compression 57 3 hp Johnson

Home Forum Ask A Member compression 57 3 hp Johnson

This topic contains 28 replies, has 11 voices, and was last updated by Avatar garry-in-tampa 1 week ago.

Viewing 15 posts - 1 through 15 (of 29 total)
  • Avatar
    scubachuck
    US Member - 2 Years
    Replies: 7
    Topics: 4
    #184797

    Top cy 40 psi
    Bottom cy 60 psi
    took head off bore looks good
    took exhaust plate off rings don’t look stuck

    Avatar
    garry-in-tampa
    Lifetime Member
    Replies: 3204
    Topics: 30
    #184799

    If the head is not warped, I would say some one has been using one hundred to one oil mix in it . . .

    Avatar
    scubachuck
    US Member - 2 Years
    Replies: 7
    Topics: 4
    #184807

    ok i’ll check the head and no to the 100 to one

    Avatar
    crosbyman
    Canada Member - 1 Year
    Replies: 1239
    Topics: 161
    #184808

    any problems at this stage…???.

    why not just run it for a while and throw in some Mercury power tune (follow directions)

    then retest compression after a few hours …who knows maybe #s will come up enough

    frankr
    frankr
    US Member - 1 Year
    Replies: 4471
    Topics: 43
    #184809

    40 psi, if accurate, is seriously sick.

    Avatar
    billw
    US Member - 2 Years
    Replies: 1039
    Topics: 37
    #184828

    60 is more like it, at least run-able. Why not try a new head gasket? Maybe the old one was leaking. If not, then she has to come apart, which isn’t a huge deal.

    Avatar
    fleetwin
    US Member - 2 Years
    Replies: 2753
    Topics: 33
    #184829

    Don’t be afraid to “deglaze” the cylinder walls using some fine emery cloth, this might help the rings seal and bring up the compression….These engines don’t have much compression, so 60 PSI isn’t that far out of the expected range. These engines have such small combustion chambers, compression readings tend to be inaccurate. You have the head off, no signs of scuffing/scoring, no signs of stuck rings/scuffed pistons with the exhaust cover off….Be sure the head/block surfaces are flat before installing the new head gasket…

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 4200
    Topics: 425
    #184852

    Maybe try spinning it over with a drill at a steady cranking speed and see what the numbers are.

    Avatar
    crosbyman
    Canada Member - 1 Year
    Replies: 1239
    Topics: 161
    #184853

    start simple and cheap…. run it with decarb and fresh gas/oil for an hour or two and report back ….the compression #s

    Avatar
    aquasonic
    US Member - 2 Years
    Replies: 343
    Topics: 31
    #184902

    With 40 PSI compression in one of the cylinders, I’m not sure if the motor will even run. My first thought is a blown cylinder head gasket. Is there any sign of a breach in the old gasket? Is there any sign of overheating such as burnt/discolored paint on the cylinder head.

    A new head gasket is fairly inexpensive for this model, and easy to replace.. If you decide to replace it, then there is a very easy lapping procedure that will give a fresh, new, and flat mating surface. I make it a habit to always lap the cylinder head when installing a new gasket. Every one that I have ever lapped is slightly warped in a concave manner in the direction of the block. I’m hoping this is your solution.

    Let us know what you decide to do.

    Avatar
    scubachuck
    US Member - 2 Years
    Replies: 7
    Topics: 4
    #184931

    ok i’m going to try a new head gasket.
    how is your easy way to lap the cylinder head?
    Thanks Chuck

    Avatar
    crosbyman
    Canada Member - 1 Year
    Replies: 1239
    Topics: 161
    #184937

    circle 8 the head on 150 sand paper over a piece of glass or any dead flat surface … (ceramic tile) circle 8 till all sections of the head a shiny

    remove all crud before starting check powerhead with a straight edge

    • This reply was modified 1 week, 4 days ago by Avatar crosbyman.
    Avatar
    outbdnut2
    US Member - 2 Years
    Replies: 1155
    Topics: 57
    #185114

    If your compression gauge has a rubber hose on it, and the checkvalve is at the gauge end of the hose, the reading will be low due to the volume of the hose, but the difference you see between cylinders indicates a problem with the one reading 40 even if the gauge is reading low.

    Avatar
    billw
    US Member - 2 Years
    Replies: 1039
    Topics: 37
    #185129

    I’ve heard this many times about the long hose and I want to start a conversation about it. To me, it doesn’t matter if the hose is 100 feet long. You keep pulling the engine over until the gauge doesn’t go any higher, and that’s the compression. The check valves in the compression gauge basically make the engine into an air compressor and it will KEEP compressing air in the hose until the cylinder reaches it’s max. Am I not right? Not to be a nit picker but I have heard this so many times and I don’t get it.

    • This reply was modified 1 week, 2 days ago by Avatar billw.
    Avatar
    outbdnut2
    US Member - 2 Years
    Replies: 1155
    Topics: 57
    #185137

    You are right if the check-valve is at the engine end of the hose, then the hose will build up compression pounds with each stroke. If the checkvalve is at the gauge end of the hose, the compression in the hose is lost each time the piston backs up. I have a gauge like that and it gives me about 20 pounds less on a 3 HP Johnson. The amount or error will vary from gauge to gauge depending on the volume of the hose. On bigger cylinders, the volume of the hose becomes less significant.
    Dave

Viewing 15 posts - 1 through 15 (of 29 total)

You must be logged in to reply to this topic.