Please note this thread has been inactive for 90 days. For the best results, please start a new thread.
Results 1 to 20 of 20
  1. #1

    Default GT johnson 150 losing power

    i have a procraft bass boat with a Johnson GT 150. it runs good for a couple of minutes at plane then power drops to idle. i can play with the throttle and it sometimes acts like it wants to have the power. a couple of times it would clear out doing that. it would act up for about a minute or two then power back to wot. i have no idea what it is but i believe it is a fuel problem. the guy i bought it from rebuilt the carbs, but don't know if they were synchronized. please help its my first boat.

  2. #2
    Admiral
    Join Date
    Feb 2008
    Location
    Canton Ma, South of Boston
    Posts
    7,155

    Default Re: GT johnson 150 losing power

    how about the yr and model number,

    its strongly suggested to get the johnson factory manual so you know whats what under the hood.
    www.outboardbooks.com

    Don't use it until its fixed, you can melt a piston very quickly like this.
    New to you, I would go through it.

    get some simple tools,
    a timing light,
    spark GAP checker from an auto parts store,
    compression guage.

    Take compression readings, all plugs out.
    Set spark gap checker to 7/16th inch gap and test each hole, plugs out.
    Change water pump.
    Re-test the engine, monitor very closely for overheating with an infrared termometer.

    Depending on the yr, it sounds like its overheating and going into SLOW mode. Need to know yr of motor to comment further.


    Also ,change the gear oil using new seals on the drain screws.
    Examine gear lube for milky appearance, report back.
    Remove prop, check shaft for fishing line, lube and reinstall prop.

  3. #3

    Default Re: GT johnson 150 losing power

    the boat is a 1987 and its the original motor. i pulled the drain screws out the carbs and the fuel had water in it. could this be the problem? i dont think its running hot cause it will do it when i first fire it up.

  4. #4
    Fleet Admiral
    Join Date
    Sep 2008
    Location
    North Central Iowa
    Posts
    8,689

    Default Re: GT johnson 150 losing power

    Water in the fuel system can cause problems. You should flush the entire system and see if it helps.

  5. #5
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    An engine will typically run/idle with a little water in the fuel, but it will have no power when under load. Pull the fuel hose off the side of the pump and fill a clear glass with a fuel sample-about 4" tall. Let is settle for a few minutes. If there is any water in the fuel, purge the tank, fuel lines and drain the carbs. Fill with new fuel.

  6. #6
    Admiral
    Join Date
    Feb 2008
    Location
    Canton Ma, South of Boston
    Posts
    7,155

    Default Re: GT johnson 150 losing power

    wow yeh thats the problem, get a water seperator, its today's fuel causing that unless you have a leaky fill hose fitting.

  7. #7

    Exclamation Re: GT johnson 150 losing power

    what is the best way to drain the fuel tank? will seafoam work and help after i drain it? my tank vent has a screen at 3 o clock and at six. can the water be sipping throuh this while in water?

  8. #8
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    Pull the incoming fuel line off the side of the pump and let it dangle over the side of the boat into a 5 gal can. Make sure the top of the can is below the bottom of the tank in the boat. Prime the hose bulb and let it go. It will self-drain the whole tank.

  9. #9

    Default Re: GT johnson 150 losing power

    it stills does it! please help!!!!!!!!!

  10. #10
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    When the engine starts to act up, do you have spark to each plug? Check each plugwire with a timing light. You can see misfires or no fires in the flashes.

  11. #11

    Default Re: GT johnson 150 losing power

    i think it is going into s.l.o.w mode. does this 1987 gt 150 have it? how could i bypass it? its not running hot.

  12. #12
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    The later model XP/GT models with the 35 amp alternator had SLOW. Not sure if it was only on the models with single power pack or not. Your 87 has two packs. Anyway-on the models that have SLOW, both brown wires from the cylinder head temp switches signal the system of an overheat at 203 degrees and then the power pack limits rpm to 2500. You could disconnect both switches at the knife connector back by the switches to test run the engine.

  13. #13

    Default Re: GT johnson 150 losing power

    my motor has only one power pack one the right side from the rear.

  14. #14
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    If your engine has only one pack, it is at least an 1989 model. It is likley that your engine does have the SLOW system.

  15. #15

    Default Re: GT johnson 150 losing power

    if it helps the motor is a white gt 150v6

  16. #16
    Admiral
    Join Date
    Feb 2008
    Location
    Canton Ma, South of Boston
    Posts
    7,155

    Default Re: GT johnson 150 losing power

    Why don'tcha post the model number so we can figga out exactly what yr we're dealing with.

    Mine is white too.
    Better than those black uns.

  17. #17

    Default Re: GT johnson 150 losing power

    alright i pulled off a brown wire from the top of motor near head that goes into my power pack. their was a other one but it was black i think. should i snip this wire? it didnt have a connecter. is the one brown wire all i needed to pull apart?

  18. #18
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    I wouldn't cut any wires. There should be a brown wire going to the temp sender by each head. There is a black ruber sleeve/boot over the knife connection. Move the black rubber boot over the wire to expose the knife connection. Temporarily disconnect the two temp senders for testing purposes.

  19. #19

    Default Re: GT johnson 150 losing power

    i unplugged the brown wires and ran the boat yesterday and it helped alot. boat did loose power but only maybe 3 times but it beats the 10 to 15 before. it quickly gained back power. do you think i need a new power pack? i also changed the spark plugs.








    thin






























    k

  20. #20
    Fleet Admiral
    Join Date
    Dec 2005
    Location
    LaGrange, Illinois
    Posts
    8,396

    Default Re: GT johnson 150 losing power

    Not sure if you are fixing the right problem yet. It is possible the engine is overheating. It is odd that you don't get a warning horn..The tan wires may be sending the power pack a signal that the engine is overheating. You have disconnected the tan wires, so now the engine (powerpack) does not know if the engine is overheating and continues to run normally. Your current loss of power could be related to a powerhead overheat. Borrow or rent a laser temp gun and check the temps on both heads when running the engine. 140-155 is normal idle, cooler when on plane. If you determine it is not overheating, then go back to the ignition as the potential problem. The power pack is a likely failure item, but I'd check the stator output first, as a new power pack is expensive.

Similar Threads

  1. 115 Johnson Losing Power
    By AUtigers in forum Johnson & Evinrude Outboards
    Replies: 17
    Last Post: July 21st, 2007, 12:00 AM
  2. 55 HP Johnson losing power
    By james_shoemaker in forum Johnson & Evinrude Outboards
    Replies: 4
    Last Post: July 1st, 2006, 01:19 AM
  3. 50 HP JOHNSON LOSING POWER AT WOT
    By ssulkazi in forum Johnson & Evinrude Outboards
    Replies: 1
    Last Post: May 26th, 2003, 05:53 PM
  4. 85 johnson losing power
    By myvitaloca in forum Johnson & Evinrude Outboards
    Replies: 2
    Last Post: July 4th, 2002, 01:05 PM
  5. Losing power - 40 HP Johnson - Need Help!!
    By Haverb in forum Johnson & Evinrude Outboards
    Replies: 2
    Last Post: May 28th, 2001, 07:19 PM
  1. iboats Forum Directory - Over 100,000 forum posts organized by topic