Yamaha 40hp develops misfire when warm.

[seahog]

My Yamaha 40 HE, 1986, 3cyl will start at the touch of the button, idle and run perfectly and powerfully for up to 15 mins. After that time a misfire develops and it will run on just 2 cylinders, opening the throttle just chokes the motor.<br /><br />Eventually, it will run on just one cylinder until it fails. When it is running on 2 cylinders, any plug can be disconnected and it will run on the other 2!<br /><br />Once the motor has cooled down and a clean set of plugs fitted it will restart and run perfectly again for another 15 mins, or so, when it will begin to fail again.<br /><br />So far, I have made sure the fuel is fresh with the correct 100:1 mixture, correct NGK BR-7HS10 plugs are fitted, the carb floats have been checked, the fuel pump has been checked, the mixture setting is correct, the bottom crank seal is new and i have swapped around all ignition coils, to no effect.<br /><br />There is no evidence that any engine compression is being lost, due to breached gaskets etc. There is no water in the fuel and no two stroke oil residue in the jets.<br /><br />I now feel that the fault may lie with the CDI but at a cost of $400.00 (in the UK) I am reluctant to spend out, if I am not sure a cure will be made.<br /><br />Any ideas anyone?

 

[Seamule]

Re: Yamaha 40hp develops misfire when warm.

Only a suggestion...I had the same motor with the same problem and did all the things you did in the same order. I ended up reading the coils for resistance and found that I had 2 bad coils. they seem to break down when the engine gets warmer. It worked for me. Good Luck. <br />P.S. The resistance specs are in most manuals and when you check them make sure that they fall within the specified range.<br />Seamule

 

[mtrafton]

Hi hog and mule,
I've got a 70 hp Yamaha 2 stroke three cylinder that's got the same symptoms. Fires out of the hole great in first ten minutes of so, then after about 12 to 15 minutes misfires. Fuel system has been gone over and is all good. All three coils checked out ok on ohm meter when cold, but could they be breaking down when hot? Does it make sense to check them on the ohm meter right after it starts to misfire, when hot? If it's not one of the three coils, what's your sense of whether it could be the trigger coil or CDI? Anybody out there have experience enough to know the most common cause. Thanks for any advice. I need to get this thing running just right and sold, or fixed and winterized and sell it in the spring.

 

[GA_Boater]

This post is 13 1/2 years old. Read the Help Tip on top of the page about posting in old threads. Your best chance to get help is by starting a new thread with your question.

This is closed.