Why does my Yamaha EX Sport stall at idle in the water and shudder at WOT?
I have a 2022 Yamaha EX Sport that runs perfectly on the flush hose. But as soon as I put it in the water, it stalls at idle. If I manage to get it going, it shudders badly at wide open throttle. What should I be looking for?
Answer
A Yamaha EX Sport that runs fine on a trailer but stalls in the water and shudders at wide-open throttle (WOT) is suffering from a load-related misfire or a restricted jet pump. In most cases on the TR-1 engine, the culprit is either fouled spark plugs failing under load or debris wrapped tightly around the impeller shaft.
Inspect the Jet Pump
Check the jet pump first. This requires no parts and solves the problem frequently. When a ski sits out of the water, the engine faces no resistance. It spins a partially jammed impeller easily. Dropping the hull into the water adds immense hydraulic drag. This pulls the engine RPM down until it stalls at idle. Opening the throttle then causes severe cavitation and shuddering.
Always disconnect the battery negative terminal before reaching into the intake grate to prevent accidental starting.
Shine a torch through the intake grate underneath the hull. Look for the following:
- Shaft debris: Seaweed, fishing line or a plastic bag can wind tightly around the drive shaft just in front of the impeller. You may need a long hook or a serrated knife to cut the obstruction free.
- Impeller damage: Check the leading edges of the impeller blades. Sucking up stones bends these blades and destroys the water flow. This causes violent vibrations at WOT.
- Wear ring clearance: The gap between the impeller blades and the outer plastic wear ring should measure less than 0.6mm. A rock can gouge a deep groove in this housing and drop pump pressure. This usually causes high revs rather than stalling.
Replace the Spark Plugs
If the pump is clear, suspect the ignition system. The 1,049cc 3-cylinder Yamaha TR-1 engine is notoriously hard on spark plugs. This happens frequently if the ski idles extensively in no-wake zones or burns off winter fogging oil. A weak spark fires a cylinder perfectly when the engine runs unloaded on a flush hose. Pushing water creates a heavy load that blows the spark out. The engine drops a cylinder, stalls at idle and shudders violently at speed.