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1971 25Hp Johnson complete power loss after warm up

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kevin_stinton

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"I have an old 25R71S outboard

"I have an old 25R71S outboard which starts well and has plenty of power when run from cold, but starts to loose power after 5 to 10 minutes at WOT.
It gets slower and lumpier until finally slowing down to a complete stop.

I have installed new Carb, fuel pump and water pump kits on it, plus cleaned out all of the cooling system thoroughly.
There is plenty of water coming out of the exhaust port on the back of the gear case, but no ‘Telltale’ at idle.
I don’t know if this model is supposed to be able to do this?

I am suspecting an electrical fault, say maybe the coil is failing when it gets warm.
Has anyone seen something like this before on a similar model?"
 
"Kevin, that is not an "un

"Kevin, that is not an "uncommon" problem.

If there is a break in the coil windings the heat can cause the copper to expand and the winding can lose continuity.

You can check a couple of different ways. Either wait until it cuts out and put a timing light on her and see if the spark is gone.

Alternately, you can disconnect the ignition coil leads and test them with an ohm meter. You should have minimal resistance (near zero) when cold indicating a complete circuit - then take a blow dryer to the flywheel and start heating it up.

If the resistance increases significantly then you definately have a problem with a winding..."
 
"Thanks for the advice Graham.

"Thanks for the advice Graham.
I have just tried the blow dryer method, but there was no change to the resistance of the coils, even when very hot
I will run the motor again until it stops and then do the spark test on both plugs, just to make sure I’m not looking into the wrong area.
Are there any other causes of power loss similar to this, but not electrical?
The symptoms are pretty consistent, always runs well from cold and only looses power after between five to ten minutes.
I am sure that the cooling system is ok and have even removed the thermostat to make sure that there is no chance of overheating."
 
"A clog somewhere in the fuel

"A clog somewhere in the fuel system could produce the same symptoms.

A blocked vent on the tank, clog in the line or filter etc.

Eventually the fuel pump would not be able to suck enough gas if no air was getting into the tank vent etc...

Worth taking a look..."
 
"carefully squirt a gas/oil mi

"carefully squirt a gas/oil mixture into the carburetor when the problem occurs....also try pumping the primer bulb..if either of these techniques makes a difference, you then know you have a fuel related problem"
 
"Yes, the fuel supply was my f

"Yes, the fuel supply was my first suspect on my list.
There is always plenty of fuel in the carb bowl (I drained it to check each time) and I have already tried pumping the primer bulb on the last two outings.

I am beginning to suspect that there is uneven compression between the two cylinders, which I have heard causes problems.
One of the cycles seems to pull through more easily than the other, so I will get the motor compression tested at the weekend to be sure.
I’m not sure how this could only cause a problem after ten minutes though?"
 
Fuel tank venting?----Impeller in good shape?----Thermostat installed and working ?----Perhaps test with another tank and hose.----Coils checked for cracks ?
 
Update: Problem solved! It's an old worn out impeller causing it to overheat! Buy a new impeller they're cheap and will fix the overheating problem unless you have a more major issue going on! Try flipping the impeller and directing the impeller arms the opposite direction of the way they're curved and that will tell you if that's the problem! CAUTION - Only do this for quick test purposes to make sure water is pumping through properly!!!
 
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