15 Johnson stalls when coming off plane

[islander9]

Hello folk's, I have a 1984 15 hp Johnson (new to me) that will stall if I suddenly slow down from being on plane. It will always start right up again and run properly afterwards. It will do it every time I come off plane if I slow down suddenly.

As a course of maintenance I have replaced my impeller and cleaned the carb and other than this irritating problem, she runs perfect.

I have confirmed that water isn't pooping over the back of the engine hood and there is no water inside the hood.
I'm thinking that somehow either water, or pressure from water is somehow moving up inside the exhaust housing and effecting the exhaust exiting the housing.

kinda stumped....any suggestions??

 

[jmendoza]

Re: 15 Johnson stalls when coming off plane

CRACK THE LOW SPEED MIXTURE NEEDLE OPEN 1/8TH OF A TURN COUNTER CLOCKWISE TO OPEN IT AND SEE WHAT HAPPENS, IT MAY BE JUST A BIT LEAN. The other thing it could be is the throttle is closing too far, check to see the throttle stop is properly set and allows the engine to maintain an idle when the throttle is closed.

Finally, old spark plugs can do this too, as can old fuel, both are easy and cheap to change, so don't overlook them.

 

[1stgenbird]

Re: 15 Johnson stalls when coming off plane

Could be the float in the carb is adjusted incorrectly.

 

[islander9]

Re: 15 Johnson stalls when coming off plane

Thanks for the tips gent's.
I tried opening the idle jet slightly to no avail, the engine idle is strong with the throttle all the way down (against the adjustable idle stop). When I installed the carb kit I adjusted the float level to the carb body.
I'll pick up a new set of plugs to see what happens.

In a effort to try and better describe my problem....you can hear the engine stall (stop firing) as soon as you suddenly turn the throttle to idle while on plane. If I turn the throttle back up a bit while it is coasting to a stop it will recover and not stall.

It never stalls if I come off plane slowly.

Other than this problem, it is one of the best running 15hp OMC engines I have been around.....I kinda wish I had other issues to use to try and diagnose the cause.

I'll try the plugs and post back.

Much appreciate the help

 

[islander9]

Re: 15 Johnson stalls when coming off plane

Just a follow up to say thanks for the tips and to let anyone else who may have this problem know what we found.

It turns out there is just enough slack in the throttle linkage that when the engine is suddenly decelerated to idle, the throttle plate is sucked closed and held closed by the volume of air going through the carburetor.
On a slow deceleration the throttle plate is just slightly supported by the linkage and can stay open enough to give the engine a bit of air and keep it running.

Thanks again for all the helpful info that led us to the conclusion.