If a Mercury outboard hesitates while accelerating, there’s something wrong with the motor. An outboard motor should never pause when attempting to increase the rate of speed.
Poor acceleration is often described as bogging during acceleration, or surge on acceleration. And this problem often happens when accelerating from a stop, or when accelerating hard. The delay in power can be brief or lengthy.
Whatever the case may be, it’s important to pinpoint the cause of the problem so as to restore the boat motor back to working order. If there is a delay in your outboard’s capacity to gain speed within a short time, refer to this troubleshooting guide to get your Mercury outboard running smoothly again.
Engine Hesitates During Acceleration
The following information is a list of possible causes and corrective actions needed to fix a Mercury outboard that hesitates on acceleration. It also includes information on how to fix a motor that idles rough and stalls, or runs uneven or surges, which is often associated with an engine that hesitates during acceleration.
- Fuel mixture not in accordance with factory specifications. Check fuel tank and replace gasoline if necessary.
- Fuel tank air vent blockage or closed. Ensure air vent in the fuel tank is free from restrictions and is completely opened.
- Loose fuel line connection, pinched line, or blockage in the fuel line. Inspect all fuel lines for bad connections, fuel leakage or damaged lines. Replace as needed.
- Restricted or dirty fuel filter. Check fuel filter for contamination or restrictions. Clean or replace filter if needed.
- Low fuel pump pressure. Refer to appropriate section in factory service manual for testing and inspection procedures.
- Anti-siphon valve. Refer to appropriate section in factory service manual for testing and inspection procedures.
- The carburetor’s needle and seat is either stuck closed or open. For reference, if the needle and seat is stuck in the open position, the carburetor will flood. If the needle and seat is stuck closed, fuel will not enter the carburetor and the engine will not start. Refer to carburetor adjustments section in factory repair manual for appropriate procedures.
- Idle mixture screw out of adjustment, restricted jet or wrong carburetor jets. Refer to carb adjustment procedures in service manual to fix this problem.
- Incorrect carburetor float level. Refer to service manual for how to adjust float level to factory spec.
- Carburetor loose on reed block housing. Use a pressurized oil can of 2-cycle oil to spray around the crankcase housing and reed block housing mating surfaces and at the base of the carburetor. If the outboard engine’s RPM changes, replace the reed block housing gaskets or carburetor base gaskets as necessary. Or torque the housing down.
- Thermal relief valve malfunctioning. If the thermal relief valve is stuck close, the motor will run rich at idle. If the valve is stuck open, the motor will exhibit cold start characteristics.
- Primer ball. Ensure the primer ball is not collapsing while under load.
Always use a repair manual when servicing your Mercury outboard, especially if there is a delay when accelerating or throttling the motor. Advanced troubleshooting procedures will be listed in the repair book.