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.