Links To Mechanical Sympathy

The full list of photos I took when disassembling the carb are available online at Photobucket.

http://s49.photobucket.com/albums/f267/WelshAl/Carb/?start=all



Most of what I know has come from Haynes Manuals, talking to people and helpful online resources such as user-generated advice pages and forums.

I link to some here, in case it helps, although most are bike-related.

http://www.dansmc.com/MC_repaircourse.htm

http://www.gadgetjq.com/keihin_carb.htm

http://www.gbbikers.co.uk/gbb/

http://www.ybw.com/ybw/home.htm

5. The Carburettor Throttle

This is the front of the carburettor, and you can see all the way through the airway because the throttle is fully open.
The vertical line is the needle valve, which governs the fuel flow.


You can see that the end of the needle valve has a point but it is also delicately tapered.
This is what increases and decreases the flow of fuel into the air flow.

As the needle rises, the taper increases the amount of fuel that can be sucked up into the mixture, and conversely as the needle is lowered it cuts down on the amount of fuel that can be used.

4. The Carburettor Disassembled

The carburettor front, from below, shows the drain screw in the bottom of the float bowl. The washer is important, as a good seal is necessary to stop fuel leaking out.

The drain screw is there to drain fuel from the float bowl without needing to remover the carb from the motor, but can also be used to diagnose fuel issues by showing the progress of fuel in the process.

You can see by eye if there's fuel in the tank, and if you have a clear in-line filter you can see if there's fuel after the tap, and if you undo the drain screw a bit you can see if the fuel is making it to the carb itself.

This screw should be accessible below the front panel of the motor, though when attached you will have to turn the motor to one side to gain access as there's usually little room between it and the transom to which the engine is attached.


Two screws hold the float bowl on to the carb body, with a gasket in between to stop leakage.



With the float bowl off, you can see the float bowl on the left, the black plastic float on the right, the float spring that holds the float needle and the brass main jet.



When the fuel passes down the fuel connector, here the lower brass-ringed circular aperture below the center hole, it runs into the float bowl.
The black plastic float rises as the level of fuel increases. The fork-shaped float spring has a small channel in it to which connects the float needle, which rises into the passage under the fuel connector.
As the level of fuel in the float bowl reaches its optimum, the float needle touches the fuel connector and stops the fuel flow until the level in the bowl is reduced and the float falls, allowing the float needle to fall and more fuel to flow.

It is important that the float is air-tight and the soft taper on the float needle is in good condition, otherwise the carburettor could flood and engine performance will suffer.

The main jet, visible here as the threaded brass nut to the left of the float spring, governs the amount of fuel that can pass to the needle valve. The hole through its center is very small and must be kept clear and in good condition, not burred or distorted. Like the needle valve, it can be changed for bigger/smaller sizes as a tuning technique, but this is irrelevant for maintenance and operation of a standard engine.

3. The Carburettor, Choke Operation

This is the Front of the carb, with the throttle fully open and the choke off. The oval-ish paddle above the main airway lowers when you raise the arm, here on the right of the picture, creating a restriction to the air flow, so enriching the fuel-air mixture.




Here we can see the front of the carb with the choke lever up, so the choke is 'on'. The small hole in the paddle allows some air to flow, but the restriction increases the amount of fuel in the air-fuel mix, so helping the motor to start in certain circumstances.

2. The Carburettor Dismounted

This is the front of the dismounted carb. Visible is the oval-ish choke paddle. The choke lever, here on the right of the picture, moves this paddle over the main airway. Restricting the air in this way makes an artificially rich mixture which can help start the engine in certain circumstances.
I'm not sure which circumstances they are though, as the use of any form of choke on any engine I've ever used has invariably made me turn up late and stinking of petrol, regardless of the weather or the time of year.

The circular airway is the chamber in which the fuel and air are mixed. The air is sucked in as the piston in the combustion chamber rises, compressing the previous charge before igniting it.

The large round slider has inside it the needle valve, which controls the amount of petrol released to the mixture.

This is the left hand side of the dismounted carb. You can clearly see the throttle arm on the far side and the choke arm on this side.

The lower brass fitting is the fuel connector, which screws in to the carb body. The fuel feeds directly into the float bowl, which is the rounded, lower half of the carburettor.

The brass fittings visible above the body are part of the throttle assembly. The throttle arm raises and lowers the slider that surrounds the needle valve.



This is the right hand side of the carb. Visible is the throttle arm, here almost fully open. The screw that is visible in the middle is what I assume to be the method for establishing the idle speed.


As you can see here, when that screw is fully wound in, it protrudes into the airway and stops the slider. A groove cut in the side of the slider works with the rounded profile of the end of the screw to provide gradual adjustment.

This provides a lower position of below which the needle valve may not go, for a certain amount of fuel to be available for the fuel-air mix, even when the throttle arm is fully down. Wind the screw out and the slider, and so the needle valve, can be lowered thereby reducing the fuel in the mix and so the engine speed.

1. The Carburettor Body

This is the carburettor for my Mercury 2.5hp 2t (two stroke).

They are mechanically simple, as are 2t engines in general. It has one jet and one needle valve, a simple air restrictor to make a simple choke mechanism and a very simple collar connection to the manifold of the engine.

I haven't had much to do with boat engines, having dealt mainly with bike engines because I seldom have enough money to pay for repairs and servicing. This first photo is a picture of the right hand side of the motor above the carb, with the top panel off of course.

The air in-take is a small hole above and our side of the front of the carb's round mouth. There does not seem to be a filter there nor the mounting brackets for a filter, although I don't know so couldn't say definitively.
On this side can also be seen a screw that I assume to be the method for setting the idle speed, although that'll be looked at and talked about in more detail later.


This is the left hand side view, where you see the fuel pipe connecting to the carb.
The arm visible on the left of the picture, just below the cast 'Open' text, is the choke lever. This moves a paddle in the mouth of the carb that restricts the air in-take, so creating a rich mixture for starting in poor conditions.