Starting From Cold

Following the new video about starting your chainsaw, I mentioned that we'd have a look at a couple of things brought up in the video. So, to begin, here's a few pointers to watch out for when starting the saw. Find out more after the jump...

The sequence used for cold starting should include those safety checks:

1. Chain Oiling: make sure the chain is being lubricated.


2. Chain Brake: the chain brake must work properly before you use the saw.


3. Chain Creep: the chain should remain stationary whilst the chain brake is off and the engine is idling.


4. On / Off Switch: ensure that the off switch really does kill the engine

But, we're getting a bit ahead of ourselves here, as I want to take a quick look at positioning and how to hold the chainsaw.

To my knowledge, there are no left-handed chainsaws which means that whether you're left-handed, or right-handed, the hold on the saw is the same - left hand on the front handle, right hand on the rear (or top) handle. This mean that when you start the saw, it's probably easier to keep hold of the front handle and use your right hand to pull the starter cord. Get your right heel on the rear hand guard and either adopt the 'crouching' or the 'kneeling' position. Try to put a little bit of weight on the saw just to stabilise it whilst you pull the starter cord.

Naturally, you could start the saw in the standing position, which you might want to do if the ground conditions prevent you from getting down on the floor. In this case, make sure that you've got a firm grip on the rear handle by using your legs to hold it in place.

For many occasions however, starting on the floor will be the easiest and most comfortable option. Make sure that you don't start the saw right next to your stash of fuel - keep that well away from the saw unless you're refuelling. Try to choose level ground, clear of any debris that the chain could flick up whilst you do the safety checks; and ideally maintain a safe working distance of 5 metres during the startup procedure.

Once the saw has been started, you'll hear that it is essentially idling very high, but as the chain brake should will be on, it'll also sound a bit strained too. You need to give a quick squeeze on the throttle in order to get the idling down to normal levels. However, if you're still in the 'correct' position, then you'll find that you can't your hand to the throttle as your foot is in the way; use this as a prompt to take up position behind the saw. You can crouch down, or kneel down, but if you elect to kneel only do it with one knee; having both knees on the ground will restrict your movement in the event that you need to get out of the way.

So, where are we in the proceedings? Well, the saw is ticking over nicely and you've adopted a comfortable position behind the saw; try to keep your head out of the line of the bar. You're ready to do the safety checks, so in the next post we'll take a closer look at the chain lubrication.

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.

Problems

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.