Forum

[Creaky45]

I recently bought a 2017 C5 Bullet and I have read how the de-compressor works. Correct me if I'm wrong. When the engine is stopped or running below 350 rpm eg the exhaust valve is held open a bit allowing the starter motor to turn easier. When it fires and the revs increase over 350 the exhaust valve works normally. So does that not mean that I should be able to push the kick starter down easily and feel no compression at all? Well I can't and I thought that was odd until I tried the same on a friend's C5 and also a brand new bike in the showroom. They both felt like they had normal compression. I have a few other issues with my bike which still has a few month's warranty left so I would like an opinion on this please.

[papasmurf]

I suspect the decompressor does not function until the ignition is switched and the electric starter button is pressed. (General comment on such systems.)

[Boxerman]

The auto decompresser is built into the exhaust cam. It just cracks the exhaust valve open slightly - not a lot.
I can feel the compression on my bike but the kickstart doesn't go 'solid' if you understand what I mean.

Frank

[Creaky45]

Papasmurf, There is no electrical connection to the de-compressor so I don't think that's it. And Boxerman, I'm not sure what you mean. Even if the exhaust valve is opened just a smidgeon the kick start lever should push down easy, the only resistance being the valve springs. Wouldn't it be like pushing the kicker down with the plug removed? No compression there.

[Haggis]

It does not hold the valve open.
I just gives the exhaust valve a little bump to let out some compression.
Give the starter an easier life and prevents kickback when switching off.

https://youtu.be/wU-QbPUCFxg

[Dan H]

The exhaust cam lobe has a small spring loaded pin/roller which turns on centrifugal force, as the rpm picks up, the pin/roller turns in to prevent it lifting the valve at normal running.



The pin/roller is there to open the exhaust valve a very small amount for a very short period of time and does not hold the valve open constantly like you would when using the manual setup on the older Bullets. As Haggis says, there will still be a large proportion of time the exhaust valve is closed and the auto decompressor is just there to give a little bump to reduce the compression as it approaches its peak and give everything a easier life.

[Tim NZ]

The auto decomp is a bit variable in the rev range it drops in at, turn the idle too low and you will hear it starting to clack, some times as low (high) as 900 rpm. Hence the advised idle speed of 1000-1100rpm


The most important feature of the auto decomp function is when stopping the motor; it prevent piston 'compression bounce back' as the slowing crank-shaft runs down, there by preventing reversal of the electric start drive and killing the sprag bearing.


Kicking the motor over slowly sees the auto decomp in 'engaged' function and you wont have any effective compression.

[Creaky45]

Tim, This was my point. I thought that kicking it over slowly would be like kicking it with the plug out, but this is not the case and Dan and Haggis have set me straight. The valve is only opened very slightly for a very short time on each revolution of the camshaft as can be seen in various diagrams I have now seen. It's hard to see how that would take much load off the starter but apparently it does. All is well though and I also know now why the idle speed is quite high. My old Bullet can tick over so slow I can count the revs if I'm quick.