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40 hp johnson won't idle when tank is less than half full.

S

sgfrye

New member
any thoughts would be greatly appreciated.

couple months ago my wife and i bought a pontoon with a 1998 johnson 40hp

we have had it out on the lake 5 times now.

it runs great from idle thru wide open throttle when the gas tank is full down to about a half tank.

after the tank is half full or less it runs great above idle but does not idle good, keeps cutting off then is hard to restart.

here is what i know about the engine's history

2 months ago the carbs were rebuilt and adjusted
new plugs
new fuel pump
new fuel line and primer bulb
new fuel filter

tank is 12 gallons, vent is open
fresh gas
seafoam was added to my last tank of gas.

idle speed seems ok

many thanks
 
That is a first! Been in this line of work as a serious minded technician since 1960 and never encountered a problem like that. I realize that this rig is new to you... BUT... if you possibly have this knowledge...... Prior to the carburetors being rebuilt... did the engine idle normally with a full tank?
 
joereeves said:
That is a first! Been in this line of work as a serious minded technician since 1960 and never encountered a problem like that. I realize that this rig is new to you... BUT... if you possibly have this knowledge...... Prior to the carburetors being rebuilt... did the engine idle normally with a full tank?
Click to expand...

thats the 1 variable im trying to verify. the fellow i bought the boat from bought it from his nephew who needed money for medical bills. he only had it a couple months. he owned a ski boat. not a pontoon person.

the work done was what i was told by him and what was on receipt from boat mechanic. in march

he took the boat out and ran it fine before he put it up for sale and had no problems

however, i do remember there being 3/4 full tank when i bought it. so he may have only run 1/4 tank out

along the way i thought i might be doing something wrong in a simple manner.
one thing i did wrong was only have the vent screw for gas tank partially open on the first 2 trips out.

gas tank is plastic
but last trips vent was fully open
i was wondering if there might be a small pinhole in tank that doesnt cause a problem till fuel gets lower?

ive had 2 other boats in past, 30hp yamaha, and 88hp evinrude. yamaha never missed a beat, evinrude was a cold natured engine but ran great after warming up

this is a new one on me too


i know the issue with back pressure on water hose muffs, but even when tank is less than half full, engine idles fine on the muffs
 
Try a different tank for no other reason then to verify it is a tank problem. Frankly your explanation pinpoints a tank problem!

A pin hole in the tank fuel pickup tube would cause that problem. <--- Encountered this one often.

Next trip... if you haven't already tried it, loosen the fuel cap to create an additional vent.
 
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