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compression test on small 4 cycles?

Discussion in 'Mechanic and Repair' started by themadcutter, Sep 7, 2011.

  1. themadcutter

    themadcutter LawnSite Senior Member
    from florida
    Messages: 914

    I have done very little work on 4 cycle engines except on the carbs.

    Do all modern 5 to 7 hp engines like brigs and intek) have compression release that would cause a low reading?
  2. BigFish

    BigFish LawnSite Platinum Member
    Messages: 4,153

    Pretty much all of em have some sort of compression release mechanism.
    You can get a more accurate assessment of condition by using a cyl. leakdown tester. Or just by pumping a little air in the spark plug hole and listening for any major leakage at the carb, muffler, oil fill etc. Just make sure you have the piston at about 1/4 " past TDC.
  3. themadcutter

    themadcutter LawnSite Senior Member
    from florida
    Messages: 914

    So if the air is passing through fast and easy then bad rings/ cylinder.

    But if it just squeaks through then the rings are tight.

    is that correct?

    And I have read on the net that you can do a better compression test by turning the motor backwards but I don't think its true.
    What do you think of that?
  4. unkownfl

    unkownfl LawnSite Gold Member
    Messages: 3,837

    Take your rockers off if your going to blow air in to make sure your valves are closed. Also, make sure the Schrader valve is out if your using your compression tester hose. If air is coming out the carb or exhaust its a valve issue. If its coming out your pcv or oil tube its rings/piston/block. It could very well be your head gasket if all those check out.

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