The procedures employed to troubleshoot an outboard are identical to those used to troubleshoot any other small engine. The potential causes of given problem are systematically checked and eliminated until the true crux of the problem can be identified. The most common outboard engine problem is no start, the same as it would be for any small engine. You know that the ignition system must produce a spark of correct intensity at the correct instant. With an acceptable spark, there also must be a fuel air mixture which contains the right amounts of fuel and air; this means that enough fuel must reach the carburetor from the fuel tank, and that sufficient air must arrive through the carburetor passages to achieve a good mix, and finally that, having achieved a good mix, it needs to flow efficiently from the carburetor to the combustion chamber where it can be burned. And there must be sufficient compression in the cylinder so that the fuel air mixture can reach high enough temperatures to ignite as it should. Here are the most frequent causes of no start:

1)No spark, weak spark, or improperly timed spark
2)No fuel, insufficient fuel, or an improper fuel air mixture
3)Weak compression

When troubleshooting an outboard, you must first determine which of these conditions is not being met. You will also need to check for mechanical failures inside the engine. Here is review of troubleshooting procedures that apply to two stroke outboard engines.


In an outboard engine, mechanical problems in the powerhead can cause hard starting and a loss of power. Reduced compression can be the result of a blown or leaky cylinder head gasket, excessive glazing of the cylinder walls, or broken, worn, or stuck piston rings. Any of these problems can make the engine difficult to start. Low compression can also procreate loss of power due to the incomplete combustion of fuel. Starting problems may also be experienced if water gets into engine crankcases or cylinders because a gasket is leaky gasket or the engine block cracked. Other conditions caused by mechanical difficulties in the powerhead include:

2)Severe vibration
3)Unusual noises during operation

Knocking can occur as a result of a loose or worn wristpin, a bent or twisted connecting rod, excessive wear on the piston or cylinder walls, or a loose flywheel. Any of these conditions may also cause massive engine vibration. On the other hand, vibration can simply be the consequence of a damaged propeller or worn engine isolation mounts, both of which would be straightforward fixes.

Overheating may be caused by a faulty thermostat or blockage within the powerhead water passages. It can also result from a defective water pump or clogged water intake.


The first step when troubleshooting an outboard powerhead is to perform a compression check. You should disconnect the ignition system prior to testing to prevent the engine from starting accidentally. To perform a compression test without a compression gauge, turn the engine over by hand(utilizing its recoil start), and gauge the amount of bounce you feel as each piston passes top dead center(TDC). In a healthy engine, the bounce should be readily detectable. Compression will usually be low if the engine bounce is tepid as its piston or pistons pass TDC; compression will generally be high if you struggle to turn an engine over as TDC is reached and passed.

If you are using a compression gauge to perform the test, a difference of more than 10 psi between any two cylinders indicates a problem. If the compression gauge reading of any cylinder is between 10 and 15 psi less than the optimum compression specified in the engine service manual, a problem condition exists.

If you observe any of the above problem characteristics during the compression test, you will need to remove the cylinder head or heads, and the exhaust cover, to diagnose the cause of the problem. If the compression in one or more cylinders is low than specified, check the condition of the cylinder walls, pistons, and piston rings. If the compression in one or more cylinders is higher than specified, check for an excess of carbon buildup on the cylinder head and piston head. Inspect the exhaust ports, cylinder head, and the pistons and their rings. If you discover an excess of carbon deposits in any of these locations, the powerhead will need to be disassembled and cleaned.


An engine is said to be binding when it is difficult or impossible to turn over. The flywheel should always turn smoothly, and with a minimum of resistance. The one point at which a moderate resistance should be experienced is when the piston passes top dead center(TDC). If an engine appears to be binding when tested, never force it to turn over; this can result in damage to internal parts.

Binding in an outboard can occur for several reasons. The most common of these is mechanical damage in either the gearcase or powerhead. Broken piston rings and damaged bearings(of the connecting rod or crankshaft) are powerhead problems that frequently arise. Binding can also occur if piston rings are too large for a given cylinder, a quandary that will be visible immediately after an engine overhaul(if oversized rings were accidentally installed on the piston before reassembly).


The problems commonly associated with a malfunctioning gearcase are:

1)Severe engine vibration
3)A noisy or seized right angle drive
4)Inability to shift gears or remain in gear

Severe engine vibration is often caused by a damaged or loose propeller, or by worn or loose rubber isolation mounts. Vibrations may also result if an engine has not been securely mounted on a boat. Stern brackets must be tightly clamped to the boat transom.

Overheating is often the result of a clogged water intake, a defective water pump, or a damaged water tube. A right angle drive problem may be related to worn or broken gears, to a damaged gearcase, or to inadequate gearcase lubrication. A gear shifting problem can be the consequence of an improperly adjusted gear shift mechanism, or by internal gearcase problems(TIPS FOR OUTBOARD MAINTENANCE).


As previously noted, the most common cause of outboard engine vibration is a damaged propeller. You should examine the propeller for bends, cracks, and breaks in its blades, and replace it if necessary. Never attempt to repair a cracked or broken blade by welding it. You can remove minor propeller nicks by filing them, however, should you file out a nick, take care to retain the original shape of the blade edge. You should also avoid removing too much metal from any one blade or propeller part, as this can unbalance the propeller and cause it to shudder as it spins. For this reason, a badly nicked propeller must be replaced.

Another potential cause of engine vibration is loose or worn rubber isolation mounts. An outboard contains a number of rubber mounts that are meant to isolate the exhaust housing from the stern bracket. These rubber mounts absorb engine vibration, preventing it from reaching boat occupants via the boat transom. A typical 25 or 35 horsepower engine will possess six mounts, two of them upper side mounts, two of them lower side mounts, one an upper thrust mount located immediately below the powerhead, and one a lower thrust mount located between the exhaust housing and lower stern bracket.

To inspect the lower thrust and side mounts, you must first remove the cover plates. Depending on the engine, you may need to remove the powerhead, and lower the engine cover, to access the upper side and thrust mounts. Any loose mounts should be tightened, and worn mounts replaced.