Carb101: Idling

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.

Makita DCS4301 Review: Under The Covers

The air filter on the DCS4301 is hidden under the grey rear housing, which is easily removed by turning the large, black dial anti-clockwise a couple of turns. The dial itself is easy to grip and the cover is also secured by two plastic prongs along the front edge that must be positioned in to the main engine housing before tightening up, following removal to gain access to the components underneath. As for maintaining the filter itself, find out more after the jump...

Removing the top cover reveals the filter, access to the spark plug and the winter / summer setting. The air filter is held in by a wire clip, and what is nice is that the filter and the clip is shaped to make it a very simple affair to remove the filter quickly. There are no screws or nuts to undo, and although I find the twist-on, twist-off air filters (a la Husqvarna 350) easier, the wire clip really is no trouble and it does hold the filter on securely.

The spark plug is just forward of the air filter and the plug cap is moulded allowing you to get a good grip on it - useful when it comes to removing the plug. You do not need to remove the air filter to get the spark plug out (users of the Stihl MS260 would love that feature, although to be fair, if you've got a stubby combi-spanner you can get away with not removing the air filter when it comes to removing the plug).

The only other bit to play with under the top cover is the winter / summer setting - although it's doubtful that (in the UK at least) you'd ever need to take it off the summer setting. However, if you find yourself working in the cold winters of Scandanavia or similar, then it's just a matter of pulling the plastic cover up, turning it over and replacing it in the new position.

Other than these three things - filter, plug and winter / summer, there is nothing else for the user to touch; indeed the moulding doesn't even allow you a peek at the cylinder housing. Makita actually advise that you use a bottle brush from the side to clean out dirt lodged in the fins of the cylinder housing. All in all, it's very neat and certainly well-suited to the target home- / farm- users.

Next post... more on the DCS4301 chainsaw.

Air Filters...

The air filter plays a critical role in the smooth running of your chainsaw, and it's important that you look after it properly. There are several different ways of going about cleaning the filter and several different styles and fitments for filters too, so let's take a slightly deeper look after the jump...

The filter has one clear, obvious function in life, and it should come as no surprise that it's to filter the air prevent debris from entering the carburettor.

In order to run, the engine needs a mixture of fuel and air, and with that in mind it's easy to see that a clogged air filter will upset this balance. During use the engine is sucking in air via the air filter, and along with the air is all the dust and dirt that you're throwing up whilst sawing - hence the need to filter the air.

With the air filter choked up with sawdust, there will be a lack of air in that fuel:air ratio and this will cause the machine to run rich.

Fortunately, cleaning the air filter is a simple affair, and it doesn't get much simpler than using a brush (a clean, unused paint brush is ideal) to swipe the dirt from the face of the filter. The idea is to gently brush the dirt off - not stab it further in to the mesh!

There are a couple of other ways to clean the air filter...

• Wash it in warm soapy water - remember to rinse it, otherwise you'll end up with a bubble machine next time you start the saw.


• Use an airline - you may need to split the filter in half in order to blow the dust off of the face (i.e. blow from the inside -> outside). Make sure that it's the type of filter designed to be split in half before trying it ;-)

There are many different styles of air filter in use, even with machines made by the same manufacturer, so you might find that you need to unscrew the filter, twist it to remove it or fiddle around with those annoying little plastic clips simply lift it off.

One things for sure - if the filter and surrounding area is pretty filthy then you'd be wise to use that brush to remove 95% of the dust before removing the filter. Once the filter is removed, you might consider putting a loosely scrunched up ball of tissue gently into the carb opening; this will prevent small children and furry animals from falling in to the carburettor whilst the filter is off. It's also quite a good way of preventing more dust from getting in to the carb.

That's about it, for now, with air filters - keep it clean and you'll be helping your engine to run at it's optimum.