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Anybody understand the Automatic Decompresor

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I have been familure with the centrifically activated Automatic Decompresor on the KTM's. It has a cam within the cam, the inner one just makes a small bump on the exhaust lobe that 1/2 way thru the compresion stroke bumps the exhaust valve off the seat. As soon as the engine reaches running speed the centrificle weight swings this small inner cam out of the way and it runs normal. This system does the job, I have felt it a bit crude in that at low RPM the centrificle weight is bobbing around both causing the clanking noise and releiving 1/2 of the compresion when you are close to stalling. Didn't know thats why you stalled, did you?

Recently got an 05 Husky 450TE, it has a similar system, I have stared at it in the parts manuel and with the valve cover off. It has a thin flat cam lobe that slides over the main cam and when slid left does approximatly the same thing as KTMs. What I can not figure out is what slides it right when the engine starts?

Oil pressure?

Electro magnet?

Anybody out there actually know?

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I don't actually know but it seems similar to the other systems that you mentioned. Below a certain RPM the decompressing lobe will depress the exhaust valve. Above that RPM it will stay put. Why does it work upon start-up?-Because the RPMs are low enough to have the lobe engage the valve. If starting the bike with a kick or the electric leg resulted in the engine spinning say 1500 RPM, the decompression lobe would not work. -just a guess

I don't think that autodecompression will cause the engine to stall. Even the slowest running engine speed is great enough to keep the decompressor in place.- again just a guess.

anyone else?

-james

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my guess is its oil pressure related as when you stall the starter will not turn motor for a few seconds i took it this was oil pressure still up and when it drops de compressor comes back on,i may be wrong but i am also interested to find out the workings

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Likely oil pressure. I didn't find a diagram in the '05 manual that I downloaded from Husqvarnausa.com (I'e got an '05 SM510R on order) but oil pressure is my best guess based on what I see.

At anyrate the system looks to be better than the cable actuated decompression system my old SRX 600 uses.

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I have just done my valve clearances and remembered this thread,

My thought on how this decompresor works is just a mechanical weight,

The additional cam lobe runs on the exhaust valve lever from zero revs and is just enough to give the starter a chance to get moving until the flywheel carries the crank round with its inertia, this extra lobe has an over hanging counter weight towards one end of the camshaft so at a certain speed it cocks the lobe over to the side and misses the valve lever, The engine should be just starting up now. When the engine stops the counter weight has a light 'bias' spring to reposition the lobe back onto the valve lever ready for the next start up. 😢

When I have removed the top cover to check my valves I position the cams by putting the bike on a stepup stand, with the motor in top gear and turning the rear wheel by hand, if you spin it briskly you can see this device opperate,

Before you try this make sure you have removed the sparkplug and disconnected the battery it is posible to start up 😢

It does come into effect at a much slower speed than normal tick-over so you may have already stalled before it starts to decompress again,

RickyT

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