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decompression valve

jsandidge

jsandidge

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Past Site Supporter
I'm working on a no start issue for a friend. It is an 08 Can Am DS450. I've been poking around on the DS450HQ forum. Through search, the symptoms are pointing to a bad decompression valve. That forum isn't very active. There has only been 3 posts in the past 2 weeks and 2 of them are from me. What I've read is that the repair involves a new exhaust cam. The cam comes with a decompression valve which is not available separately. I haven't pulled the valve cover yet, but I am completely unfamiliar with how this works. Does anyone know about these things?
 
Inside the exhaust cam is another shaft which when rotated raises a plunger. At very low rpm's, this extended plunger causes the cam follower (in most cases) to be lifted above the base circle which in turn lifts the exhaust valve off it seat slightly. The leakage created by holding the valve off its seat effectively lowers the cylinders compression allowing the engine to rotate without much effort.

The de-compressor is 'deactivated' when the weight(s) located on the cam gear sling outwards, against spring pressure, (above a certain cranking speed) rotating the inner shaft that causes the plunger in the lobe to retract. The plunger is pushed back into it's bore strictly by exhaust valve spring pressure via the cam follower. (This may be incorrect, It could be by the weight of the plunger while spinning or being mechanical pushed back by a small spring. Sorry, it's been a while since I've had one apart.)
 
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If your engine follows the usual Rotax setup, Rusty should be correct as to how it works. There's not a lot to go wrong.
Possibly the lever has seized on it's pivot pin. It's pretty basic engineering.

I'd be tempted to check across the other users of Rotax singles for the parts - incl the dirt track parts suppliers.
 
What I've read is that the repair involves a new exhaust cam. The cam comes with a decompression valve which is not available separately.
Click to expand...
I based my comment off the above information.

If it were me I would pull the cam and have a look at it.

NOTE*** the inner shaft MAY have the ability to be assembled two different ways. The incorrect way allows for it to work exactly opposite as intended. Pay attention to the orientation of the inner and outer shafts when you disassemble it or mark them for proper alignment.
 
Thanks guys. I will take a look at it this weekend. I also need to fix the starter clutch (sprag). Right now the starter spins without engaging the engine.
 
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