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Author Topic: Decompressor  (Read 797 times)

Propellor

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Decompressor
« on: July 26, 2014, 07:04:56 AM »
How does the auto decompressor work on the xbr and 250 RS? I'm guessing it lifts an exhaust valve a small amount? How does it know when to lift? Presumably it is timed by gears to the crankshaft? The timing would presumably be 2:1?

All guesswork!

Cheers.
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guest564

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Re: Decompressor
« Reply #1 on: July 26, 2014, 08:30:06 PM »
Its timed off the camshaft.
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Propellor

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Re: Decompressor
« Reply #2 on: July 27, 2014, 06:09:47 AM »
Quote from: Rhinoman on July 26, 2014, 08:30:06 PM
Its timed off the camshaft.

Hi rhinoman. Thanks for reply.

So, the levers are geared to the kick start, but the top lever only actually does anything when in appropriate alignment with the exhaust valve?

Cheers
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Steve H

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Re: Decompressor
« Reply #3 on: July 28, 2014, 09:04:36 AM »
On the 250RS and SRX my impression was that the decompressor wasn't timed in anyway, if the exhaust valve is already open it as no effect and it is closed then it just holds the exhaust valve off its seat. No timing required. It only does this for the first part of the kickstart stroke and closes the valve towards the end of the kickstart stroke
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Propellor

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Re: Decompressor
« Reply #4 on: July 28, 2014, 05:30:45 PM »
Quote from: Steve H on July 28, 2014, 09:04:36 AM
On the 250RS and SRX my impression was that the decompressor wasn't timed in anyway, if the exhaust valve is already open it as no effect and it is closed then it just holds the exhaust valve off its seat. No timing required. It only does this for the first part of the kickstart stroke and closes the valve towards the end of the kickstart stroke

Makes sense. Fiendish.
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guest564

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Re: Decompressor
« Reply #5 on: July 28, 2014, 08:05:16 PM »
Maybe my memory is failing me. I'll have to have a look, I thought that there was an extra lobe but it could be that one of the exhaust rockers has a 'tag' on it for the decompressor to activate.
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Moto63

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Re: Decompressor
« Reply #6 on: July 29, 2014, 08:13:29 AM »
Morning chaps,
Just been in the workshop and whipped the head off the ol cafe racer, and the way I understand it is that the decompressor shaft has a key way on the end of it, the shaft then slides thro the centre of the exhaust rocker which has a corresponding key way inside it. When the cable is put under tension via kick starting the shaft is turned and the rocker lifted.  Regards Michael.      Ps the bit about whipping the head off was a joke. Chortle chortle
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Moto63

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Re: Decompressor
« Reply #7 on: July 29, 2014, 09:21:09 AM »
Hi again,
I was,nt happy with my first inspection of the decompressor assembly so I went back into the workshop, whipped the head back off and took a more detailed look. I was correct with the decompressor shaft with it,s key way, however on further inspection the shaft appears to sit where the rocker arm meets the sub rocker arm. Then it carries out it,s function as I said in my first post. Hope my detailed analysis helps out in some way. Gotta dash I,ve got a cylinder head to pop back on. Toodle pip
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