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Stihl MS 291 idle speed rapd increase


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I've got a Stihl MS 291 which I use fairly infrequently for small jobs around the farm. It was proving difficult to start the other day so I took the carb off, and used an air hose to blow out everywhere I could! Before I stripped it down I noted the number of turns needed to close the idle screw (5 turns) and the low running (1 turn)


Put it back together, reset the idle low running screws to the original setting (5 and 1 turn open respectively)  and eventually with a bit of adjustment  got it firing up OK. However after a period of use - say 15 minutes or so the engine starts to race uncontrollably. Taking the cover off to adjust the carb (I find this easier than trying to poke a screwdriver through the adjustment holes when the engine is vibrating) I noticed that the idle speed screw had closed in quite a lot, so I opened it out again to 5 turns open and started up again. As I started blipping the throttle I noticed that the engine vibration was causing the idle speed screw to close by itself causing the engine to speed up - you could see the screw turning!


As I said the idle speed screw seems to need to be 5 turns open , which does seem a lot, and at this opening it does seem very loose. My initial reaction was that there must be a spring missing which I've seen used on other carbs to keep the needle secure, but checking the online parts diagrams, none of them show a spring.

So a bit mystified and wondered if anyone could offer advice.


Thank you




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The idle screw shouldn't be that loose, use a little thread lock on it or superglue just to hold it. I have never set the idle screw like you do, you usually set the H&L screw to typically one turn out each (may be different on some saws and depends if limiters are fitted) and then adjust the idle to suit. If the carb has had a major rebuild, I usually turn the idle right out until you cant see light around the throttle valve plate and then turn it in until you can start seeing light around it so you get a reasonable start position.

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Had the exact opposite of this on my Makita DCS6800i. The thing kept stalling, I'd wind the idle screw in a bit more and 2 minutes later it would stall again, I was convinced a bearing was eating itself up. This repeated over and over, would have been hilarious to watch the grumpiness unfolding.  Finally twigged the idle screw wasnt wound in as far as it should have been - given the number of quarter turns it had up to that point....

Tillotsen carb on that, idle screw and thread in carb seemed fine. Think I deformed the thread in the carb to add a bit of friction, been good since.


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