Carb101: Setup & Troubleshooting

In this final article about carburettors we will take a quick look at setting up the carburettor and how to spot some of the problems that can arise due to carb failures. Find out more after the jump...

In the last post we saw that the amount of fuel drawn through at idle was limited by the "L" low speed screw. The "H" high speed screw has a similar function when the saw is running at full revs - controlling the amount of fuel entering the air stream. This obviously affects the fuel:air mixture and setting this screw incorrectly has the potential to cause catastrophic damage to the saw.

The high speed jet needs to be adjusted with the saw at maximum throttle (fitted with the bar and chain, but otherwise unloaded); because this jet affects the fuel:air mixture, it's very important that adjustments are made with a clean air filter (setting the high speed screw with a dirty air filter will potentially leave you with a weak mixture when you do clean it). The picture on the left shows the high and low jets.

To set this screw, you really need a tachometer to measure the engine revs - refer to your manual to find out what the setting should be. For the Stihl MS260, the "H" screw should be set to allow the engine to run at 12,500rpm - but if you screw the adjuster in and out you'll find that by turning anti-clockwise the saw starts to 'four-stroke' (you'll hear the difference) and the revs will drop off; screwing it the other way and the revs are liable to increase beyond the recommended maximum as the fuel gets weaker. However, be warned, with the saw running at it's (unrecommended) maximum, it will lack cutting power and worse still, due to the weak mixture, it will not be getting enough lubrication either. Result? An overheating engine that's liable to seize. If you don't have a tachometer, set the "H" screw to the manufacturers recommended setting and leave it there.

The idling screw (not to be confused with the "L" screw) makes no difference to the fuel:air mixture settings. It's connected to the throttle and depending on where it's set, will hold the throttle open just enough to allow the saw to idle normally but leave the chain stationary (with the chain brake off).

So, to set up the carb, set the "L" and "H" screw to the manufacturers recommended setting (usually, but not always) this equates to screwing them all the way in, then backing them off 1 turn. With this done, fire up the saw and adjust the "L" screw until the revs reach their maximum, then back off about 1/4 turn. Now set the idle screw, so the saw idles happily, but the chain remains still when the chain brake is off. With this accomplished, open the saw up (full revs) and adjust the "H" screw until the tachometer reading shows the setting recommended by the manufacturer.


There are a number of problems that can be caused by the carburettor and if the saw is playing up - for example, it keeps stalling (especially when turned on its side), suffers from 'lumpy' running then I would recommend that you reset the carb settings back to normal, clean the air filter and start from there. If resetting / cleaning do not solve the problem, then it could be the diaphragm has had it; or the needle in the metering chamber has started to wear. In these cases, just purchase a carburettor kit which contains everything you need, replace the lot in one go and try again.

Well that's it for Carb101, I hope that you've enjoyed the series as much as I have writing it.
- DV -

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: The Design

The design of the carburettor is quite interesting in itself and came about to overcome some problems in using the chainsaws (although the roots of the problem go back further still). Find out more after the jump...

The early chainsaws could be operated by blokes that could scale tall trees in a single bound, rip up sizable trees straight out of the ground with their bare hands and thought nothing of cycling 30 miles to chop down a tree with just an axe and then tow it back home (before chopping it up to use as firewood). Apparently.

In fact, if you are one of those very people who used those early saws, drop me a line via the comments as I'd love to hear of your exploits with those chainsaws.

Anyway, the early chainsaws used engines that had to be kept upright, and so they made use of an arrangement that allowed the bar to rotated from a vertical position to a horizontal one - whilst the engine remained upright.

When you think of the angles that the climbing saws get used at, you wonder what changed to make this possible. Answer: the carburettor.

Those early engines used carburettors fitted with a float chamber, as the fuel filled the chamber up the float would close the needle so restricting flow. It worked well - as long as the saw remained upright. Once the saw got used at an angle the float didn't work as it was supposed to, with the result that the saw didn't run smoothly (or at all).

The design of the carburettor had to change, and it did. The float chambers went out, and in their place came the new diaphragm carburettors that allowed working at any angle. Using 2-stroke engines also helped as lubrication to the engine is supplied via the fuel, so now a saw would run properly and receive adequate lubrication irrespective of what angle the operator used it at.

As we go through Carb101 we'll be stripping down a diaphragm carburettor and seeing how it works.

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.