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2002 40hp Johnson low power at WOT

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njko14

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I could use some help identifying a problem with my 2002 Johnson 40hp. (J40PLSNF).

Background: The engine is on a 16ft fiberglass center console, not underpowered for the boat but definitely at the low end of HP for it.

The boat will usually start fine, sometimes I need to rev the engine to get it running if it won't start right away.. Performs well at low speeds, but when i bring it to WOT the engine stays at around 3000rpm and will not plane or get up to top speed. No excess smoke or anything, just sounds like the engine is stuck in between gears almost. In the past, when the boat has performed properly it will do 18-20kt in glassy conditions, when it won't reach top speed at WOT it is topping out at 9kt. I only get to use the boat 5-10 times per season and this low power situation happens about 90% of the time.

Occasionally, but very rarely, when the boat is performing poorly like this after a little while at the 9kt WOT, the engine will start to rev up on its own and reach top speed, i can't seem to replicate this performance on command however!

I feel like it could be a possible fuel problem, so i have replaced fuel tank, Racor fuel filter/water separator, fuel line and connectors leading up the engine with no luck. I replaced the VRO last season and have brand new spark plugs at the beginning of this season. This week I put a fresh tank of gas mixed with 1pint SeaFoam and ran it for 30 minutes with no results. There is an inline small white plastic fuel filter inside the engine which i have not replaced but didn't know if that was necessary or not.

Not sure where to turn to now, any advice would be greatly appreciated.

Thanks!
 
Pull that white filter& blow threw it, replace as necessary. Have u tried squeezing primer bulb while running, If issue improves,possible fuel restriction or faulty pump. Has entire fuel line been replaced? Primer bulb firming up on startup? Compression test ,followed by spark test,big blue lightning snap!!
 
As "Boobie" suggests... do that drop test. Any rpm, pull a plug wire off and replace, one by one to see if the response is the same on all cylinders. If possible, do the same at the rpm where the problem exists... use insulated pliers please (we prefer you stay with the boat) :)
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Shop work: Remove all spark plugs. Do a compression test as "johnny" suggests... What are the psi compression readings of the individual cylinders.

Also as "johnny" suggests, with the spark plugs still removed, rig up a spark tester with a 7/16" gap.... the spark at cranking speed should jump that gap with a strong blue lightning like flame... a real SNAP! Does it?

The timer base under the flywheel advances the spark as you increase the throttle. Check to make sure that the timer base advances smoothly to its full advance spark position without sticking at any point. DO THIS TEST with the engine NOT RUNNING and in gear. Spin the propeller with your foot to allow the engine to be put into forward gear (aligning the shifter lobes between the shifter dog and the gears).

Also with the above test, check to see that the throttle butterflies just start to open when the scribe mark on the cam is aligned dead center with the center of the throttle roller... AND... at full throttle, the butterflies are fully horizontally and set at the full throttle position.
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If you or someone else haven't done any work to the engine between the last time it ran normally and when this problem popped up... AND... all of the above is as it should be, I'd suspect that the carburetors are somewhat fouled, and if so, clean and rebuild them as needed. Manually (and carefully) clean the high speed jet that is located horizontally in the bottom center portion of the float chamber, way behind that drain screw plug as solvent just doesn't do that job properly.

Let us know what you find.
 
Well i think i found the problem! Still need to do a lake test to make sure but have run it on muffs, started right up, ran strong and sounds 100% better than it has in a long time.

I was getting ready to perform the cylinder drop test but decided to do the easy thing first and take off the air silencer to make sure the butterflys were opening at full throttle. Once i got the air silencer off i discovered a severed supply line into the bottom carb (pics attached). I'm not sure if this was already completely severed or if getting in there a little bit made it come fully off, but i was able remove the nipple, make a clean cut in the supply line and put it back into the carb. Does anyone know exactly what this supply line does? i know its fuel but not sure if its the main line or not.

Ill update after the lake test to make sure that was 100% the problem. Thanks for everyones advice.

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Ok so i took it out today for a lake test after fixing the choke/enricher - not the problem. Boat did the same thing it always does, starts up fine but won't open up.

After trying a couple different things I unplugged the fuel line from the tank while the motor was running to let the motor die out. When i plugged the fuel line back in and started up the boat, it got up to top speed in seconds and was running perfect. I tried this three separate times and got the same result each time.

The only problem was that when i was in WOT and the boat was in top speed and i would pull back on the throttle to slow it down, then try to get it back up to top speed it wouldn't get back up to top speed, it would rev and bog down. So the only way for the boat to get up to top speed is to pull the fuel line, let it die, plug it back in and run it on my first throttle out.

I'm completely lost now.
 
When bogging, press in on key to activate choke/enricher. I bet it takes off.
Feather the key presses in/out/in/out to find that perfect spot where it goes.

If so, dirty carbs, specifically the idle and midrange fuel circuits.
 
Remove carbs, disassemble,soak, clean every nook & cranny,passageways,circuits,high,mid low,idle thoroughly,rinse with high pressure water,followed with high pressure dry air. More than likely fuel pump is gummed up also. Might want to clean it thoroughly!
 
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