Snowmobile Repair: 79 skidoo 440, bombardier rotax, disk valve

Question
have a basket case given to me, just wondering about reeds? shouldn't there be reeds inbetween carb and engine? there isn't any now. every other two stroke i have seen had reeds and i am baffled as to how a two stroke could work with out any?

Answer
Hello Lee,

  Until the early 90's, Ski-Doo did not use any reeds on their machines. The same goes for nearly every other 70's and 80's sleds, with the exception of a few high performance sleds designed for racing. Your sled uses Bombardier-Rotax's twin cylinder, central rotary valve  first seen on the 1972 Blizzard 400 and 440. In this design, a single, rotating disk valve controls the opening and closing of both intake ports. The disk valve is driven at crankshaft speed by a shaft that is gear driven off the crankshaft. The design is very compact and light and allows the engine designer total control over the intake timing. The intake port can, for instance, be timed to open 140º BTDC and close 70º ATDC giving a total intake duration of 210º without a "peaky" power curve. When the disk valve clears the intake port, it exposes a wide open passage to the crankcase, there are no restrictions to the flow as is the case with a reed valve. There are many sleds that do not have any type of reed valve, and as surprising as it is, are ported so well, that they do not need any type a valve. I hope this answers your question. Let me know if you have any other questions. Good luck with the sled! --Arcticatman