Compression, whats acceptable?

Discussion in 'Bubba's Outboards' started by tbull, Mar 2, 2009.

  1. tbull

    tbull New Member

    Messages:
    3,318
    State:
    SW Ohio
    Just wanted to get an idea from some of you guys on here. I have always heard that you do not want anything more than 15-20 psi difference between cylinders on a compression check. Now to me that indicates some type of problem, given it may not mean its time for a trip to the machine shop, my motor has always had lower compression in 1 cylinder since I have had it, I know there are grooves in the cylinder, but they dont run the entire length of the bore, and the motors still does run like a scalded ape!

    What I am getting at here is I am looking to purchase a bigger motor, and had a mechanic go over it today, since it was already down there. The compression on this motor was perfect 110,110,110,110,112,112. But what threw me is he said anything over 80psi was acceptable. I always thought you were gauging the difference between the same set of cylinders, and not basing it off of a certain psi range. what do you guys think? Especially you Bubbakat?:0a31:
     
  2. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Anything over 80 within 10 to 15% of each other will run but here is the kicker. at 80 psi it will run and run smooth but is an indication that the engine is worn out evenly and therefore will be hard to start.
    Now the numbers you gave up above would tickle the heck out of me if I was buying that motor. The first thing I would do is decarb the heck out of it and then install new plugs and run her like I stole her.
     

  3. roadkill636

    roadkill636 New Member

    Messages:
    551
    State:
    warrenton misso
    thoes #s you posted are normol for a omc looper ,and a bit low for a merc,very good for a crossflow