right revolution, move the flywheel back an forth while watching the valves until you get them both slightly open. Then look where the TDC mark on the flywheel and pointer line up: you should be VERY close to TDC. If you're off a bunch, then that's the problem.

    Jeff.

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