Boat Motor Loses Power at Full Throttle: Here’s What to Do

September 24, 2021

They say, “A bad day of boating is better than a good day at work.” Well, it wouldn’t exactly feel that way if you were adrift in the middle of the ocean, with a boatload of grumpy passengers, and your engine decides to lose it at full throttle! 

If you’re handy at troubleshooting engine issues, this is the place for you. There’s a multitude of potential reasons to consider, but no worries – we’ve got you covered! Here’s what you need to save the day if your boat motor loses power at full throttle.

First, Let’s Get to the Root of This Snag

To troubleshoot the problem, you need to first check and eliminate the following possibilities one by one until you finally come to the exact root cause. 

1. Fuel Shortage or Leakage

We could actually lose track of our fuel consumption just as we lose track of time, so do check your tank as a first step. You may need a refill to run at full throttle.

If, however, there’s a leak somewhere, you need to quickly fix it to avoid the risk of a (costly) fuel spill, as well as more serious catastrophes, like fires or explosions—perish the thought!

If you find plenty of fuel and your tank isn’t leaking, then your next move is to inspect the fuel filter.

2. Clogged Fuel Pump Filter

If you have a spare in-line filter, great! If not, you can remove your current filter and check for any debris you can clear. Also, make sure to drain any accumulated water if you find any in your fuel. 

Own an inboard/outboard (I/O) boat? Make sure to thoroughly ventilate the engine box before you restart your engine. Missing this critical step can make your clogged filter seem like a minor problem compared to the new one!

In the case of a clogged filter, you can easily clean it with a few simple steps. We also recommend getting a larger aftermarket filter.

3. Fouled or Corroded Spark Plug Wires

Now, if you do have fuel in your tank and your filter is just fine, it’s time to check the spark plugs. Some points to keep in mind:

  • Check them one by one so you won’t mess up the order when putting them back.
  • Inspect the wires carefully.
  • If you find any fouled plugs, clean them up and remove all the gunk as best you can.
  • Use a rag or a piece of cloth to remove as much build-up as possible.
  • A knife or anything similar can help you scrape off large chunks of build-up, but be careful not to cause any damage to the spark plug or accidentally adjust the gap.
  • To inspect the plug wires, simply check them for signs of age, like brittleness or cracking.

4. Clogged or Worn Carburetor

Speedboat in sunset

Carburetors provide the mixture of air and fuel into your internal combustion engine, controlling their flow and ratio. 

If the carburetor is clogged or worn, it fails to efficiently balance the air and fuel mixture required for your engine to run properly at full throttle. This could cause stalling. 

Also, your carburetor could be all clean but adjusted incorrectly, preventing sufficient fuel reach into the engine, and thereby causing power loss at full throttle. 

Check your carburetor for any grime or dirt and clean it thoroughly. When it’s perfectly clean, adjust the carburetor’s settings to provide your engine with the proper fuel-air mixture, then try again riding at full throttle.

5. Debris in the Gas

If you have a relatively old fuel tank, look for any debris at the bottom. As the fuel level drops, debris can get stirred up down there. Tip: Increase filtration to avoid this issue.

Fuel is a volatile substance, so make sure to regularly clean your tank in a ventilated area, away from sparks or open flames. 

Here’s how you can clean the debris:

  1. Take off the fuel line.
  2. Remove the fuel cap and plug a vent pipe (tank should be well-sealed). 
  3. Use a vinyl tube; place one end in a container where you should keep the fuel debris and the other end in your fuel tank.
  4. Connect your air compressor to the tank and switch it on.
  5. Gradually increase the pressure until you fully drain your tank.
  6. Using a wet/dry vacuum, clean any remaining debris.
  7. Add a fresh supply of fuel.
  8. Start your engine.
  9. Damaged Fuel Line

Is the engine still refusing to function properly? Have a closer look at your fuel line; is it damaged or cracked? The fuel line can collapse over time, causing poor engine performance, especially at full throttle.  

It could be preventing the fuel from flowing properly, so inspect it well to make sure there’s no problem there. Leaving fuel in the line for many months can cause the fuel to gum up and form blockages. 

Try removing any debris from the line using a screwdriver. If you can’t, it’s time to replace your fuel line to save your boat motor from any potential damage.

7. Water Buildup

Another possible cause is water buildup in the gas. Condensation can be a major issue with gas tanks. 

When there isn’t a lot of fuel, air is drawn into the void in the tank. The moisture in the air then condenses inside the tank, diluting the fuel.

As expected, as soon as the water-mixed fuel goes into your engine, performance issues arise. Condensation can also cause bacteria and fungi to grow inside your tank.

To clean water buildup in your fuel tank:

boat in water
  1. Check for water in the fuel.
  2. Siphon it out. 
  3. Fill your tank almost completely with fuel to avoid condensation, making sure to leave at least 2% of the tank empty to allow for expansion. 
  4. An easier way out is to simply drain your tank completely and refill it with fresh fuel.

Consider installing a water-separating filter between the tank and motor to keep your engine free of water buildup. Also, if you’ll leave fuel in your tank for a long period, you might want to get a fuel stabilizer.

8. Low Compression

Low compression is also a potential cause of power loss at full throttle. You’ll need a compression gauge to do a compression test

  1. Disable the engine to prevent it from starting while you’re testing the cylinders.
  2. Take off the spark plug and ignition coil from the cylinder you’re testing.
  3. Screw-in the compression gauge extension.
  4. While someone cranks the engine, watch the gauge closely till it reaches maximum compression. 

A healthy engine typically has 100 PSI per cylinder. If two cylinders alongside each other both have low compression, then a blown head gasket is probably the one to blame.

So, what should you do if you have low compression? You’ll need to replace the leaking part, which could be the head gasket, valves, camshaft, piston, or piston ring. 

More than one of these could be faulty, so make sure to check them all. Depending on what needs replacement, you might be looking for some costly repair!

9. Bad Fuel

Another overlooked cause is as simple as, you’ve gotten very unlucky and unfortunately purchased a bad load of fuel. In such a case, you’ll need to urgently replace the fuel to protect your engine from damage. 

If the fuel is contaminated, siphon it from the tank and replace it with fresh fuel to see if that fixes your issue.

Still Clueless on Why Your Boat Motor Loses Power at Full Throttle?

If the problem still isn’t fixed by this point, you’ll need to check the terminals and connection points. Noticing any damage, breakage, or corrosion? It’s time to replace a few terminals.

Checked all of the above and still stuck? Well, boatie, it’s time to call a repairman!

Spares and Tools You Should Carry Onboard

In case of any at-sea surprises, you need to be prepared. Make sure to have these with you to facilitate troubleshooting:

  • Spare fuel filter
  • Oil filter wrench
  • Clean rags
  • Socket and ratchet set
  • Proper crimping tool and terminals
  • Fuel stabilizer

A Multimeter Could Save You

motor boat screws on sea water

As a boatie, you probably have a multimeter stashed somewhere aboard your boat but have no idea what massive help it can be! 

Using a multimeter can help you quickly determine whether there’s an underlying electrical problem. Digital multimeters monitor and test a boat’s electrical system, so consider using one if you face any snags at sea!

A Final Note

We hope this guide could help you troubleshoot any underlying issues if your boat loses power at full throttle.

Now you know the most common causes and what you can do to fix them. If the problem persists, however, we’d advise you to contact a certified boat mechanic to help you out with your engine hiccups!

Also, we highly recommend that you choose a high-quality engine for your boat to avoid any snags while on board. We hope you enjoyed this article on what to do if your boat motor loses power at full throttle.

A quality marine engine guarantees reliable long-term service with rare setbacks. It won’t, however, remind you to fill up your fuel tank or to stop draining your battery by over-playing your stereo! Happy boating!

Happy Hunting!

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