... Also look for welds that were not made at the factory. Check the prop for any signs of damage and wobble. Don’t forget to ask for the owner’s manual. You’ll need a compression test to check the cylinders. Compression between cylinders should not vary by more than 10%. Any more than that indicates excessive cylinder and ring wear or possible valve leaks on four stroke engines. Hire a mechanic to perform the inspection if you are not experienced with used boat motors. A good mechanic could prevent you from making a very costly mistake. The money spent is well worth the price and will make the purchasing decision a little smoother. ... Buying a used boat motor is a lot like buying a used car. You can save quite a bit of money and enjoy reasonable service if you know what to look for. The opposite is also true. You can have a ton of problems and find yourself stranded in the water. Believe me, this is no fun at all. It can also be dangerous if the motor decides to fail when you are a long way from shore or trying to outrun rapidly changing weather. When buying a used boat motor, there are several important items to pay attention to. If it is currently mounted on a boat, take it for a test ride. Run the motor through all ranges of power. Make sure it idles smooth and comes up to full speed without hesitation. Look at the water pump discharge to see if the pump is working properly to keep things running cool ...