Alchad

Thanks both for the ideas, will try a little Loctite first. Alchad

Hi, 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 Alchad