A Comprehensive Approach to Diagnosing Carburettor and Fuel System faluts in Outboard Engines

Hi Guys, this is one of the most common faults that we see in outboard engines – fuel related issues are the bane of every mariner’s life, so Symptoms first

1. Old/stale/watery fuel

Modern fuels are crap – to put it bluntly. E10 is akin to having a water sponge in your fuel tank -the ethanol just hoovers up water in the surrounding air in spades. then it just dumps it in the bottom of the tank due to the relative density of water and fuel. This manifests as a sputtery engine, or one that appears to run for a few minutes then just inexplicably stops and won’t restart.
Diagnose this by draining the fuel tank into a white or clear bucket – you will be able to see the water in the bottom, it will appear like a submerged bubble of a different colour to the rest of the fuel. The fuel will often change to a browny colour too. Fix it by fully draining and cleaning the fuel tank, then refil with fresh fuel, also flush out all your fuel lines nadd engine carb etc with fresh fuel.

2. Dirty Carburettor – three possible faults here:

A) Idle Circuit:
Manifests as an engine struggling to maintain a steady idle, with symptoms including rough idling or the engine stalling when throttled up from an idle state.
B) Stuck-Open Needle Valve:
Results in an overly rich fuel mixture, evident through black smoke from the exhaust or a rich exhaust smell, along with a rough-running engine.
C) Dirty Main Jet/Emulsion Tube:
A clogged main jet or emulsion tube restricts fuel flow, causing the engine to run lean at higher speeds, which can lead to a lack of power and responsiveness.

3. Faulty or Tired Fuel Pump:

A malfunctioning fuel pump can lead to insufficient fuel delivery to the carburettor, resulting in poor performance or difficulty starting the engine. Symptoms may include a lack of power under load or an engine that sputters or dies during operation.

4. Blocked Fuel Filter:

Fuel filters play a crucial role in keeping contaminants out of the engine. A blocked filter can severely restrict fuel flow, leading to symptoms similar to those of a dirty main jet, such as an engine that runs lean, struggles under load, or fails to reach full power.


1. Disassembly:

Best Practice: Begin by carefully documenting each step of the disassembly process. Taking photos or videos with your phone can be incredibly helpful for reassembly.
Common Mistake: Rushing through the disassembly and not keeping track of where each part belongs. This can lead to confusion and potential damage during reassembly.

2. Inspection:

Best Practice: Inspect all parts for wear and damage, particularly the needle valve, gaskets, and o-rings. Look for signs of corrosion or debris.
Common Mistake: Overlooking small components or failing to inspect them thoroughly, which can result in unresolved issues even after cleaning.

3. Cleaning:

Best Practice: Use a carburettor-specific cleaner to thoroughly clean all parts and passages. Use compressed air to blow through the passages to ensure they are clear.
Common Mistake: Using aggressive tools or wires to poke through the jets and passages, which can damage them and alter fuel flow.

4. Soaking:

Best Practice: For heavily soiled components, a soak in carburettor cleaner can help loosen stubborn deposits. Ensure that the cleaner is compatible with your carburettor’s materials.
Common Mistake: Soaking parts that are not chemical-resistant, like certain plastics or rubber components, which can lead to damage or degradation.

5. Drying:

Best Practice: Allow all parts to dry completely after cleaning. Compressed air can speed up this process but do so gently.
Common Mistake: Reassembling the carburettor while parts are still wet with cleaner or water, which can lead to immediate or future problems.

6. Reassembly:

Best Practice: Refer back to your documentation for the correct placement of all parts. Replace any worn or damaged components, particularly gaskets and seals, to ensure a good seal.
Common Mistake: Overtightening screws and bolts, which can strip threads or crack the carburettor housing. Use a torque wrench if possible, adhering to the manufacturer’s specifications.

7. Adjustments:

Best Practice: After reassembly and reinstallation, make necessary adjustments according to the manufacturer’s guidelines. This might include idle speed and air-fuel mixture adjustments.
Common Mistake: Skipping final adjustments, leading to suboptimal engine performance or further issues.

Additional Tips:

Stay Organized: Keep screws, bolts, and small parts in labeled containers during disassembly to avoid losing them.
Use Quality Parts: If replacements are needed, opt for high-quality parts that match OEM specifications.
Safety First: Always work in a well-ventilated area and wear gloves and eye protection when handling chemicals.
Don’t forget to not only clean the carburettor but also inspect and potentially replace the fuel pump and fuel filter if they show signs of wear or blockage.
By tackling both the carburettor issues and the broader fuel system problems, you ensure a thorough restoration of your engine’s performance, safeguarding your time on the water against unexpected interruptions.
Hope that helps and if you need a carb repair kit then shout us up!!
Oh and if all this feels like a bit too much trouble then why not book your engine in with Higham’s Heroes who will be very happy to get your prop spinning again!

or those of you visual learners – here’s the video

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