The classic complaint goes like this: the ski runs fine at the ramp, fine at idle, and then, twenty minutes out, it stumbles for a second or two and recovers. By the time you are back at the dock everything is normal and the fault memory is either empty or holds a vague entry. This is the situation the data logger was built for, and it works on every supported Yamaha four-stroke, from the 2002 FX140 to the current 1.9-litre models.
Set up before you leave the dock
Connect the dongle and start a recording. The app logs the live values during the ride, so you can read them afterward. The phone has to stay within Bluetooth range of the dongle, which on a jet ski means a waterproof pouch in the storage compartment. Do not read the screen while riding. Note roughly how many minutes into the ride the stumble happened, since you will need to find that spot later.
Pick the values that can explain a stumble. The useful set is engine speed, ignition timing, injection duration, throttle angle (TPS, and APS where fitted), battery voltage, intake pressure, intake air temperature and engine temperature. Fewer values make the curves easier to read, and the ones above cover both the electrical and the fuel and air side.
Reading the curve
Find the moment of the stumble. It will show as a dip in engine speed. Then look at what else moved at that exact moment, because the cause is usually something that moved with it.
- Battery voltage dips at the same time: look at the charging side, the battery terminals and the main plugs.
- The throttle or accelerator signal jumps or drops out although you held the lever steady: a contact in that sensor or its plug.
- Temperature climbs steadily toward the moment of the stumble: heat is involved, and the water path comes before any sensor.
- Nothing else moves, only the engine speed dips. That points at ignition or fuel hardware, and it is where the component tests come in.
The last case is not a failure of the log. It is a useful result, because it rules out the sensors and the voltage and narrows the search to the coils and the injectors.
Narrowing it down by cylinder
I would not expect a log of these values to name the faulty cylinder by itself. For that, the app can test each ignition coil and each injector individually, and the cylinders can then be compared with one another. Details on that are in the page about checking an ignition coil and an injector. On the FX140 and the early FX models, two coils serve two cylinders each, in the pairs 1 and 4 and 2 and 3, so a bad coil can look like two misfiring cylinders at once. Working near fuel, keep the area ventilated and free of ignition sources.
What the log can miss
A recording shows what the sensors saw, and it cannot show problems that leave no electrical trace. A tired impeller, cavitation, low compression or a pump that delivers less than it should all hide from it. The fuel pump test in the app only tells you that the pump runs. It does not say whether it delivers enough, and that matters for a stumble under load. Seized spark plug threads are another thing nobody can read from a log, so go carefully when you pull a plug.
The Yamaha edition of the M-JET Diagnostic records test rides on all of its 37 supported models, and the data logger and test ride page explains the recording in more detail.
