15HP evinrude losing power at full throttle

[johnyeh]

1988 15HP evinrude shortshaft runs fine at lower RPM, but when I open it up all the way the engine will not maintain constant RPM, and loses power like it is going to stall, then gains more power, and repeats this cycle until I back off on the thottle. The previous owner had no knowledge of the history of the motor so neither do I, and I have not done any work myself on it yet.<br /><br />John

 

[baxter66]

Re: 15HP evinrude losing power at full throttle

For help you are going to have to give the guys some history. Is the motor new to you. last carb rebuild waterpump replacement things like that.

 

[Solittle]

Re: 15HP evinrude losing power at full throttle

I think you can plan on rebuilding the carb.

 

[joaklay]

Re: 15HP evinrude losing power at full throttle

Could be at least 3 things. I had "fun" with the same outboard last summer. The fuel pump might need attention (dirty screen, bad gaskets), the carb might need rebuilding, or as I found (besides a carb problem), the gas tanks intake filter was partially clogged. It'd run fine for a few minutes then slow down, stall, was almost impossible to re-start. I switched gas tanks with a known good one, problem solved.

 

[johnyeh]

Re: 15HP evinrude losing power at full throttle

Hey guys thanks for all the advice. My manual just arrived in the mail and I am going to overhaul carb, then check fuel pump.

 

[Boatist]

Re: 15HP evinrude losing power at full throttle

Welcome to the board John. <br />You have gotten good information so far. I belive you can eliminate a couple of thing with out spending any money.<br />To start with loosen the cap on your gas tank and see if there is any suction inside. Then go on a test run and when it starts to fail squeese your primer bulb on your fuel line and see if problem goes away. If problem is a fuel pump or filter then the problem will go away while you squeese the bulb.<br />Another thing that will cause your problem is a prop with too high a pitch for your boat. I have the same motor as a kicker on a 21 foot boat with a high thrust 10 inch 9 pitch 4 blade prop. It did the same thing your motor does. The prop is too high a pitch but only prop I found so far that will back my boat up against the wind.<br /><br />I ran Evinrude/Johnson engine tunner / decarb and that problem went away.<br /><br />Here a very good link for that motor.<br /><br /> http://www.pugetsoundanglers.org/Fishing Info/Johnson 9.9_15.htm

 

[tdp63]

Re: 15HP evinrude losing power at full throttle

I find the comment interesting about the prop size. I have 3 74 -76 Johnson 15's (in various states of repair!)one has a prop on it that is obviously bigger by the eye when side by side. It is stamped as a 91/2 x 10 as are the other two but they are stamped inside on the hub with raised lettering. The bigger one is stamped on the outside. May just mean not original. My 'expert'mate says it is just a newer prop and therefore not yet worn down to the size of the others. It's condition corooborates this statement. Is 91/2 x 10 the right size for this motor? I'm thinking I might put the smaller looking one on and see if any of my probs go away or change.

 

[joaklay]

Re: 15HP evinrude losing power at full throttle

If and when you remove its carb, be carefull to keep the bolt inside all the starter parts when loosening, removing the manual pull starter as you don't want all that to come apart. It's actually easier than the instructions say but one needs to keep that in mind. I then find a nut to thread on the end of that bolt so that it can't possibly come apart and just let it hang off the front. I've had the carb off mine a few times. Pay attention to the position of the spring clip on the throttle side of that starter too so that it's correct when you re-assemble it.

 

[OBJ]

Re: 15HP evinrude losing power at full throttle

The 9 1/2 X 10 is the standard prop that comes with the engine. Getting a rpm reading at WOT is really needed to prop the rig out right plus engine set up on the boat has to be correct also. 9 1/4 X 8 are available for increasing r's at WOT. There are a couple other sizes available also.<br /><br />It's interesting to me that so many folks that own these engines do not address prop size and set up. I guess they feel it's a small engine and it doesn't make a difference. But it does.....

 

[tdp63]

Re: 15HP evinrude losing power at full throttle

Thank you OBJ- you have put my mind at rest in that regard. So was my mate right about one prop being more worn than the other OR can some after market props not be what they claim? (in regard to size)

 

[OBJ]

Re: 15HP evinrude losing power at full throttle

Props can get worn down after a period of time.....hitting sand bars, objects in the water, etc., all take a toll. Just running the engine in "clean" water can do this. The water is full of abrasives, dirt particles and such that act like sand paper against the blade edges of the prop. That's why a the leading edges of a prop get rounded over.<br /><br />Now wether or not generic props are exactly the same size, I couldn't just tell you. But if I got a 9 1/2 X 10 prop from Michigan Wheel, I would expect it to be just that, 9 1/2 X 10. I've never really compared blade width. But my guess is they would be close. There are some of the other guys here that could probably answer your question better than I could. Maybe one will chime in here.