Hi.
I have had a 2003 15 horsepower 4-stroke Mercury tiller controlled outboard for about two years. I don't remember when this problem started, because when we originally bought the motor we had some fuel delivery problems that probably masked the problem until it was fixed. For about the past year, however, the motor has, when run at high throttle (above about 75% according to the markings on the tiller control), suddenly dropped RPM's as the throttle is turned any higher. The throttle point at which the motor begins to bog down varies somewhat. When the motor is run on high throttle for the first time that day, it will run well at nearly 100% throttle, only running slightly roughly when turned to full 100%. However, as time goes on, the motor will bog down at 95%, 90%, 85%, etc... To keep this concise, I'll list all the symptoms we've observed and the attempted repairs.
- Spark plugs: spark plugs have been changed twice in the past 12 months. Every time the spark plugs are inspected, be it before a change, after a change, or just randomly, heavy dry black carbon deposits are observed. The spark plugs were properly gapped when installed
- Fuel pump: the fuel pump was replaced in its entirety within the last month
- Fuel filter: the fuel filter was replaced about 2 months ago
- Carburetor: the carburetor was in the shop for a rebuild about a year ago, then was fully replaced about a month ago
- Pilot Screw (on carburetor): the pilot screw on the carburetor was screwed in fully, then unscrewed 1 and 1/4, as per the repair manual specs, then the boat was taken out for testing on the bayou and I tried screwing in and unscrewing the pilot screw by 1/8 turns, testing the motor at high throttle after each change
- Throttle linkage/accelerator pump: the throttle linkage gap and the accelerator pump gap on the carburetor were adjusted fairly accurately to the gaps called for in the repair manual
- Water impeller: likely unrelated, but the water pump impeller went bad about 6 months ago and was replaced
- Prop: the prop initially installed (outboard was bought used) had a slightly higher pitch than was called for in the repair manual, so was replaced with the propeller suggested in the manual. I don't recall the specs of either propeller, but I can find them if requested
We always use ethanol free gas, albeit not always with stabilizer. The fuel is usually run out of the motor after use, but we've only recently started draining the carburetor bowl of fuel after use.
Any assistance in fixing this issue would be much appreciated.
I have had a 2003 15 horsepower 4-stroke Mercury tiller controlled outboard for about two years. I don't remember when this problem started, because when we originally bought the motor we had some fuel delivery problems that probably masked the problem until it was fixed. For about the past year, however, the motor has, when run at high throttle (above about 75% according to the markings on the tiller control), suddenly dropped RPM's as the throttle is turned any higher. The throttle point at which the motor begins to bog down varies somewhat. When the motor is run on high throttle for the first time that day, it will run well at nearly 100% throttle, only running slightly roughly when turned to full 100%. However, as time goes on, the motor will bog down at 95%, 90%, 85%, etc... To keep this concise, I'll list all the symptoms we've observed and the attempted repairs.
- Spark plugs: spark plugs have been changed twice in the past 12 months. Every time the spark plugs are inspected, be it before a change, after a change, or just randomly, heavy dry black carbon deposits are observed. The spark plugs were properly gapped when installed
- Fuel pump: the fuel pump was replaced in its entirety within the last month
- Fuel filter: the fuel filter was replaced about 2 months ago
- Carburetor: the carburetor was in the shop for a rebuild about a year ago, then was fully replaced about a month ago
- Pilot Screw (on carburetor): the pilot screw on the carburetor was screwed in fully, then unscrewed 1 and 1/4, as per the repair manual specs, then the boat was taken out for testing on the bayou and I tried screwing in and unscrewing the pilot screw by 1/8 turns, testing the motor at high throttle after each change
- Throttle linkage/accelerator pump: the throttle linkage gap and the accelerator pump gap on the carburetor were adjusted fairly accurately to the gaps called for in the repair manual
- Water impeller: likely unrelated, but the water pump impeller went bad about 6 months ago and was replaced
- Prop: the prop initially installed (outboard was bought used) had a slightly higher pitch than was called for in the repair manual, so was replaced with the propeller suggested in the manual. I don't recall the specs of either propeller, but I can find them if requested
We always use ethanol free gas, albeit not always with stabilizer. The fuel is usually run out of the motor after use, but we've only recently started draining the carburetor bowl of fuel after use.
Any assistance in fixing this issue would be much appreciated.