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?