Solving for stalling at idle on a Honda BF15a 4 stroke Outboard
This post is a quick share on my findings when solving idling issues on my Honda 15hp outboard. A fairly common problem for small carbureted outboards, and engines in general, is problems idling, especially after long periods without use, e.g. winter lay up. Think lawnmowers that won’t start or stay started!
Start with the easiest checks – Fuel, Spark & Fuel Filter
Step one is to ensure the engine has fresh fuel (modern petrol is said to “go off” quite quickly), and that we have a good spark. New spark plugs are also cheap and very easy to replace (NGK DR6HS). If that doesn’t do the job then we turn to the fuel system.
The first port of call for the fuel system is the fuel filter. If fuel can’t flow easily then stalling is more likely including at idle. In my case this is situated on the rear right hand side of the engine. Unclipping the filter and carefully blowing through onto a rag, should reveal if it’s gunky. A new filter will blow through quite easily. If it’s older than a couple of years it’s likely worth replacing anyway.
If that fails to rectify the issue we turn to a very common cause of problems with small carburated engines, both two and four strokes. A gummed up carb. Especially when the carb is left sitting for some time, e.g. months over the winter with some fuel still in the carb, this can partially evaporate and go off leaving varnish like deposits in the carb.
The basic theory on carbs and idling
Carbs are theoretically fairly simple in function. They provide the right mix of fuel and oxygen to the engine to run. As the throttle is opened, more fuel/air mixture is provided to the engine and as the throttle is closed, less. The throttle controls the volume of air flowing through the carb, the venturi effect (speeding up of air through a narrowing) providing a pressure differential to suck in the fuel. The right amount of fuel is provided at different engine speeds through some clever design of nozzles/jets to ensure fuel is vaporised at just the right level. Commonly carbs feature an idle jet setup designed to offer the appropriate mixture for slow running.
This all relies on tiny holes in brass jets which can easily become clogged. If the slow/idle jet system is clogged in any way the engine may lose the ability to idle smoothly.
The carb also has an idle adjustment screw who’s job it is to keep the throttle slightly open and ensure some airflow. This doesn’t always work though and is something of a setting to a properly functioning carb, not a fix to a malfunctioning one. If you have a carb that won’t idle appropriately despite adjusting this it stands to reason that the carb is not able to provide an appropriate level of air/fuel mix at low air flow levels.
Cleaning the carb
Having freshly fueled, changed the plugs and fuel filter I did see some improvement with better idle although still some stalling. So I set about removing and cleaning the carb. On the Honda this is very straightforward. More so than I expected. The tools required are:
- A 10mm socken on an extension bar, I use a Bahco 1/4 set,
- A pair needle nose pliers to release the fuel hose clips
- A posidrive screwdriver
- A large flathead and small flathead screwdriver.
- Carb cleaner spray.
- A rag and ice cream tub.
- A small sharp, brass wire is ideal, to unclog a jet if required.
The steps to remove and clean the carb are as follows:
- Find the carb. On the BF15a facing astern this is on the right hand side of the engine. It is clearly identifiable by the attached black plastic air intake.
- Remove the hoses. There are two to remove before the carb can be removed. A breather hose which is just pushed onto the top of the air intake. A fuel hose which is clipped to the rear of the carb.
- Unbolt the carb. The air intake and carb are bolted to the manifold with long 10mm bolts. These are easy to reach with the 10mm socket and bar although a spanner could be used.
- Once the bolts are loosened the carb will still hang in place on the throttle and choke linkage wires. To remove it fully gently pivot it up and out to lift off the linkage.
- Once the carb is removed it can be gently seperated from the intake and inspected. If there are clear signs of gunge around the airways etc. these are good to remove with the carb cleaner spray and rag. Note: the bowl on the carb will likely still contain some fuel so handling over the ice cream tub is handy to contain spills.
- To clean the jets/nozzle it is necessary to remove the float bowl. This is held on by 4 posidrive screws on each corner of the bowl.
- Once removed you can see and check that the float and needle valve are cleaning and undamaged. The pin holding the float in place can easily be withdrawn with the pliers.
- Next taking the large flat blade screwdrive you can unscrew the central brass cap, screw plug, which conceals the nozzle. This can be gentle removed with the central tube (this is held in place with a small o-ring, and all holes inspected for blockages. A good spray of carb cleaner should suffice with a quick poke with brass wire if required.
- The main jet which is screwed in at right angles to the base of the nozzle, can also be unscrewed with the fine bladed screwdriver. It can also be inspected for blockage.
- Have checked and cleaned all of these parts everything can be reassembled in the opposite way to dissassembly.
- NOTE: It is possible that despite the above the carb still contains deposits that are not easy to shift. For this the recommendation is to clean the carb in an ultrasonic bath.
Having cleaned the carb, and ensured fresh fuel, plugs and fuel filter the engine should now idle nicely. If it doesn’t then other problems would be anything else that are restricting fuel flow. Bad fuel lines, air leaks, fuel pump.