Editor's note: While this guide generally applies to all small two-stroke outboards, in this case a 4-hp Mariner, there may be some servicing differences between makes. However, the majority of procedures are similar for most modern small outboards.
Servicing a modern outboard motor is not a difficult task and generally involves changing the gearbox oil, cleaning or changing the plug, and greasing the few points that are still equipped with nipples. It is also worthwhile checking the condition of the propeller, shaft, and shear pin where fitted and greasing them before reassembly, while the fuel filter benefits from an occasional clean. Checking the condition of the starter cord and recoil assembly help to prevent unexpected and sudden failure while attempting to start the motor. Apart from keeping the motor clean, this is about the extent of routine servicing, but a couple of hours lavished on the outboard at the end of the season will ensure trouble-free running throughout the next.
Starting from the bottom and working up, one job that should ideally be performed every year is removal of the propeller. On small engines this allows the shear pin to be checked for signs of wear and avoids premature pin failure. More importantly it prevents the propeller becoming seized to the shaft due to the effects of corrosion and lack of grease.
Setting up the engineFigure 1: Before commencing any work on an outboard motor, set it up firmly where it is convenient to work on and cannot fall over.
Figure 2: Remove the locking pin from the propeller nut.
Figure 3: Holding the propeller securely, loosen the nut.Remove the nut and slide the propeller off the shaft. If it is corroded in place, set the outboard down with the shaft vertically upwards and run Plus-Gas or a similar penetrating oil down the shaft and leave to soak overnight or longer. A puller may be required to finally get it moving. If it refuses to budge, heating with a blow-lamp may break the corrosion seal, but care is needed to avoid heating the propeller shaft ring seal in the bottom of the leg. This procedure may best be left to a professional.
Figure 4: Remove the shear pin and check it for cracks or bending. If it shows signs of serious wear, replace it with a new one of the correct size and type. Do not substitute it for a high-tensile bolt with the idea of preventing it breaking. It is there to protect the propeller from excess damage in the event of the blades striking hard bottom or floating debris.
Smear the propeller shaft generously with waterproof grease prior to reassembly. The propeller nut should be tightened sufficiently to hold the propeller firmly against the shear pin before the locking pin is fitted. Other makes of motor may differ in this respect.
Changing the gearbox oil
The gearbox oil should be changed once every season, not only to ensure that the gears are properly lubricated, but also that the seals are in good condition and no water is entering the gearbox.
Figure 5: Place a receptacle beneath the leg and remove the drain screw located just below the propeller shaft housing in the motor skeg.
Figure 6: Next, remove the level plug located just above the cavitation plate and next to the water flushing plug. Both are clearly marked. Once the level plug is removed the oil will drain into the receptacle. Check the condition of the oil as it drains out. If it is dark like this example there should be little to worry about inside the gearbox. If it is white or milky, indicating water contamination, the "O" rings and oil seals will need replacing and an examination of the gear assembly for damage and corrosion is necessary. These procedures are beyond the scope of this guide. While the oil is draining check the condition of the seals on the drain and level plugs and replace them if damaged.
Figure 7: Once the oil has drained completely, refill the gearbox with the correct grade of lubricant. Quicksilver gear lube is the recommended type for the Mariner 4-hp and the squeezy container makes filling easy. Cut off the end of the gear oil container spout and insert it into the oil drain hole. Squeeze oil into the leg until it appears at the level hole. This ensures there is no air in the gearbox and that the correct amount of oil has been added. Refit the level plug while maintaining the level, followed by the drain plug. Tighten both plugs and clean up any excess spilled oil.
Spark plug cleaning and setting
Figure 8: Remove the spark plug using a proper spark plug socket with rubber insert. These prevent the ceramic part of the plug being broken. If one of these is not available use a standard deep socket, taking care not to apply any sideways pressure to the ceramic part of the plug with the top of the socket.
Figure 9: Use a feeler gauge to set the gap of the new plug to 0.020-0.025 inches (0.5-0.6mm). The feeler gauge should be a snug fit in the gap so that slight resistance is felt as the feeler blade is pulled through. Fit the plug, taking great care not to cross thread it. Screw it the first few turns by hand to ensure that it is running freely down the thread before finally tightening it with the socket, again taking care not to crack the ceramic if an ordinary deep socket is used.
Cleaning the fuel tank and filter
The fuel filter is located within the fuel shut-off cock under the fuel tank and is accessed by removing the fuel tank.
Figure 10: Ensure the fuel cock is in the off position. Undo the two bolts holding the air-intake housing onto the carburetor and remove the housing. This gives access to the bolt holding the fuel shut-off shaft. Remove this bolt and pull the shut-off shaft forward to disengage it from the fuel cock. Disconnect the fuel line from the cock by releasing the spring clip and pulling the hose off the fitting. Next, remove the two bolts securing the fuel tank. These are located in a recess under the engine. Lift the fuel tank off and give it a swirl to pick up any sediment within the tank. Drain the fuel into a receptacle for disposal, remembering that petrol is an explosive liquid.
Figure 11: Unscrew the shut-off cock and check the filter screen for damage and dirt. Either clean or replace the screen as necessary. Refitting of the tank and reconnection of the fuel line and shut-off shaft are the reverse of the fore-going.
Replacing the recoil starter cord
Figure 12: Remove the three bolts securing the recoil assembly and lift the assembly off. There is no spring pressure to contend with at this stage.
Figure 13: Pull the starter cord all the way out against spring pressure and while holding the pulley to prevent the spring recoiling, examine the entire length of the cord.
Figure 14: If the cord is frayed or in any way damaged replace it with a new length by undoing the knot in the pulley recess, removing the old cord, and fitting a replacement. Tie a stopper knot in the engine end and push it flush into the pulley recess with the free end secured in the clip provided in the recess. Tie a figure of eight knot in the handle end of the cord. If the old cord was broken, fit the new one in exactly the same way after first tensioning the recoil spring 4 1/2 turns counter-clockwise with the recoil assembly upside-down.
Figure 15: There is only one greasing point on this engine requiring the use of a grease gun, and that is the gear shift arm. Other areas that need to be greased manually are the clamp screws, which should be screwed right out, greased, and then screwed in to distribute the grease throughout the threads and the throttle control inside the tiller handle.