Choke Control Knob Two Bases for Two Engines

What you pinch at cold start is not the throttle, it is the choke. The 35438 red knob is the one that handles the choke.

It is a pair with the throttle knob. On the engine, two knobs like this sit next to each other; the 35438 takes the choke and the 35440 takes the throttle. They look alike, same slots, but they do two different jobs. Squeeze the 35438 and the choke plate closes, the mixture goes rich, and a cold engine fires. Squeeze the 35440 and the throttle opens, the speed goes up. Do not buy them mixed up, the two shafts are shaped differently, and a crossed fit will not go in.

What is the inner shaft inside the knob for? The knob looks small, but there is an inner shaft in the middle, the hole shaped like a gourd, pressing on the choke push rod of the engine. You pinch the knob once, the shaft presses the rod once, and the choke plate in the carburetor closes or opens. The shaft and the rod meet hard, no cable, no spring, which is why the knob feels solid under the hand.

Why two bases, one flat and one pointed? The flat one with the locating block fits the flat seat with the locating slot on the engine, and the tapered pointed one fits the boss seat with the hole. The same knob, two bases, two kinds of machines, so look at what the bottom of the old knob is and take the same.

What are the red slots for? The three horizontal slots are the anti-slip grips for the hand, and they have to work even with winter gloves on. The red is not random, choke knobs are made eye-catching by habit, so on the machine you can tell at a glance which is the choke and which is the throttle.

When to replace it? It will not pinch, it goes all the way but the choke does not follow, the inner shaft is worn round and slips, the plastic is cracked, or the base is worn flat and will not sit, then swap it. The knob is a wear part, the plastic aging and the shaft wearing fail from both ends, a cracked shell can be replaced, a worn shaft means the whole knob goes.

What to check when swapping it? Line up the base shape and the direction of the gourd hole in the shaft on the new knob against the old one. Engines in the same series mostly fit, but fit is not guaranteed for every one, so comparing against the old knob is the safe way. On install, the shaft mouth on the push rod, pinch twice and see if the choke follows, and only then close the machine.