A slow-feeling Yamaha can be slow for three unrelated reasons: the engine isn't making its power, the electronics are holding it back, or the pump isn't turning engine power into thrust. They look alike from the saddle and need different fixes. The order below sorts them, and it applies to every four-stroke the M-JET Diagnostic supports, from the 2002 FX140 to the 2024 1.9-litre skis.
Step 1: does the engine reach its rpm in the water?
Look at the rev counter at full throttle on a straight run. Three cases come out of this.
- Rpm is normal or even higher than usual but the ski is slow. The engine is fine and the pump side is the suspect: cavitation, a damaged impeller or foreign matter in the intake. No diagnostic tool sees those, and they don't set codes.
- Rpm stops at a ceiling well below the rated range, often around 5000. Something is limiting the engine, or the engine has a problem. Go to step 2.
- Rpm is low and the engine feels flat all the way through. Go to step 3.
Step 2: is something limiting the rpm on purpose?
Connect the app and read the fault memory before anything else. According to the EX manual, overheat and oil pressure warnings reduce engine speed automatically. The learning mode (L-Mode) and the no-wake mode also cap rpm and feel exactly like a defect, so check whether either one is switched on.
An owner of a 2006 VX1100 described 5000 rpm in the water while the same ski revved to 8000 on land. The cause was an air filter housing soaked in oil after an overfilled oil change, which no scan will show, so look inside the housing. TR-1 skis behave in a similar way: too much oil costs around 500 rpm according to owners, and the oil level is only trustworthy after about six minutes of running on level ground.
Step 3: throttle and intake pressure
Compare what you ask for with what the engine gets. On models with the electronic throttle the live data shows the paired signals, TPS1 and TPS2 and APS1 and APS2, and each pair should track together. If the accelerator signal says full and the throttle angle stays short, the system is holding back or a sensor is lying. The intake pressure value shows whether the engine is breathing: on a naturally aspirated ski it should rise toward ambient pressure at full throttle, and a clear shortfall points at the intake side or a leak.
On the supercharged FZR, FZS and SVHO, a ski that hangs at about 5000 to 5500 rpm with intake pressure near ambient fits the supercharger clutch wear that owners report, and power that falls off above about 6000 rpm is another version of the same story. The app shows the intake pressure but can't prove the clutch is worn, and the hand test of the clutch is considered unreliable.
Step 4: voltage and heat
Low battery voltage can produce odd behavior, so the voltage value belongs in every power loss check. Heat soak on supercharged skis is real and shows up in the intake air temperature. One FX SVHO owner reported rpm dropping from about 7500 to 4500 after 20 to 30 minutes with no alarm, and the cause there is unknown. Log the ride and see which value moved first.
If the electrics are fine, throttle and intake pressure look plausible and the ski is still slow, the pump and intake go on the list: grate, impeller, wear ring. The page on power loss and finding the cause walks through the electronic side again, and a logged test ride shows where the curves differ from a healthy ski.
