We've pretty much come to the end of this series, having followed the fuel being pumped from the tank onwards. But, as mentioned in the previous post there's a problem. The fuel is drawn in to the engine by the low pressure in the venturi caused by the air flow speeding up as it enters the carburettor; the problem is at low speeds there is not enough air speed to create that low pressure to draw the fuel up. Find out how it was overcome after the jump...
Air speed is crucial in creating the low pressure to draw the fuel in to the air stream and on in to the engine. When the throttle is closed, the butterfly valve is also closed and no air flows - the result is that no fuel is drawn and the engine dies.
To overcome this problem, someone figured out that if a small notch was cut in to the butterfly valve, the air speed past this valve could be increased - enough to draw a small amount of fuel through a jet positioned close it it.
Take a close look at the image on the right (click on it to enlarge), you'll see that the brass butterfly valve that can be seen through the main venturi has a small notch cut in to the bottom right of it (underneath the numbers "285"). That's the notch that allows air to flow through at idle speeds, increasing the air speed. As an aside, that valve is not a circular disc, it's oval in shape and the sides of it are chamfered - if you're taking your own carburettor apart I'd recommend very heavily that you leave this bit well alone! It's difficult to set it back exactly right and you could be faced with a saw that doesn't run too well after the event :-(
So we have now got a means by which we can increase the air speed, causing a low pressure to form - all we need to do now is to get the fuel out of the metering chamber in to that air stream.
Looking through the venturi with the throttle (butterfly valve) open, as shown in the photo on the left, you can see a series of small holes. This is where the fuel is drawn through at idle speeds, but the actual amount of fuel allowed to pass through these is controlled by the "L" (low) setting screw on the carburettor. Turning this screw clockwise, reduces the gap, restricting the fuel flow (and also adjusting the fuel:air ratio). You'll find that screwing this in all the way will restrct the flow so much that the saw will stall, then as you turn it anti-clockwise the revs will pick up, reach a maximum, then die off again. Check your owners manual for the correct setting.
In the final part of the Carb101 series, we'll take a look at some of the adjustments when setting the carburettor up and little look at troubleshooting.