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  1. #1

    Default 1985 johnson 70hp runs WOT first 5 minutes then dies

    Hey all,

    Boat was running fine for 2 years now. Ran into this issue on the lake today.

    Went out, fired up no problem, went full throttle and was cruising. After 5 minutes the motor sounded like it was dropping a cylinder. I throttled down and was able to maintain about half speed just fine. I nursed it back in to safety.

    On the way back in, I noticed middle cylinder did not have spark. (pulled wire quickly to check to see if any rpm dropped on all cylinders). The cylinder would get spark after I let the motor sit for a little bit. I had new coils and spark plugs sitting around so changed those.

    I launched the next morning and ZOOM full speed, thought i fixed the problem. 5 minutes later, same thing. Felt like it dropped a cylinder, it slowly idled itself down and would not be able to have power past a slow troll. I checked spark again and spark was there on all 3 cylinders. Possible intermittent spark problem?

    I ran the boat and squeezed the ball throughout, did not fix the problem.

    I ran the boat with the air cover off, i did notice A LARGE amount of fuel pumping out of the top of the top carburetor. Looks like it was from the idle orifice?

    I need to run compression and spark test when I have the time this week. Anything based on the above to look for/start with? Possible carb rebuild? Powerpack? Stator?

    Any and all help is greatly appreciated.

  2. #2
    Join Date
    Sep 2010
    Posts
    18,012

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Float / float valve on the top carburetor needs to be looked at.

  3. #3

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Quote Originally Posted by racerone View Post
    Float / float valve on the top carburetor needs to be looked at.
    Hi racerone,

    Thank you for the help.

    I pulled the top carb. Clearly it was not setting properly. see image link below. I guess it would be best to just rebuild all 3 carbs? What would you suggest next steps?

    https://ibb.co/rKB31Vp

  4. #4

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    problem is still here.

    Ok so I rebuilt the carbs and tested today. Same issue. Launched the boat. Took off right away and was cruising for about a minute. Then heard it starting to what sounded like loosing power/dropping a cylinder. Tested all spark again on all 3 cylinder and we were good.

    if I let it sit for about 15 minutes I will be able to go WOT again for about 30seconds. Then I lose power and can just idle. If I go wide open throttle it just sounds bogged out and does not have any power at all.

    what would be next to trouble shoot.

    thanks!

  5. #5
    Join Date
    Sep 2010
    Posts
    18,012

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Well, ----What did you find in the top carburetor that was leaking ?-----Find / corrected anything or not sure ?-----Checked the fuel pump ?----Checked fuel tank and hose ?----Checked the water pump ?---Is motor overheating perhaps ?

  6. #6

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    I rebuilt the carbs and made sure everything was clear of leaks on all lines. Made sure carbs opened horizontal at WOT. When I ran the boat no problems on that end. Pumped the bulb while WOT to be sure and nothing. Tapped in the choke and nothing.

    I had a spare fuel pump and threw that on when I first had the problems. It did not solve the problem. Is there something should adjust on that? I do not have a Vro pump.

    motor is not beeping and the tell tale is strong. I can touch the head with my hand without problems. Something to check there?

    Is the water pump the impeller? That was changed last season.

    Motor would die immediately when I pulled the middle cylinder spark plug. N
    would not die in the others.

    I pulled spark plugs, replaced with new one as was going to do anyway. Nothing changed.


    Hope that was helpful.

  7. #7

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Ran compression -
    120/120/115

    the wire harness looked pretty shot. I took it apart and whoever had the boat before definitely jerry rigged it together. Wires were showing signs of wear and barely connected with wire connectors. I have a 1982 75 evinrude for parts so swapped out the Harness.

    i was tempted to swap out the power pack as well but it looks like the 1985 power pack has an extra wire from the stator (3wire plug) compared to the 2 wire plug on the 1982. Is that 3rd wire necessary? It looks like a kill switch wire.

    going to take the carbs apart again and due another thorough cleaning. They looked great the first time when I cleaned and rebuilt but want to make sure.

    im going to run it on the muffs later today to really observe the spark and make sure it’s strong and firing.

    is there anyway to check the power pack while running on muffs?

  8. #8

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Ok so update - solved then new problem arose

    replaced the power pack and problem was solved! Ran for about 2 hours no problem. Started great, WOT great, cruised great.

    Pulled in midday and tried to go back out and ran into problems again. When going WOT motor would bog out and die unless I hit the choke. I could nurse it to WOT but it was running 5mph less at top end. I suspect this is a carb issue? I just rebuilt the carbs so maybe I missed something or maybe bad gas? Any suggestions are greatly appreciated.

  9. #9
    Join Date
    Sep 2010
    Posts
    18,012

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Compression numbers should be higher.----Perhaps test with another gauge.----Or pull cylinder head to inspect.----Did you remove the brass , 5/8" long , high speed jet from the carburetor bowl for cleaning, yes or no ?

  10. #10

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Is the high speed jet the jet behind the drain plug? If so then yes I removed that and ran a thing wire thru it to clean.

  11. #11

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Quote Originally Posted by racerone View Post
    Compression numbers should be higher.----Perhaps test with another gauge.----Or pull cylinder head to inspect.----Did you remove the brass , 5/8" long , high speed jet from the carburetor bowl for cleaning, yes or no ?
    when I pull the jet do I need to count the rotations and place it back in on that or just screw back in until tight?

    i inspected the carb just on the outside and the bottom carb has some fuel on it in the outside of it on the port side. I’m trying to figure that out cause the other carbs are dry on the outside. I’m thinking possible gasket leak on the carb.

  12. #12
    Join Date
    Sep 2010
    Posts
    18,012

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    The jet goes in snug / tight.-----All the fuel goes through the calibrated hole in that jet.

  13. #13

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    Ok pulled off spark plugs. Top spark plug is milky. As mentioned above likely the head gasket?

    Will replace the head gasket and hopefully get out on monday. Anything else to check before getting out especially knowing that the top cylinder spark plug is milky?

    Thanks!

  14. #14

    Default Re: 1985 johnson 70hp runs WOT first 5 minutes then dies

    It looks like the head gasket is caked on the head pretty good. What is the best way to take this "blue" left over residue off?

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