Compression question

Discussion in 'LOCAL TENNESSEE TALK' started by TennesseeJugger, Dec 16, 2009.

  1. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    What should the compression be an a early 80's model 3 cyl. 70 HP johnson?
     
  2. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    hi there,
    compression on that johnson should be around 120 or above and within 10% of each other. New mine was around 140-150. If you had readings of around 120 to 140 i would think it is in good shape. You could run on 100 but i wouldnt go any less than that. You need no more than 10 percent difference on each cylinder. for example #1-140 #2 135 #3 130,which would be within 10 percent of each other. Sometimes you can have a head gasket that is leaking that causes those umbers to go a little wacky..hope this helps
     

  3. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn

    My # 1 is 100, #2 is 135 and #3 is 60. My brother in law and I are going to rebuild it, Dingger66 offerd to help but my bro in law is close to me and has rebuilt the same motor as mine before. So we are going to do it in my garage.
     
    Last edited: Dec 16, 2009
  4. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    seems like you got a problem going on there..first thing first.. i would suggest taking some oil and shoot in those weak cylinders 100 and 60 see if compression goes up if it does i would think rings.. if it really does no good take head off and see if gasket is screwed up.. not really hard and can save alot of money ..
     
  5. SCcatfishing

    SCcatfishing New Member

    Messages:
    889
    State:
    SC

    60 was on bottom cylc right? I have seen this alot of times on people trying to sell em and saying they where fine as they will still run on muffs but not well in the water..
     
  6. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    I was out in it and it started to bog down and then just shut off and would not start back up. When it did start up it would not run under load.
     
  7. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    That what mine would do, run great on muffs but not under load.
     
  8. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    it was running that way because of low compression.
     
  9. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    So do you think that it may be just a piston ring or reed valve or..?
























    ?
     
  10. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    if it was me i would shoot some oil in low cylinders see if readings come up if it does the rings are wore.. simple thing to kinda give you an idea of whats wrong..also its a long shot but ive seen it before. carbon build up can sometimes cause wacky readings. you could run some sea foam through it and check again. never know..
     
    Last edited: Dec 16, 2009
  11. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    I am a newbie here, how do I do that?
     
  12. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    just get you a squirt can and put some oil in the can.any oil will be ok.give it a good shot in those weak cylinders..then turn engine over by hand as to lubricate the cylinder walls good. then after that install guage and check again. if the rings are wore the oil will help the gap that would exist between the cylinder wall and rings thus uping the compression..
     
  13. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    So how do I get the oil into the cyl? Remove sparl plugs and pour in that way?
     
  14. catin38

    catin38 New Member

    Messages:
    272
    State:
    alabama
    yes remove plugs and give it a good shot of oil..turn maybe 2 revolutions by hand then squirt some more.. see what happens
     
  15. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    Thanks
     
  16. SCcatfishing

    SCcatfishing New Member

    Messages:
    889
    State:
    SC
    You prob need new rings but hey if your going to do all that work you should just bore and all new internals.... Alot of work for what 1 week of running or 1/2/3/4 years you never know... But if you do a full rebuild you should get 10 years out of it if you keep it up..... Or even faster a new/rebuilt power head bolt on and throw acc's on and you running in a day....:wink:
     
  17. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    What would a rebuilt power head cost?
     
  18. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Guys before tearing one down you need to find out what caused it and then fix both problems. They had a bulletin out on the three cylinders that noted that it had a cooling problem. Now guys this is where a pressure gage comes in handy. The water pump on those 3 cylinder wasn't designed bad and needed to be replaced and the tell tell hole needed to be moved to the top of the head. What was happening was the water was getting air bubbles in it and not cooling it like it should. The cylinders are running at top performance the will become elongated and if it is running a few degrees hotter it will cause some of the cylinder walls to heat more then the rest of the wall.
    If you pull the head you will most likely find the wall on the lowest cylinder wrinkled or waved where it didn't cool down proportionately like it was suppose to.


    I'll get you that info you needed tomorrow.
     
  19. SCcatfishing

    SCcatfishing New Member

    Messages:
    889
    State:
    SC

    I'm going to pick up 2 that hasn't been rebuilt from a marina in up state sc for 100 each that has all acc on them including the carbs... But I will call him in the am to check on some other pats and I will ask... I think he has one in the 900 range which is cheap and also a used good one for I think a few hundred bucks... Pm me a contact number and I will call you some time in the pm..
    As I will be on a 30+ hour workday start at 6am in the morn...
     
  20. TennesseeJugger

    TennesseeJugger New Member

    Messages:
    2,029
    State:
    Watertown, Tenn
    Well I think there were two things that may have caused it, one the water pump was spitting alot and the full flow and then spitting. two my fuel bulb was not staying firm and maybe not enough gas and oil mixture to lube motor.