2 Stroke Outboard Fuel Problems

Find & Fix 2-Stroke Fuel Problems

2 Stroke Outboard Fuel Problems

Two Stroke Outboards - when working well - are fantastic. Reliable, economical and portable.

But they can be tempermental things, especially older two strokes where the rigours of time and a lack of love from previous owners can lead to an unwillingness to start, not idle well or a lack of power.

Blocked Two Stroke Outboard Carburettor Fuel Problem
Stale Two Stroke Fuel turned to Rice Crispies in this Yamaha Carburettor Float Chamber

In this article we'll explore common fuel problems for older two-stroke outboards i.e.

  1. Dirty or Defective Carburettors
  2. Stale or Contaminated Fuel
  3. Fuel Delivery and Flow

Keep in mind that 2-stroke outboard fuel problems typically fall into one of two categories; Fuel Starvation & Flooding.  Both will impact performance and both can potentially turn a good engine into a non-starter. Both are intrinsically tied to the correct mixing of the fuel.

As we're talking about older marine engines with this invariably be job of the carburettor. So what does this look like?

Video of the Carburettor Throttle Lever and Main Jet in action

This short video shows what happends when you opening and close the throttle on a Mariner 2HP carburettor.

Viewing the main carburettor jet move up and down to supply fuel through the air inlet port with the choke flap wide open

Do Carburettors 'Break'?

Two Stroke Carburettors can go 'out of tune' i.e loose the correct adjustments or have a component failure but this isn't that common. Most often they have got dirty or degraded to the point of malfunction as a result of poorly mixed, old or contaminated fuel.

The good news is that carburettors and are actually quite simple mechanical components (particularly on smaller engines) but it's important to start with an understanding of the  the basic elements that make up a carburettor:

  1. Inlet pipe - attached to fuel supply hose from fuel tank
  2. Float bowl or chamber - self-regulating store of fuel ready to supply the main jet
  3. Main Needle and Jet - the needle and jet combo that open and close as you adjust the throttle
  4. Venturi space or chamber - the area where the fuel and air are actually mixed as the needle allows fuel to vaporise in the air flow
  5. Exit port - the fuel/air mix enters the combustion chamber ready for compression and ignition.

Stale or Contaminated Outboard Fuel

If an outboard hasn't been run in a while -- anything over a couple of months -- or you have recently aquired an outboard containing fuel and don't know the history it's likely there has been some degradation of the fuel in the system due the impact of aging.

A note on Ethanol or '5% Fuel'

More recently the addition of greater and greater proportions of alchohol or ethanol to the fuel, e.g. 5% ethanol or even 10% ethanol under the guise of 'environmental reasons' causes problems. Especially for outboards that may not have been origianlly designed for such fuels.

Crucially these alcohol additives absorb moisture over time reducing the performance of the fuel and degrading in your engine.

See this useful article from MB Marine for more on Ethanol Fuels

Drain Down - Start from a good base

Often the best course of action is to simply drain down the fuel tank, delivery hose, fuel filter and carburettor float bowl (there is usually a small drain screw on the underneath of the bowl - often accessible through the cowling of easy access)

A simple inspection of the fuel tank and filter will give you a clue to the level of deposits or particles in the system but with old or ethanol-based fuel any degradation will be harder to detect. The safeest bet is to drain down and dispose of the old fuel (i.e. local recycling centre)

Contaminated 2 stroke outboard fuel
Contaminated 2 stroke outboard fuel with numerous rust particles

With clean fuel in the tank, good supply through the fuel filter and past any isolation tap and delivery of said fuel to the carburettor further starting or running problems due to fuel will be down to the carburettor.

Time to remove it ;o)

Dirty or Defective Carburettors

Removing the carburettor is quite simple, they usually have a ring clamp or perhaps 2 or 3 bolts attaching it to the inlet side of the crankcase. Isolate the fuel supply via the tap, remove the input hose, disconnect the throttle and choke inputs and then unbolt/unscrew the unit from the engine.

A note of caution at this point. The carb contains a number of very small pieces that are easily lost through slippy fingers. It's highly recommded to work at a bench with sufficient space and over a towel or better a metalic parts tray to catch any parts that may attempt to escape.

With the carb on the bench it's time to start dissembling it, starting with removal of the float chamber.

Checking the Float Chamber

The float chamber feeds the mixing jet so needs to be continously 'topped up' to the pre-requisite level without overflowing. This is typically achieved via the float that rises and falls as the fuel level changes allowing the needle value it's attached to open and allow more fuel in or shut off the supply until more is needed.

Small Outboard Carb Float Chamber with Fuel
Base of carbeurettor removed revealing a clean float (black plastic) 'floating' on clean fuel.

Check the bottom of the float chamber of dirt or crusty traces and that the twin pin hinged metal arm it pushs moves freely up and down with the needle valve.

Carb Float Bowl with Swing Arm and attached Needle Valve
Carb Float Bowl with Swing Arm and attached Needle Valve

Assuming the float chamber and float are clean with a freely moving needle valve attached to the swing arm it's safe to eliminate this as a cause of problems and move on to the carbuerretor jets.

More likely to the source of the problem.

How to Check Carburettor Jets

Carburettor jets have a very fine tolerance or 'operating range' so they one of the most sensitive engine components to the scenario of dirty, stale or incorrectly mixed fuel.

There are hundreds of designs for carburettor jets so in this article we are going to continue using the basic design on a Mariner 2HP. The concepts are the same in larger enginess.

With the bottom of the carburettor (float bowl) removed we can see the brassing housing containing the main mixing jet.

Yamaha Mariner 2HP Carburettor Base
Mariner 2HP Carburettor showing bottom of brass mixing jet housing

In this simple carburettor we have just one jet to deal with, on larger outboards they may be separate jets for main throttle vs. idle etc.

Carefully unscrew the jet from the base of the brass housing.

Be gentle when removing and replacing carburettor jets as they are easy to over-tighten and stripped the thread - often being made of soft brass.
Main Jet from carburettor of Mariner 2HP 2 stroke outboard
Main Jet from carburettor of Mariner 2HP 2 stroke outboard

Hold the jet up to the light to check the opening. You should see a clear (small!) consistently round gap through the middle. This is the tiny gap that powers your entire engine!

How to clean a blocked Jet

If yours has a build up of crust around it or is partially blocked or worst no hole at all then this will impact the engine performance. It may be the entire source of your problem but more often than not just one part of the fueling problem.

To clean your jet spray on a cleaning fluid and gently just brush or poke out any build-up or blockage. You can purchase specialist carb jet cleaning tools with different gauges but and old piece of wire with the correct diameter and soft wire brush will suffice.

Good example of before and after cleaning of outboard carb jets from a professional service outfit.

Flooding - Too Much Fuel

At the other end of the scale to fuel starvation there is flooding. i.e. too much fuel entering the combustion chamber. A flooded engine is unlikely to start as instead of a blend of vapourised petroleum to correct air volume mixture there is liquid fuel saturating the combustion chamber.  Likely 'wetting' the spark plug to point it is unable to generate a spark to generate combustion.

A flooded engine can be caused by:

  1. Over zealous use of the choke mechanism
  2. A another restriction of the air supply through the inlet port (e.g. fluff, bugs, spiders in the air inlet)
  3.  Too richly oiled 2-stroke mix making it to heavy to vaporise through the carb.
Choke Flag on 2HP Mariner Carburettor
Full Open i.e. 'No Choke' Flap on 2HP Mariner Carburettor

You can often tell if an engine is flooded as you will start to smell the fuel. On manual pull chord outboards this will be present within 3 or 4 pulls. You can also remove the spark plug and check the tip. When an engine is flooded engine the tip will be wet.

The best remedy for a flooded engine is to just leave it for a while. perhaps 5-10ins. The fuel will naturally evaporate over time.

Starting Procedure - Check your Owners Manual

If you engine consistenly floods on start when you use the manufacturers procedure from the owners handbook then it's likely there is something amiss. Check the air inlet flow and carburetter supply jet.  Also, confirm you have the correct spark plug as some run hotter than others.

Thats it, hope this article helps. Any comments/requests add them below.


  1. Thanks for the post. My outboard has been sat all summer with no use. When I drain it down is it worth changing the fuel filter at the same time?

    • Hi Aaron, you don’t mention which brand or model but assuming your comfortable with it then yes I would definitely change the fuel filter[s] at the same time as a full drain down. The reason I mention brand and model is that larger two strokes often have multiple fuel filters and or strainers. Check for a strainer/pre-filter inside the fuel tank, plus an inline filters in the supply hose or the outside of the carb housing.

      Good luck!

  2. Just acquired a Mariner 6hp cira early 90s. Not sure how long it has been sat but the fuel has a distinctive dark blue colour to it and wondered if I should get rid of it.

    Is there some kind of additive that would make it blue?


    • Hi Brad, sounds like there may have been some kind of additive used or it may just be an overly oily mix. (i.e. too much oil for the volume of fuel).

      Either way I would definitely do a full drain down of tank, lines and carb and flush with new fuel before trying to start it. You don’t want to pull crap fuel through and make your job harder.

      Good luck

Leave a Reply

Your email address will not be published.