Carb101: Metering Chamber

For those of you following this Carb101 series, you'll be aware that we've followed the fuel from the tank, through the impulse chamber and it's just about to arrive at the metering chamber. So let's see what happens here, after the jump...

With the fuel having come via the fine gauze filter as it left the impulse chamber, it now finds itself entering the second chamber. However, access to the metering chamber is controlled by a second diaphragm.

To get access to this area, we need to take the cover off of the other side - the large 'pipe' coming from this cover (see photo right) is essentially a vent pipe that is open to the atmosphere. As this carburettor is from a Stihl MS260 (for those of you au fait with this model), the air filter fits over this pipe. The cover is removed by unscrewing the four small screws holding it in place. Once the cover has been removed you are faced with the top side of the metering diaphragm.

Notice that there is a small metal 'rivet' in the centre of this diaphragm and if you push it down gently then you should feel that it is sprung. The photo below shows the top side of the diaphragm, underneath that is the view of the carburettor with this diaphragm removed.

The fuel is allowed in to the metering chamber when the diaphragm pushes down on the other side of the sprung lever. This has the effect of pulling the needle up, and away from it's seat - thereby allowing fuel past it. In it's normal resting position the needle is held shut by the pressure exerted from the spring on that lever. Using the photo on the right as our reference, off to the left of the needle mechanism, is a brassy area with a hole in it. That's the main jet where fuel is sucked up and in to the main air stream, to be fed in to the engine.

You can see this jet in the photo on the left. We've now followed the path of fuel from tank to engine, but there's a problem - with the saw running at low speed (idling), there's not enough draw on the fuel to pull it through this jet. We'll have a look in the next post to see how this was overcome.

Carb101: Impulse Chamber

In the last post we took a slightly closer look at how the diaphragm carburettor works - now we're going in even closer, with a look at the impulse chamber, straight after the jump...

We have seen that the diaphragm is used to pump fuel from the tank, in to the carb, and that it's controlled by pressure pulses from the crankcase. In this post we'll take a closer look at the impulse chamber.

The bit we're interested in can be seen on the photo on the right - that small brass pipe is connected to the crankcase at one end, and feeds in to the impulse chamber at the other end. Removing this cover (a single screw) reveals a gasket and the actual impulse diaphragm - I'm not exactly sure what material the diaphragm is made of, but it looks a little like mica.

The diaphragm has a couple of small flaps cut in to it, these cover two small holes that lead in to / from a small chamber. Those flaps are controlled by the pressure pulses being sent down that brass pipe.

On the left, we can see the actual diaphragm, with the two flaps - one just above the central screw thread, and the other slightly down and to the left of the central screw thread. The flap at lower left covers the input from the fuel line (linked to the fuel tank). On the right hand side of the photo, you can see a chamber underneath the diaphragm - consider this as a little reservoir for the moment. The flap above the central screw thread is the one controlled by pressure pulses (we'll call it the impulse flap) - as these pulses are received, the flap will open and close, the effect of which is to draw fuel from the fuel line, through the first flap and down the small hole that you can clearly see next to this first flap. The fuel is then held in that small reservoir area, until it gets pumped up through the impulse flap and down the small hole next to it. You can just make out a large circle under the diaphragm at this point...

...which has a very fine gauze filter in it - as you can see in the photo on the right; which shows the area with the diaphragm removed. This is actually the top end of a hole drilled straight through the carburettor, leading to the metering chamber on the other side; in fact at the other end of that hole is actually a needle controlled by another diaphragm.

So far then, we've followed the fuel being pumped from the fuel line, through the flaps cut in to the diaphragm and through a fine gauze filter to be delivered to the metering chamber. In the next post we'll continue to follow the fuel's journey through this second chamber and out through the main jet.