Why Does My Outboard Sputter at Full Throttle?by Dan Harkins
If your outboard engine is sputtering at any speed, idle or full throttle, it could be the result of several common malfunctions. You may need to make a few timing adjustments or replace a spark plug, or you may need serious mechanical inquiry.
Check While Running
If your marine engine is sputtering, use a timing light to check each spark plug wire individually, while the engine is operating, for the proper voltage output and ignition spark (your individual model's manual will have these readings). This could indicate which plugs and corresponding fuel cylinders are giving you problems. Remember which plugs are giving you the worst voltage readings.
Remove your boat from the water and place it on a trailer. Remove all your spark plugs with a ratchet and appropriately sized socket. Clean any residue from each plug contact and plug port. If you notice any erosion, replace the plug. Use a plug gap tool to set the correct measurement on your plug gaps, according to your manufacturer. Test out your boat's operation at full throttle again.
If you notice an excessive amount of wet and black carbon on your plugs, you may have a bad fuel-water mix. There also could be a leaky head gasket. See a technician.