Husqvarna T540XP Mark II Primer Bulb (Air Purge)

This Husqvarna T540XP Mark II primer bulb, often called the air purge bubble, replaces a cracked or soft bulb so the saw primes properly before a cold start. It is a small part, but a split bulb can leave a top-handle saw hard to start or leaking fuel.

Why the purge bulb fails

The air purge bulb pulls fuel through the carburetor and pushes air out of the system before you start the saw. On a top-handle that rides on a climber's saddle all day, the bulb takes knocks against bark and gets baked in the sun, and fuel slowly hardens the material.

Signs it is time to replace it: the bulb stays flat after you press it, does not fill with fuel, or shows cracks and fuel weeping around the edge. Any of those can leave the saw hard to start right when you are hanging in the tree.

Because the T540XP Mark II is a top-handle saw that often starts in the tree, a dependable prime matters more than usual. A fresh bulb is a two-dollar fix that saves a lot of pulling at height.

Swapping it out

Confirm the bulb matches your saw before ordering, since the Mark II and earlier T540XP models are not always interchangeable for every part. Work on a cool saw, drain or close off fuel as needed, and seat the new bulb fully so it seals.

Browse other power equipment parts if you are doing a broader tune-up. Wear chainsaw protection, eye and hearing protection, and follow the operator's manual and ANSI Z133 when running or servicing a saw.