Boat won't start

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Every combustion engine needs three things to start and run: fuel/air, spark, and compression. It's best to start in the broadest sense and then work your way down to specifics.


Here is a general procedure to follow to diagnose nearly any motor's starting issues. We use this order because of how common these problems are to occur. If you have an idea where the problem is stemming from, then use the order at your discretion until you've eliminated all options.

1. Fuel/Air[edit]

  1. Do you have fuel? This seems obvious, but double check. Perhaps you're trusting your fuel gauge (when you shouldn't), perhaps you've just forgotten, or perhaps someone you share the boat with didn't fill it back up as you thought.
  2. Have you primed your fuel lines? Most outboard motors have a primer bulb that you're supposed to squeeze until hard before starting your motors. This is especially true if the boat hasn't been running for several hours or more. While it's not always necessary to prime the line, you always should to avoid overcranking the motors and relying on the vacuum.
  3. Is the tank properly venting? Imagine turning a milk jug filled with water completely upside down: As the milk pours out, if it is not replaced with air then the flow of milk slows until the jug catches it's breath. A fuel tank is not flexible like a milk jug, however, so if there is vacuum pulling fuel, but a badly clogged vent then it will cut off fuel to the motor.
  4. Have you checked your fuel line connections near the motor? Pull the main line connecting to the throttle body or carb and make sure there is fuel present. Put the line in a spill safe container, squeeze the primer bulb, and make sure that fuel is coming out.
  5. Have you checked the air intake? Are there any obstructions?

2. Spark[edit]

Besides fuel, spark is the next most common issue when a boat won't start.

  1. Does your battery have enough charge to both crank the motor and produce strong spark? (It should be noted for accuracy that the battery doesn't produce spark directly. It cranks the motor which if not cranking fast enough, will not produce strong spark)
  2. Are your spark wires connected to the distributor and the plugs?
  3. Have you visually checked spark?
  4. Check spark
    1. Pull one of the plugs
    2. Connect a spark checker to the plug and make sure you see a strong spark
    3. Work your way through all of the cylinders (or at least enough that you're confident you have spark on all cylinders - at least 3/4. Generally, even if 1/4 of cylinders don't have spark, a motor will fire, but not run well)
  5. If you don't have spark, check all wiring connections to make sure that there are no loose wires or something that has been jolted loose.
  6. If you're seeing weak spark, check your battery again. If the battery is fine, then it's likely an issue with the stator.

3. Compression[edit]

Finally, compression. To produce the explosion inside a cylinder that is necessary to create the chain reaction of a running motor, fuel and air are compressed creating a higher energy potential when spark is applied. If you don't have compression, then you're not going to have a well running motor nor power. Depending on how many cylinders lack compression, this could also be a reason that the engine is not firing.

To run a compression test[1]
  1. Disconnect all spark plug wires, disconnect the main wire to the coil and spark plug wires; remove spark plugs.
  2. Start the threaded end of the compression gauge in a spark plug hole by hand.
  3. Turn the ignition on, and crank the engine four revolutions. This should result in a stable reading; if not, crank up to 10 revolutions, but do the same with all cylinders.
  4. Mark the pressure reading for each cylinder
  5. For a cylinder well below what it should be, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems (if 4 stroke).
  6. The most important thing is that all cylinders are consistent - within 10-15 psi of each other. Consult your engine's service manual for proper compression readouts, but generally you'll know you have a problem if 4 cylinders read 150-160 psi (simply an example. Every motor's spec varies) and 2 read 100 psi.
  • Note: With the example above on a 6 cylinder outboard, the motor will still likely start - just run roughly and not have good power. It's possible it's the starting issue, but not certain

If you have a compression issue, then there is no way around a rebuild. Take your motor to a certified mechanic and have them take a look.


  1. [1], Popular Mechanics