Modern outboard motors are sophisticated pieces of equipment. However, they are also very expensive. This is why many boat owners opt to buy a reconditioned two-stroke motors. Occasionally, you may notice that an older two-stroke outboard motor begins to misfire. Below is a guide to two likely causes of this problem.
Damaged shear pin
The propeller on your outboard engine is driven using a shear pin. The shear pin is designed to break if the propeller hits an object so that the shock does not travel further into the motor where it could cause significant damage. If the shear pin has been broken, the outboard motor will begin to misfire. To replace the shear pin, you should shut down the engine and then lift the outboard engine up out of the water so you can remove the propeller and replace the damaged pin.
Issues with fuel delivery
When an engine misfires, it is often a result of the fact that the system is being starved of fuel. The most likely cause of this issue is the fact that if fuel has mixed with engine oil, it can leave behind a sticky black residue as it evaporates. Over time, this residue can build up inside the fuel lines which run from the tank to the engine, restricting the flow of diesel and causing the engine to misfire. The lower level of fuel reaching the engine will also result in a loss of power. If your engine loses some of its power and begins to misfire, you should open the choke as this will increase the volume of air entering the combustion chamber, helping any available fuel to burn. This should provide enough power to get you back to port.
Once you get back, you should arrange to have the engine and fuel delivery lines and tank on your vessel serviced and cleaned by a professional outboard motor service centre. The boat mechanic will flush the lines to remove any residue which is causing a blockage. They can also provide you will a fuel stabiliser, which will help to reduce the rate at which any gas in your tanks evaporates.
There is a step you can take to prevent future problems. If you will not be using the boat for a few weeks, one solution is to drain any fuel from the boat's delivery system. Doing so will prevent fuel which is sat in the lines from evaporating and creating the oily residue. Most boats have a drain plug fitted near the carburettor which can be used to remove fuel.
For further advice, contact an outboard motor specialist.