Can' get WOT on Johnson 25 HP

My Dad's 2000 Johnson 25 HP (J25RSSE) will not get full throttle when on water.

It would not start so I began with new gas, seafoam, cleaned out all fuel lines, cleaned fuel pump, rebuilt the carb, (twice) (sprayed with carb cleaner, soaked, and blew out with air hose.) The carb was filled with junk, so I this would be an easy fix. Also checked compression, OK.

Before servicing, i was barely able to get it started, and once it was I did, it was coughing, sputtering, and running very rough.
After servicing, and adjusting, the sputtering was gone and it idled smooth. I am able to get it to idle up from idle arm, but when in water and give it throttle I can't get any speed out of it.

This seems to be some kind of fuel restriction problem to me but was wondering if anybody has any ideas on what the problem may be, or what i may have overlooked.

Thanks for any help,

Was item #22 the high speed jet, removed from the carburetor bowl for cleaning ?

Yes, the high speed jet, and whole carb was thoroughly cleaned, soaked, and blown with high pressure hose. Just checked plug cable, got fire, and changed new spark plugs. Thanks

What happens when you open the throttle? Does the engine want to die out and shut down OR does it simply run slowly?

Yes i cleaned the high speed jet and carb thoroughly, Thanks

when i put in forward and open throttle, it does not gain much speed, simply runs slow. If I manually throttle it from carb, it revs up. Thanks

shannon123 said:

when i put in forward and open throttle, it does not gain much speed, simply runs slow. If I manually throttle it from carb, it revs up. Thanks

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I believe what you're saying here is that the engine hits a high rpm if you manually open the throttle butterfly? When you do this (engine in water, running, in gear), does the engine function as it should?

If so, and if you're using remote controls (throttle & shift cables), it's possible that the throttle cable is not adjusted properly. And if tiller operated, the engine linkage is improperly adjusted.

Yes that is correct, it does hit high rpm in neutral only when manually opening the throttle butterfly or idle lever arm... checked cables, linkage, etc. Don't think that's the problem because i bypassed that and maxed it to wide open with throttle butterfly by hand in neutral. But when i manually open throttle butterefly in gear, no speed. Seems to be some type of fuel restriction when in gear. Thanks Joe

I don't know if this would be the cause of your problem but I had a similar issue a few years ago and finally discovered that the fuel line from the tank was partially collapsed inside and restricting the fuel flow. Those hoses are made of a number of layers of various materials but the innermost one that carries the gas is actually an extruded thermoplastic/rubber compound tube. Gasoline always eventually wins the chemical battle with rubber and, even though the hose looks good on the outside, the tube may have separated and collapsed. Good luck and Happy New Year.

shannon123 said:

It does hit high rpm in neutral only when manually opening the throttle butterfly or idle lever arm. But when i manually open throttle butterefly in gear, no speed. Seems to be some type of fuel restriction when in gear. Thanks Joe

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This is a new piece of information, In Neutral Only. BTW... running the engine at a high rpm in neutral is inviting a runaway engine (rpm increase steadily and deisels) and by the time you figure out how to shut it down, connecting rods are flying out the side of the crankcase.

This indicates that the engine is more than likely not running on all cylinders. Check the compression first (have all plugs removed), then with the s/plugs still removed, check the spark which should jump a 7/16" gap with a strong blue lightning like flame... a real SNAP! Note that the 7/16" gap is important.

If you have good compression and spark, the usual cause of that problem is that the high speed jet, located in the center bottom portion of the carburetor's float chamber is somewhat clogged. I've found it better to clean that jet with a piece of single strand steel wire as solvent just doesn't do the job properly.

Since I retired in 1991, my knowledge of that engine's design is somewhat lacking... how many cylinders and carburetors? I suspect it is the normal two cylinder, one carburetor engine but I know there are some three cylinder models out there.

The model # indicates that it is the tried and true 2 cylinder machine.------Is the rod from towershaft to the carburetor in place ?--------Is it opening the carburetor the full amount ?-----Not much is proven by winding a motor up in nuetral.--That motor will do that running with just one cylinder working.----Did you look at the reed valves when carburetor was off ?

Thanks Joe. It has 2 cylinders, and 1 carburetor. I checked compression, at about 100 in each. Put in new Champion plugs (QL77JC4), and checked spark, all seems to be fine. I did clean the carb thoroughly...(twice).

Thought I may have missed a lick when it didn't plane out, so i pulled the carb out and checked it and re cleaned.

That's why this is so frustrating, just wondering what experienced people like yourself think.

Thanks Racerone,
The carb is opening the full amount. Not sure what the reed valves are, but will research. If the reed valves are within the carb, those babies are clean.

Yes, I was considering replacing all hoses, possibly a vacuum leak... I checked fuel flow through all hoses involved in the fuel system, and put a high pressure hosr to blow them out. Fuel flow seems to be fine..Thanks

Reed valves are not in the carburetor !------They are one way check valves found in the intake manifold.