25hp Johnson won't run at at full throttle

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Nov 30, 2009
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Warner Robins, GA
I have been working on an old jon boat for a while now. Had it on the water for the first time today. It will idle fine, but at full throttle it is only running about 30% of the rpm it should (guessing). Engine runs smooth, and doesn't smoke, just will not turn up to rpm. Linkage is good and it will rev up at idle. Just no power under load. Any ideas? I'm thinking I may have a stopped up 'jet' in the carb. Its a 1975 25hp Johnson.
 
Sounds like the main jet is clogged. I maintain a 76 6hp Johnson, and an 85 20 hp Johnson. When the main jet clogs, sounds exactly as you describe. Fine on idle or low throttle, nothing past that.
 
Thanks folks! Not sure on the compression, but considering how smooth it was running at low rpm I don't think I have any issues with that. I cleaned out the carb about 8 months ago, but it has been sitting with gas in it since then. I'll rebuild it again. Also it has the original (as far as I know) fuel pump on it. Could that cause any problems?
 
How much gas have you run through it since the 8 month layup??

This is a common problem on small outboards, but it clears itself up - maybe 90 percent of the time - by the time you run half a tank of fresh gas (a 6 gallon tank) through it.

If the carb was properly rebuilt, and has a good fuel filter the chances of it being a piece of dirt is quite small..... the chances of it being a thin layer of gummy gas is huge.

Running the carb dry in the fall seems to greatly reduce/eliminate this problem.
 
Before going through the expense and time of overhauling the carb again try running some OMC 2+4 fuel additive through it. That stuff works wonders on gummed up 2-stroke carbs. Have seen it bring many a 2-stroke OB, weedwhacker, lawn mower, snowblower, etc... back to life saving a carb overhaul.

The fact your OB actually starts and will stay running is a big plus. Add a good shot of it to the fuel tank( I would add a double dose of what it says on the bottle for normal use due to your problem )and then go out and run it on the water. Should only take approx 10 minutes, or if really plugged up maybe as much as 20-30 minutes, for it to clear out. As long as this is not a mechanical issue and is due to gummed up internals this stuff should clear it out.

Run it at idle and part throttle( to where it will go without issue )for say 5 minutes then start trying to rev it higher every minute or two. If no change go back to taking it easy for a few minutes. I usually see it slowly break the crud up as I go through a few attempts to rev up. I know it is getting close when instead of stalling or fading out it starts to run rough and sputter as I hit WOT. Once it starts to break up and clear out it isn't usually long before it smooths out. Then a good long sustained WOT run finishes the job. After that you should be good to go.

Use the 2+4 at every fuel up and it will help keep things cleaned out. A big tip to keeping those old Evinrude and Johnson 2-strokes running good period, and especially when stored for long periods of time, is to be sure and run them dry before pulling the boat. I always pulled the fuel line as I neared the ramp and ran the motor out of fuel( I had a mid 90's Evinrude 30HP - great motor ). Never had problems with it using that method plus the 2+4 at each fill up.

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well I rebuilt the carb again, and the high speed jet is 100% clear, float is set correctly, and everything seems to be as it should. I also put on a new fuel pump and headed to the lake today. No change whatsoever. It is like its governed. It does not bog down or smoke like it is getting to much fuel or air. It just does not turn up any rpm. The throttle plate is fully advancing and the butterfly on the carb is opening all the way. I'm at a loss honestly. Any thoughts on where to go next? Also pumping the bulb on the fuel line while running has no affect.
 
Originally Posted By: nwjones18
well I rebuilt the carb again, and the high speed jet is 100% clear, float is set correctly, and everything seems to be as it should. I also put on a new fuel pump and headed to the lake today. No change whatsoever. It is like its governed. It does not bog down or smoke like it is getting to much fuel or air. It just does not turn up any rpm. The throttle plate is fully advancing and the butterfly on the carb is opening all the way. I'm at a loss honestly. Any thoughts on where to go next? Also pumping the bulb on the fuel line while running has no affect.


How old is the fuel tank and line? Might have a leak in it somewhere and that can effect how the engine runs at idle or under load.
 
Went through the motor today and did some trouble shooting. I have 120 compression on both cylinders. Good strong spark on the top cylinder and nothing on the bottom cylinder. I swapped coils and that changed nothing, also checked wiring back to the ignition switch and it is all good. Only thing left is a problem with the points/condenser? I recently replaced them, so I suppose I could have done something wrong, or had a bad new part. I suspect a bad condenser, or bad ground. Thoughts?
 
Just going through and updating some of my old post. My problem was that the wire coming off of the condensor was shorted to a hold down clamp. Motor is now running like a champ. Thanks everyone!
 
-sorry, missed your post... -

think that you found where the problem is, good! Start by verifying all wires and contacts maybe using a mulitmeter.
Then go on and see that you assebled everything right.
Then check all components, points, cond, plug, switches etc.

I have had a bad plug wires on some small eninges lately and that can be a pain to find especially if it is just semi broken and works sometimes. Check the plug cable thoroughly. On/off switches can also be a pain. Condensors may also turn bad regardless of age.
 
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I had an 18 HP Evinrude years ago that had similar issues.

The Coil wire was slipping in and Out of the coil as it was throttled up (timing advances). The coil wire Keeper had fell off. You will have to remove the flywheel to check this. I found it by moving the throttle back and forth with the flywheel off . Couldn’t believe my eyes. I hope your is this simple to fix.
 
G'day
I just wanted to say THANKS, I was looking at a carby rebuild but found a simple earthing problem at a wire clamp thanks to your post.
 
Old thread here, but I was kind of surprised at all the dig into the carburetor suggestions.

A fouled (or non-sparking)spark plug has got to be the most common cause of issues by far with these older outboards. Besides being out of fuel, it's the FIRST thing you want to check. If one plug feels cooler than the other after some run time, that's your culprit.

#2 is a stuck/shot water pump impeller.

#3 is a bad coil(s).

Joel
 
Originally Posted By: JTK
Old thread here, but I was kind of surprised at all the dig into the carburetor suggestions.

A fouled (or non-sparking)spark plug has got to be the most common cause of issues by far with these older outboards. Besides being out of fuel, it's the FIRST thing you want to check. If one plug feels cooler than the other after some run time, that's your culprit.

#2 is a stuck/shot water pump impeller.

#3 is a bad coil(s).

Joel



E10 is causing a lot of problems hence the carb suggestions.
 
Another thought since this seems to be a new boat to you based on your statement "first time on the water". Check that the propellor and lower unit gear ratio are correct for that model. I once had a boat that could not get up to proper RPM. After much $ and head scratching, I discovered the wrong lower unit gear ratio. It had a V8 gear ratio in a 4 cyl boat.
 
Problem: My 2006 5 HP Mercury Outboard started easily and idled perfectly first trip this year. Wouldn't rev to full power. Assumed Ethanol affected carb or small amout of gas left from previous year. Cleaned carb and replaced, same problem. Installed new carb, same problem. Replaced exterior tank, tank line w/bulb, filter and all interior gas lines. SAME PROBLEM. Any ideas before I have to put in shop for $$$ and 2-3 month wait? I'm not a mechanic, just a parts changer with 2 left thumbs. HELP!
 
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