My chainsaw starts without muffler but not with it on, what's the fix

If this is the Husqvarna 440, there are two allen headed screws sunken down inside the holes in the front of the muffler, its either a 5mm or 6mm long arm allen key required.

quick check would be to pull muffler from engine check the screen if the screen is ok start engine if it starts without the muffler the muffler needs to be replaced.if it still dont start do pressure and vaccum checks on carb.

try running it with out the muffler if ok without the muffler open it and clean the internal screen

try and run it without the muffler there is a screen inside that plugs up if ok with the muffler off open it up and clean the screen off

muffler probably needs cleaned could be gummed up blocking exhaust flow

remove the muffler and run it if it runs good without it open the muffler and clean of the screen inside

There was an issue with the muffler bolts comeing loose, to that end a muffler with a third fixing hole was introduced, if the oil tank below the muffler is melted to the point that the chain oil is leaking out, then it will need a new crankcase.

I would first check your muffler and screen in muffler to make sure it's not partially plugged. Personally I think it sounds like a fuel delivery problem,I would make sure you change your Fuel filter and check your lines for being cracked and or collapsing from ethonal in gas these days. then I would put fresh gas in it and see if it helps.you also need to clean carb and then readjusr carb after cleaning carb

Check air filter- is it clean?
Some plugs are without cilinder ok but if in place dont work. cahange the spark plug.