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  1. #1

    Default Johnson 40 Idle Jet spits fuel

    1988 Johnson 40

    Getting fuel spitting back out the idle air jet. Does it at idle and also at WOT.

    Motor doesn’t run well at WOT. Used to get 32mph. Now only 20 and it struggles to get there.

    130psi in both cylinders. Less than 3% leak down in each cylinder.

    New Carson Reed valves
    All new ignition components from under the flywheel to the plugs

    Rebuilt both carbs but the bottom one is the one spitting and like I said it’s spitting out the idle jet.

    Just took that one apart again and cleaned again, running wire through the orifices. Removed the lead plug again and did the same thing. Lots of carb cleaner.

    At what point do you start thinking the carb itself is “bad?”

    Thanks

  2. #2
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    Default Re: Johnson 40 Idle Jet spits fuel

    Exhaust side of bottom piston scuffed
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3

    Default Re: Johnson 40 Idle Jet spits fuel

    Wow, that’s a very specific diagnosis. Previous experience?

    The motor did overheat last summer due to the headgasket failing where it directs the cooling water and a piece fell and blocked the water flow.

    What can I do to verify that?

  4. #4

    Default Re: Johnson 40 Idle Jet spits fuel

    Just to clarify, the fuel is not coming out of the carb throat, but the idle jet at the top of the carb.

    I’m going to swap the carbs tomorrow and see if the problem migrates or stays with the lower cylinder.

  5. #5
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    Default Re: Johnson 40 Idle Jet spits fuel

    Seen it several times only on looper motors
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  6. #6

    Default Re: Johnson 40 Idle Jet spits fuel

    And the fuel spitting out the idle jet is the give away?

  7. #7
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    Default Re: Johnson 40 Idle Jet spits fuel

    With some effort this should be easy to resolve.

  8. #8

    Default Re: Johnson 40 Idle Jet spits fuel

    Update:

    Swapped the carbs this morning and the problem stayed with the cylinder. So it’s not a carb issue. Yesterday I also swapped the reed assemblies so I ruled them out as a problem.

    Thanks for the comment racerone…I don’t mind effort…point me in the right direction and I’ll go there.

  9. #9
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    Default Re: Johnson 40 Idle Jet spits fuel

    Fast, where did you get your info about the exhaust side of the bottom piston from ????

  10. #10
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    Default Re: Johnson 40 Idle Jet spits fuel

    OMC service school (Morrow GA) and field reps. (Big Bird Dan)
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  11. #11

    Default Re: Johnson 40 Idle Jet spits fuel

    Quote Originally Posted by faztbullet View Post
    OMC service school (Morrow GA) and field reps. (Big Bird Dan)
    So, short of a rebuild is it safe to say this motor is “toast”?

  12. #12
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    Default Re: Johnson 40 Idle Jet spits fuel

    How did it run for you last season ?----Was it spitting before you started working / replacing reed valves ?----The oil recirculation passage way is confirmed to be clear ?----Did you use any sealer when you did reed valve work ??-----Inspection / rebuilding is far cheaper than a new motor.-----Don't forget the inspection of gearcase.----Those gearcases are not robust in my opinion !

  13. #13

    Default Re: Johnson 40 Idle Jet spits fuel

    Had it two years. First year it ran great. Last year the season started off with less power…lost like 5mph. Head gasket failed at the end of the summer (piece fell and blocked water flow) causing overheat. Noticed fuel spitting out of the carbs then (which I remember this year and looked at the reeds), but I don’t remember if it was out of the throat or the idle jets.

    I think it spit fuel the whole time I’ve owned it because the lower cowling always had unburnt mixed fuel splatter on it, and it would run down the front of the midsection.

    Oil recirculation passage way…not familiar with that on a two stroke. How do I check it?

    Inspection of gear case… with the reeds off I shined a light in there but couldn’t really see anything that looked “bad.” But then I’m not highly experienced so I could’ve easily overlooked something. I spun the flywheel while looking inside and didn’t see any wiggle or play, or loose particles, etc.

    Reed valves…I replaced the gasket, and per the manual didn’t use any sealer on the gasket when reinstalling.

  14. #14

    Default Re: Johnson 40 Idle Jet spits fuel

    Oh…interesting. Just did some reading on the recirculation system. Did not know about that.

    Would a choked line cause loss of power?

  15. #15

    Default Re: Johnson 40 Idle Jet spits fuel

    Also, how do I test this and verify it’s clear? Pull the line? Remove the check valve?

  16. #16
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    Default Re: Johnson 40 Idle Jet spits fuel

    Post #12 says it is a passageway not a line as on some motors..----If you did not use sealer it is unlikely that it is plugged.

  17. #17

    Default Re: Johnson 40 Idle Jet spits fuel

    Mentioned a line because there’s a small tube coming off the block going into the carb right where it connects to the intake manifold.

    So then, in your mind it’s still likely a scuffed piston?

  18. #18

    Default Re: Johnson 40 Idle Jet spits fuel

    Should say both carbs.

    If that were somehow choked off…would that explain loss of power?

  19. #19
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    Default Re: Johnson 40 Idle Jet spits fuel

    That line from airbox to intake manifold is not a recirculation line.----Scuffed piston ?----Sorry I can not see , heel or hear your motor.-----Folks will jump up and down around a boat and motor all day long.----And they are reluctant to act on advice.

  20. #20

    Default Re: Johnson 40 Idle Jet spits fuel

    Oh my bad. Someone else made the comment about a scuffed piston.

    The lines I’m talking about aren’t the one going to the air box. These connect to the top of the carbs right where they connect to the intake manifold.

  21. #21
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    Default Re: Johnson 40 Idle Jet spits fuel

    Those lines come from the electric primer valve used to start the motor.----Takes the place of choke flappers.----And again those lines have nothing to do with oil recirculation !!

  22. #22

    Default Re: Johnson 40 Idle Jet spits fuel

    Quote Originally Posted by racerone View Post
    Those lines come from the electric primer valve used to start the motor.----Takes the place of choke flappers.----And again those lines have nothing to do with oil recirculation !!
    Gotcha. Thanks for the input.

    Based on what I’ve described so far what would be your thought as to what’s going on?

    Im happy to follow any recommendations you’ve got because I’m certainly frustrated and beyond my own limited knowledge.

  23. #23
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    Default Re: Johnson 40 Idle Jet spits fuel

    Your location ?-----Yes you need to study a factory manual.-----Just a lot of guessing on the operation of this motor so far.

  24. #24

    Default Re: Johnson 40 Idle Jet spits fuel

    Central Illinois. Decatur.

    Got the service manual. Used it to test coils, power pack, etc. Spark wasn’t jumping 7/16”, etc.

    Used the troubleshooting chart to find the issue with the reeds. Was only getting 7-9mph until I changed the reeds and then got the 20mph I’m getting now.

    Beyond that I’m just using advice from searches I find on here or from a guy I’m in a group with on Facebook. Swapping the carbs around and the reed valve assemblies was my own idea to see if the problem was cylinder specific or one of those things. As I said the spitting out the idle jet is only on the bottom regardless of which carb is there.

    I’m moldable clay. No agenda other than just want to get this thing fixed or determine if it’s not worth fixing or whatever.

  25. #25
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    Default Re: Johnson 40 Idle Jet spits fuel

    Other than tearing down for inspection you might find a camera that adapts to your phone if head is small enough. Pull head lower top piston and snake thru exhaust port to looke at bottom piston.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  26. #26

    Default Re: Johnson 40 Idle Jet spits fuel

    Well, inspection is on hold until Monday. I decided to double check the ignition components. Followed service manual instructions using the DVA multimeter adapter I’ve got.

    250 volts coming off the stator
    Same multimeter settings tested voltage coming off the power pack (hooked up per the manual) and was getting 4.5 volts on one power pack I’ve got which is new out of the box, and 3.8 volts off the one I put on it new last summer. Per the service manual I should be getting over 200 volts.

    CDI brand. I have read of ignition components being faulty out of the box…but two in a row?

    Third time’s the charm? Got another unit arriving Monday. I’ll test it with my DVA and see what it reads. If I get the proper voltage then I’m hoping that was the issue all along. Although I guess it doesn’t hurt to rebuild the carbs…clean them several times…and my reed valves were definitely bad, so I’m glad I changed them. And my stator had melted in places so I’m not sad about replacing the flywheel guts.

    I’ll report back.

  27. #27
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    Default Re: Johnson 40 Idle Jet spits fuel

    Fast, is Big Bird Dan still around ? I used to work with him.

  28. #28
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    Default Re: Johnson 40 Idle Jet spits fuel

    Yep...Just talked to him couple days ago. He was at a outboard swap meet somewhere in Mississippi. One of the best OMC tech reps there was. Goes by T2stroke on the antique sites.
    Last edited by faztbullet; 06-12-2022 at 08:39 AM.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  29. #29

    Default Re: Johnson 40 Idle Jet spits fuel

    Just tested the new power pack using the service manual’s procedure for “running test”:

    Positive DVA lead clipped onto exposed primary lead on the coil.
    Negative DVA lead clipped onto clean ground.

    Meter set to 500v

    Reads 6.5v

    What the….?

    Same setting gets 250v coming off the stator

    This is a brand new out of the box CDI Electronics power pack. Is it possible to have back to back bad units?

  30. #30
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    Default Re: Johnson 40 Idle Jet spits fuel

    Yes, is possible !!

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