You might pull the valve adjustment cap off the front of the engine so you can look inside to see how the lever works.  When down the exhaust valve rocker can move up and down without restricition.  Then the lever is pulled up it should hold the exhaust rocker open.....after the cam has lifted it.

When you see how it works.....you will have a better understaning of how to operate it.

Dave

You're prolly not actually engaging the exhaust valve. It's pretty hard to move that small lever all the way. Once you get it into place and open the exhaust valve, trust me, you'll notice a difference. The engine spins really fast because it no longer has any compression. Also, the lever will pop right back as soon as you hit the starter button - you have to hold it in place when you first crank the bike. This is why you'll want a custom one that's longer and has more leverage.

Thanks everyone for the quick replies! I looked into the engine by removing the cap on top and I could see the exhaust valve. So you actually have to apply a great deal of force onto the leaver? I thought you only had to move the leaver just a tiny bit to open the exhaust valve? I'm afraid I might break it...

That depends, in my case I have the foot pedal mod like the Ryca bikes. The original shifter pedal is used to decompress the engine.

Option 2, you could mount one of those manual leavers directly onto the decompression leaver. But in that case you would have to modify the wiring and disengage the clutch from the wiring so that the engine turns over without having to pull in the clutch.