the lever-weight enigma

Today I tried fitting the nose with the lighter spring augmenting the heavier one by rotating it spirally down inside it, and this improved things. 

It meant that the car stood largely horizontally, but unfortunately it sank forward about 1/2" with me sitting in it. So I took the front axle off and weighed the car at the front to work out what kind of work the ideal spring is going to have to be able to do.

My weight is around 84Kg and the kart is about 70Kg (with the 17Kg engine in front of the back axle), so our combined weight is about 150Kg. How, then, can it be that with me sitting in the kart and with scales under the nose of the kart (front axle removed), the total that the front spring must hold up (without compressing, until it is bounced) is only 68Kg?

I know this is about levers and moments and all that jazz, but I still find it a mystery. Can anyone explain?