Yamaha 15hp 4-stroke idle vibration

[Rudderman]

I just bought this engine recently, its a 1998 model as far as I know. Its electric start.<br />When I bought it a month ago, it idled beautifully in my test tank. Recently when I start it, it runs ok but "shakes" at idle. Even when the engine warms up, it is no less shaky at idle. <br />Also, the transition from idle to mid-throttle is spluttery, you have to gently advance to mid throttle or it will choke and die. <br />When running at idle, and when I pull the choke out slightly, it does not help the situation, it just cuts out. Having said all that, the engine always starts very easily.<br />How can I get my engine back to purring like a kitten at idle?

 

[rodbolt]

Re: Yamaha 15hp 4-stroke idle vibration

hello<br /> how and when did it start this? you described a carb problem. test all the basics then do the carb. good luck and keep posting

 

[Rudderman]

Re: Yamaha 15hp 4-stroke idle vibration

Thanks for replying.<br /><br />The strange thing is that the engine has been sitting on the test tank for a month. At the beginning of the month it was fine...and suddenly one day when I started it, its shaking!<br />I have a clymers manual, but I'm a bit afraid to start dismantling the carb since I'm not sure what I'm looking for.<br />One thing is that the fuel tank was almost empty when the problem started....is it possible that it picked up some dirt from the bottom of the tank?

 

[Rudderman]

Re: Yamaha 15hp 4-stroke idle vibration

Tonight, using my clymers manual, I removed the carb and totally dismantled it, cleaned all the parts with carb cleaner, blew everything out with an air compressor and reassembled it. Everything looked spotlessly clean and new in there. I don't think the carb had ever been taken apart before because there were no screwdriver marks anywhere. Float height is dead-on correct. <br />BUT....it didn't make any difference...it still shakes...<br />Maybe this is normal for a 4 stroke outboard? I've never had an outboard before so maybe I am making a mountain out of a molehill?<br />What is an outboard supposed to be like at idle?<br />The only reason I'm concerned is that I plan to put it on a mahogany runabout that I made from scratch myself, and I'd like to go easy on my transom!<br />There are some photos here of my boat and engine>> http://www.sum-it.biz/squirt/website

 

[Rudderman]

Re: Yamaha 15hp 4-stroke idle vibration

Update: I found the problem, it was only firing on one cylinder! A faulty spark plug was the problem.