Engine Reeds 101

Reed valves might be one of the most neglected parts on a two-stroke snowmobile. That’s too bad because many snowmobilers might be giving up performance or dealing with an engine that’s hard to start because of worn reed petals.

Reed petals are situated inside a specially designed contraption known as the reed cage that attaches between the throttle body/carburetor and engine. Most modern, two-stroke snowmobile engines are either case-reed inducted or cylinder-reed inducted. Case reed means the cage is bolted to the crankcase, cylinder reed means the assembly is bolted to the cylinder.

Early reed petals were made of metal, but now stock petals in snowmobile engines are made from fiberglass. Carbon fiber is an option from the aftermarket. Rotax engines in Ski-Doo snowmobiles are cylinder reed, while Polaris and Arctic Cat each use a case reed design. Yamaha used to make cylinder- and case-reed two-stroke engines for its snowmobiles.

According to Steve Tassinari of aftermarket reed manufacturer Moto Tassinari, reed petals open and close 133 times per second to let air — or fuel and air, depending on the fuel induction system — into the combustion chamber of a reed-inducted two-stroke engine. The petals close to prevent the fuel and air from escaping the engine. Piston port two-stroke engines do not have reeds.

An engine that is suddenly hard to start might have chipped reed petals.
An engine that is suddenly hard to start might have chipped reed petals.

Signs There Is A Problem

Hard starting is a symptom of low compression, and since chipped reed petals allow compression pressure out of the combustion chamber, it might take a few extra pulls on the recoil handle to fire the engine. Poor throttle response, also due to lack of compression, is another sign that reeds petals could be worn.

Reed petals wear out not only from their constant flexing to open and close, but fuel that passes by them softens the solidified resin used to manufacture the petal, which causes cracks, chips or curled edges. Other than babying the throttle and keeping engine speeds low, there’s nothing snowmobilers can do to prolong the life of reed petals because their movement is, obviously, an essential part of what makes a two-stroke engine run.

Absence of the black line around the perimeter of the center petals indicates they aren’t seated against the cage.
Absence of the black line around the perimeter of the center petals indicates they aren’t seated against the cage.

When Should Reeds Be Inspected?

No different than hyfax, drive belts or brake pads, reed petals wear out. Experts generally recommend pulling out the reed cages and examining the petals every 2,000 miles, so depending on how often you ride, this might need to be an annual inspection. Reed cages don’t wear out.

When visually inspecting the petals, look for chipped areas or cracking, especially in the corners of the petals. The material might start to flake off, too. Petals should be seated tightly against the cage, but to test, lift them with a fingernail and release them. They should make a firm ‘snap’ noise when released. If the sound is dead, the petals might need to be replaced because they’ve lost too much tension.

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