Mercury Outboard Hesitates Upon Acceleration

Mercury Outboard Hesitates Upon Acceleration

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.

  1. Fuel mixture not in accordance with factory specifications. Check fuel tank and replace gasoline if necessary.
  2. Fuel tank air vent blockage or closed. Ensure air vent in the fuel tank is free from restrictions and is completely opened.
  3. 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.
  4. Restricted or dirty fuel filter. Check fuel filter for contamination or restrictions. Clean or replace filter if needed.
  5. Low fuel pump pressure. Refer to appropriate section in factory service manual for testing and inspection procedures.
  6. Anti-siphon valve. Refer to appropriate section in factory service manual for testing and inspection procedures.
  7. 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.
  8. Idle mixture screw out of adjustment, restricted jet or wrong carburetor jets. Refer to carb adjustment procedures in service manual to fix this problem.
  9. Incorrect carburetor float level. Refer to service manual for how to adjust float level to factory spec.
  10. 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.
  11. 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.
  12. Primer ball. Ensure the primer ball is not collapsing while under load. 

Download-Online-Factory-Service-Repair-manual-PDF

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.

4 Comments

    • For the most part it’s universal, some instructions “might” also apply to a Yamaha, Suzuki, Johnson and Evinrude carbureted motor. Here are some models that might apply:

      1969-1978, 1979-1985 7.5HP Mercury 2-Cylinder
      Mercury 9.9 HP Sailpower, XR10/MAG 10/Viking 10
      Mercury 75 HP Jet
      Mercury 80 HP Jet
      Mercury 75 HP Marathon
      1979-1985 9.8HP Mercury 2-Cylinder (Model 9.8)
      1992-1998 Mercury 135
      1984-1985 25HP Mercury 2-Cyl. (Model 25XD)
      1986-1989 25HP Mercury 2-Cyl.
      1984-1989 2.2HP Mercury 1-Cylinder
      1968-1972, 1973-1990 20 HP Mercury 2-Cyl. (Model 200)
      Mercury 100 HP
      1979-1983 40 HP Mercury 2-Cyl. (Model 40)
      1984-1985 18HP Mercury 2-Cyl. (Model 18XD)
      1992-1998 Mercury 150 XR6
      Mercury 8 HP
      1992-1998 Mercury 150, 200, 225 Horsepower Super Magnum
      Mercury 6 HP
      1971-1974 40 HP Mercury 2-Cyl. (Model 402)
      1984-1989 35 HP Mercury 2-Cyl. (Model 35)
      1975-1978, 1979-1985 4.5HP Mercury 1-Cylinder
      Mercury 9.9 HP
      1965-1968 3.9 HP Mercury (Model 39) 1-Cylinder
      1992-1997 Mercury 175 HP
      Mercury 65 HP Jet
      1969-1974 4 HP Mercury (Model 40) 1-Cylinder
      1992-1998 Mercury 175 EFI
      1999-2000 Mercury 150 hp Optimax DFI Direct Fuel Injection
      1992-1998 Mercury 200/225 Pro Max
      1980-1983 25 HP Mercury 2-Cyl.
      1988-1990 15 HP Mercury 2-Cylinder
      Mercury Sea Pro/Marathon 10
      1992-1998 Mercury 200 & 200 EFI
      1983-1985 3.5 HP Mercury 1-Cylinder
      1968-1987 6 HP Mercury 2-Cylinder
      1965-1969 35HP Mercury 2-Cyl. (Model 350)
      Mercury 8 HP Sailmate
      Mercury 75 HP
      Mercury 90 HP Jet
      1992-1997 Mercury 150 Magnum III
      1998-2000 Mercury 135 hp Optimax DFI Direct Fuel Injection
      1965-1978 9.8 HP Mercury 2-Cylinder (Model 110)
      Mercury 75 HP Sea Pro
      Mercury 15 HP
      Mercury Sea Pro/Marathon 15
      Mercury 125 HP
      1970-1971 40HP Mercury 2-Cyl. (Model 400)
      1981-1983 18 HP Mercury 2-Cylinder
      1992-1996 Mercury 150 HP Pro Max
      1976-1980 4HP Mercury 2-Cylinder
      1986-1991 9.9 HP Mercury 2-Cylinder
      Mercury 90 HP M
      1990-1991 2.2 HP (Model 3.0) 1-Cylinder
      Mercury 115 HP
      1992-1998 Mercury 150
      1980-1982 3.6 HP Mercury 1-Cylinder
      1986-1987 4HP Mercury 1-Cylinder
      1986-1990 8HP Mercury 2-Cylinder
      1992-1997 Mercury 150

  1. I have a Mercury 115hp and it runs perfect when out of the water. But when the boat is in the water and throttle it up it bogs down and stalls out. What’s wrong?

    • If it runs great out of water but bogs or stalls out while advancing throttle, I’d say you’re not running on all cylinders. If the motor is not firing on all cylinders, it could be a result of low compression, fuel starvation or no spark. Does it also happen with a slow acceleration?

Leave a Reply

Your email address will not be published.


*