Weird top end power problem with 9.9 Johnson

[Jagzilla]

I know this is off topic for this forum, but thought I'd ask anyway. I have a 9.9 hp Johnson, which starts great, idles great, trolls great, but seems to be missing a horse or two on the top end. It used to run perfect, but now when I have it full throttle, it seems to not produce full power at first. I'll be at full throttle for 10 or 15 minutes, and then all of a sudden, another horse or two, and a few rpm's more kick in. It never used to do this, but the last two years it's been doing it. I've changed fuel filter, and made sure to have good gas. It has new plugs, and generally is in good shape. I don't think it's the high speed jet in the carb acting up, cuase if there was a blockage I figure once it went out, the problem would be cured, but this seems to happen the same way each time I run the motor. This happens after I've been trolling for a while as well, so it's not like the motor isn't warmed up. Anyone have any suggestions about all this, before I have to resort to paying money at a repair shop? <br /><br />Also, another question, I understand the only difference between the 9.9 hp and the 15 hp models is the carburetor. Is it just a straight across swap to put a 15 hp carb on the 9.9 motor? I think my motor is a 1981 model, if that adds more info. <br />Thanks for all the help.

 

[Solittle]

Re: Weird top end power problem with 9.9 Johnson

Jag - Check the bulb in the fuel line. It should be firm when running. If it isn't you may have an air leak somewhere in the fuel line. Your symptoms also describe crud in the carb which will require a rebuild.

 

[Boatist]

Re: Weird top end power problem with 9.9 Johnson

Jagzilla<br />I would do the basic checks first. Gap a old plug to 7/16 ground it and move each spark plug wire to it to make sure have good spark. Gap your new plugs to .030. If have weak spark the time for points and condenser unless your motor is new enough to have electronic systems. Good idea to decarb the engine and do a compression check. Make sure your motor is warming up. Motor should be hot to touch about 140 degrees but should be able to put finger on the head for about 4 seconds.<br />Yes on the early motors the Carb was only difference between the 9.9 and 15. On later motors also have some exhaust tunning. Check the web site below, I think he did a very good job for the 9.9 and 15 hp Johnson/Evinrude.<br />New carb around $240.<br /><br /> http://www.pugetsoundanglers.org/Fishing Info/Johnson 9.9_15.htm

 

[yrag]

Re: Weird top end power problem with 9.9 Johnson

My 76 9.9 has always done the same thing I think you are talking about.When I was alot younger and lighter if I took out Jon boat by myself it would do it (used to call it getting in gear)rhe rpm would go up and speed would increase, But with 3 people in boat it never did it but still ran fine(pulled hydra slide) After 27 years all I could think of was maybe the engine needed to be a little higher on the transom (boats long gone)For less drag.If you come up with something please let us know.By the way I believe that these engines are about as indistructable as you could ever find.

 

[Jagzilla]

Re: Weird top end power problem with 9.9 Johnson

I am starting to look closely at the thermostat as the culprit now. Might be stuck wide open, and engine not warming up till run wot for a while. I'll know next time I take it out, as I'll be checking the temp of the water coming out of the "pee hole" from the waterpump. <br /> I don't think its the height from the transom thing you were mentioning though.