Intermittent power on 15hp 2-stroke

I've got a Johnson 15hp 2-stroke, model J15REEA. Sometimes I can get full power (you can feel the difference), but most of the time it's kind of weak and you can hear it almost sputtering at WOT.

I did some experimenting and found that if I do a hard left turn, where everything in the boat shifts to the right, I can get full power! Then if I do a right turn, it seems weaker. Straight forward is somewhere in between.

I'm thinking it has to do something with not enough fuel making it into the cylinders, and the right/left thing could relate to fuel sloshing in the carburetor bowl? Is this a simple issue of fuel starvation, or more complex like adjusting the carb float? I've got the shop service manual but am no expert with 2-stroke engines.

Any thoughts would be useful.

Jonathan

Try pumping the primer ball when that happens. If that makes no difference take the carb off and clean it. Or are you running a long shaft motor on a short shaft boat?

flyingscott said:

Try pumping the primer ball when that happens. If that makes no difference take the carb off and clean it. Or are you running a long shaft motor on a short shaft boat?

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It's a regular motor, on a hard bottom inflatable dinghy. What if pumping the ball does make a difference? That would indicate fuel starvation to the carb?

If pumping the ball makes a difference it is the fuel pump. So do you have a long shaft on a short shaft boat or vice versa. Put up a pic of the bot and motor from the side. Have you tried a different tank and hose?

Water in your gas tank, brother. Right, like Mr. Scott says.....try another tank and hose. Plastic tanks often leak in water if left out in the weather.

flyingscott said:

If pumping the ball makes a difference it is the fuel pump. So do you have a long shaft on a short shaft boat or vice versa. Put up a pic of the bot and motor from the side. Have you tried a different tank and hose?

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I cleaned out the tank and put new gas in it. I can try another hose but this is actually my second hose and I'm still seeing issues.

One other point to add is that the top cylinder spark plug gets consistently wet fouled black. I've confirmed both cylinders are firing by removing spark from each one, and it will run on each of the cylinders alone. But even with new plugs, the top on is getting fouled. If I swap plugs, the bottom one gets clean and the top one fouls again. I'm wondering if this could be a sign of poor compression in the top cylinder, or something to do with the carburetor feeding too much gas to one cylinder (fouled carb). The fact that I do get full power sometimes makes me think it's not bad compression though, then I'd never get power. Or that's just a red herring that's unrelated to my power issues. Thoughts?

Regarding the pic: It's a dingy for my sailboat so I don't have a way to raise it out of the water with the engine on it. I'll see what I can do.

Does the spark plug get shiny clean? Do a compression test.

Perhaps take fuel pump apart to inspect diaphragm.

Thanks all for the tips. I rebuilt the carb and fuel pump, cleaning both. The condition was overall good, didn't notice anything wrong. I also replaced the fuel tank and fuel line, and fuel. I put it back together and it runs mostly the same as before: smooth, consistent, but at WOT you can hear that it's not "all there". Like it's rough or stuttering slightly. I confirmed that one of the plugs is still getting oily (pic attached), and it still has the behavior of running with more power if I do a consistent flat left turn (and worse if I do a right turn). I have a theory.

The dirty plugs and rough running would indicate a rich mixture on the main jet. And, now that I've seen the inside of the carb, I see that the main jet pickup in the carb bowl is on the port side of the engine. Therefore when I turn left consistently, the fuel sloshes to the right and there's less fuel on the port side of the bowl where the pickup is, so it runs leaner! The shop manual even says that if it's running rich, to adjust the float and thus achieve lower fuel level in the bowl. So I might be crazy but it's possible that turning hard left is like adjusting the float in the bowl. So my next idea is to do the float adjustment.

My only other idea is that the top cylinder has poor compression, thus the oily plug, but that wouldn't explain why I can get it to run better under certain conditions. I also don't have an easy way to test compression. Is anyone aware of a way of doing that without specialty parts?

Thoughts? Thanks in advance.

Seems no matter what happens with the float level the effect should be the same on both cylinders if things are all normal on that 15 hp. So you should borrow a compression tester to help diagnose this problem. The upper cylinder could have a leak. This certainly doesn't explain why turning to the left or port, helps in speed .......other than boat dynamics, trim, or load distribution. Also, if there was a vacuum leak causing a lean lower cylinder, a change in motor pitch would not likely be significant.

timguy said:

Seems no matter what happens with the float level the effect should be the same on both cylinders if things are all normal on that 15 hp. So you should borrow a compression tester to help diagnose this problem. The upper cylinder could have a leak. This certainly doesn't explain why turning to the left or port, helps in speed .......other than boat dynamics, trim, or load distribution. Also, if there was a vacuum leak causing a lean lower cylinder, a change in motor pitch would not likely be significant.

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Yes, my theory about mixture doesn't really explain the difference between cylinders. My only thought was that as the mixture richens, one cylinder fouls before the other...?

I'll try to find a compression tester, but I'm in Panama at the moment so getting access to one here may not be possible. Even if I do, I'm not sure what I can do about it. If compression is poor, then what can I do? I'm not going to want to, or be able to, do a full engine rebuild. Since I've found certain conditions that the engine appears to run with more power, I was hoping to pursue that track.

Could there be an air restriction coming into the cowling? Maybe when you turn the motor, it is allowing more air to enter.

timguy said:

Could there be an air restriction coming into the cowling? Maybe when you turn the motor, it is allowing more air to enter.

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Thanks for the feedback, but no luck. I've run it with the carb intake fully exposed, no luck. The strange thing is, it seems *sometimes* I can get it to run with more power. Then as soon as I think it's fixed, I let it sit, and the next time I run it, it's back to it's old self.

The fact that the top cylinder is always fouled, must mean there's either a compression or ignition problem in the top cylinder. Sparking seems OK, I can hold the plug about a half inch or so from the block and still get a spark. As for the compression, I don't have a way to test accurately, but simply pulling the starter cord slowly allows me to feel that the compression on one of the cyclinders is less because it pulls through more easily on one of the compression strokes. I can also take off one of the spark plugs, pull through slowly on the starter, and hear what sounds like air hissing when I start the compression stroke. So it doesn't "hold air" for long. Looking inside the cylinder, there is definitely more "black goo" on the top cylinder itself.

So I thought about having to do an engine teardown to service the pistons/rings. The service manual says before you give up and rip it apart, to use "OMC Engine Tuner" to clean gunk from inside the cylinder and potentially free the piston rings in case they are stuck and therefore not sealing to the edge of the cylinder. If the rings are "clogged" and periodically come loose, that could explain why I have times where power is apparently much higher, right? But I can't get any of that here in Panama. Any clue on homemade solutions to get piston rings unstuck?

So, my best ideas for path forward are:
1. Try to verify the strength of spark on top cylinder. Any ideas on how to do that?
2. Thoroughly clean the inside of the cylinder to try to free possibly stuck rings. Any clue what to use?
3. If all else fails, open up the engine and service the piston rings.

Have you serviced water pump at all ?-----Remove exhaust cover and have a look at pistons and rings !---Easy to do.

racerone said:

Have you serviced water pump at all ?-----Remove exhaust cover and have a look at pistons and rings !---Easy to do.

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I have not. I didn't know there was an easy way to see the pistons/rings. I'll try that.

As for spark testing, you should create a simple gap tester to check for good reliable spark when pulling over the motor briskly....both plugs out, at 7/16th of an inch nice blue white crackling spark. Do so with no spark plug, directly from the spark plug wire to ground. If you have a way to check compression, then this can easily be diagnosed. Sounds like you have very few resources there.