ZX-5 - Weird sound when accelerating

[supah]

I got a weird sound coming from my Lazer ZX-5 when I accelerate. Especially noticable on grass and gravel. I drove it through the grass out in the garden, and it kept making a "clicking sound" like crazy - I give throttle, and the clicking appears and it will not move that much forward.

I have uploaded a short clip of it: http://www.youtube.com/watch?v=7InCkRhMuYI . Is this a normal sound, from the slipper clutch or something? I have tried tightening it a lot, and it's gotten quite better. Am afraid of tightening it too much though. Still makes that noise, like a *clicking* sound, especially when accelerating on gravel. Only barely when accelerating on the asphalt. And as you can hear from the vid clip, it makes it when blocked.

Is this normal? Or has something broke? Should I tighten the clutch even more?