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 -

1 comment:

Anonymous said...

Very well done. Within a few minutes I come to a whole new understanding of that mysterious little bit, the two cycle carb (I read all of carb 101).
If I was a buxom female wood cutting chain saw mechanic I'd give you a huge hug, but I am not. And unfortunately, search as I may, I find none in my area.
Thank you, DavidG.