The symptom pattern

Everything an owner is told to check comes back fine: fuel is getting to the carb, the air filter is clean, and pulling the plug shows a healthy spark. And yet the saw won’t start, or only fires briefly before dying. It’s a frustrating position to be in, because the usual troubleshooting checklist has already been worked through with nothing to show for it.

What’s actually happening

Fuel, air, and spark are the intake side of the engine — but a saw also needs to expel exhaust cleanly for the next intake stroke to draw properly. Stihl saws include a fine spark arrester screen in the muffler, required in many regions to stop hot exhaust particles starting fires. Over time it clogs with carbon, and once restricted enough it can choke exhaust flow badly enough to stop the engine drawing a fresh charge at all — which looks exactly like an ignition or fuel problem from the outside, because the actual blockage is downstream of everything the standard checklist tests. Old fuel left over winter is the other strong candidate: it leaves a varnish-like residue on the carburettor diaphragm that produces the same “everything checks out individually, but it won’t run” picture.

Working through it

Rule out stale fuel first — it’s free and extremely common on a saw that’s been sitting. Then work through the standard checks (plug gap and condition) before moving to the exhaust side: pull the muffler cover and look at the spark arrester screen directly. A screen caked in carbon is usually obvious on sight, and cleaning or replacing it is inexpensive. If the saw has been idle for a long stretch, check the carburettor diaphragm for the tacky varnish residue that old fuel leaves behind — this is a separate but similarly overlooked cause with the same symptom picture. Only once the spark arrester and diaphragm are both ruled out is it worth moving to a full carburettor rebuild or testing the ignition coil.

Verdict

Check the spark arrester screen before assuming a deeper carburettor or ignition fault — it’s a five-minute inspection that resolves a real share of “fuel, air, and spark all check out but it won’t start” cases, and it’s far cheaper than a carb rebuild that may not have been the actual problem.