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.

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.

Carb101: Diaphragm Carburettors

We saw in an earlier post that the diaphragm carburettor overcame some of the issues concerning the earlier carbs that used float chambers. Let's take a closer look at these carbs after the jump...

The diaphragm carb actually uses two diaphragms - an impulse diaphragm and a metering diaphragm. The role of the impulse diaphragm is to pump the fuel from the tank in to the metering chamber, with the impulses being provided by a line running from the crackcase to the carburettor; as the piston moves up and down the cylinder, changes in the air pressure within the crankcase are transferred to the carburettor. These pulses of air essentially cause the diaphragm to pump up and down, thereby pumping the fuel.

As this diaphragm is driven by pressure pulses from the crankcase, the pump will work at any angle.

A second diaphragm resides in the metering chamber which could be considered a reserve of fuel, waiting to be mixed with the incoming air dragged in via the air filter. The photo to the left shows the metering chamber, with the rubber diaphragm removed (but shown next to the carburettor).

That air is pulled in by the action of the piston moving down the cylinder. The carburettor is constructed so that the air flow speeds up as it enters the carburettor via the venturi (it's akin to squeezing a hose as water travels through it - restricting the flow speeds the water flow). The really clever bit is that increase in air flow actually creates a low pressure that drags the fuel in to the air flow, and hence is passed in to the engine. The more the throttle is opened, more air flows in, more fuel is combined and the engine speeds up. Closing the throttle shuts off the air flow, less fuel is drawn in and the engine slows down.

There is a problem with this though - at idle speeds the air speed is not fast enough to create enough of a low pressure to pull the fuel out, so a slightly different approach is taken. For now though, that's it - we'll take an even closer look at exactly how it all hangs together in the next post.

Carb 101: Fuelled Up & Ready To Go.

The carburettor is kinda crucial to the well-being of our chainsaws, so I figured it might be interesting to write up a new series looking at this vital element of the chainsaw. Find out more after the jump...

The carburettor mixes the air drawn in through the air filter, with the fuel being pumped in to it, but it's crucial that just the right amount of air is mixed with the fuel. There's three common adjustments made - the Hi setting, the Lo setting and the idle speed, so in due course we'll take a look at those as well as some problems that had to be overcome with designing the carburettor.

For example, years ago when the big Danarm saws ruled the roost, the engine had to be kept upright - but nowadays we can use chainsaws at any angle; so what changed? There are also problems to be overcome at tickover speeds too, so we'll look at that as well.

Makita DCS4301 Review: And To Cap It Off...

Something that really annoys me personally about the Stihl saws is the filler caps - they seemed to have engineered a technical solution to a problem that didn't really exist. Husqvarna and Makita have taken a more traditional approach to filler caps. OK, so it's not the most exciting of subjects, and I may have been a little unfair on Stihl, but let's take a look after the jump...

I should make it clear that the college where I work is a very heavy user of Stihl equipment, and boy does it take a battering out in the field with the students. There's no doubting the excellent build quality of the professional Stihl saws, but those filler caps...what were they thinking?!.

Stihl have manufactured a twist-on, twist-off cap that means that you don't need any tools to remove it and that's a big advantage over the traditional screw-on types. The screw-on caps tend to get tight during saw use and then become difficult to remove with just your fingers - with the Stihl caps, it's not an issue. On the downside, I've seen so many people think that they've fitted the cap (especially the oil cap) correctly, picked the saw up and then walked off to use the saw... en-route they stop, look down in disbelief as all the oil drains out of the tank as the cap was not on securely.

Makita (and Husqvarna) use screw-on caps - but the Makita version is actually really well thought out. It allows you to screw on the cap with your fingers, and undo it too - but if the cap is too tight, you can use the combi-spanner (screwdriver end) to loosen the cap. What you can't do is use the screwdriver to tighten the cap up (see the accompanying photo).

It's the small things that make ownership of anything a happy experience, or not (and I'm not hinting that the Stihl caps make ownership of their saws an unhappy one!). The Makita filler caps are well designed and simple to use.

Welcome to DriveLink

Hi, and welcome to Drive Link - the new home of chainsaw maintenance and usage. Over the next few months we'll take a look at how to go about safely maintaining and using your chainsaw. More after the jump...

This blog is all about doing it properly and keeping safe whilst you look after your chainsaw - they are a potentially dangerous piece of kit and it's not just about not holding the pointy end filling it up with fuel and mullering a few pieces of wood.

We'll start by taking a look at assessing the risks while maintaining and using your chainsaw, before we take a fairly in-depth look at how to actually go about maintaining it. After that... well, once you've all got clean, sharp saws maybe we'll take a gander at actually using it.

Finally, as we go along, if you've got any thoughts, questions or comments just use the comments section.

Cheers,

DV.