the Gaz shocks have adjustment for both bump and rebound I think? Bump will be at the top of the shock absorber, rebound at the bottom. You should be able to get a scew driver on both with the springs in situ. If not then support the rear of the body and jack under the de dion elbow (or close by) so that you can move the suspension up and down relative to the body. Then vary the suspension compression until you can reach the adjusters through the spring.
The principle of setting up a shock absorber is simple, pactice a little more difficult. If you had a coil spring without any shock absorber then you would be able to get the rear of the car to bounce on its own for ages. I've tried this with mine, whilst moving the car during rear end work. The effect is quite comical, like sitting on a childrens see saw! So imagine putting a sudden push up onto the rear suspension. Without a shock absorber the spring wil tend to push the car up beyond the final static position. Then the car will fall back, and again the spring will allow the car to go beyond the static position in the downward direction. And so on through quite a few cycles! You sometimes see a car on the road with a dead shock absorber and the wheel "patters" between the body and the road, sometimes leaving the road altogether!
The objective of the shock absorber is to make this motion "sticky" so that the car goes only a little beyond the static position and falls back exactly to it. If you adjust the shock absorber too tight then it acts as a rigid strut and you feel every imperfection in the road. Effectively the only spring you have is the tyre! Too slack and you get back towards the Zebedee situation. In Zebedee mode you don't get consistant grip from the tyre, sometimes none at all, so traction and cornering both suffer badly.
A rule of thumb is that if you deliver the body a shock load downwards - lean on it suddenly with your full weight - the body should spring back to just beyond the static position and then settle back down directly to static with no further movement. The initial rise of the body is the rebound motion, the subsequent fall bump (you've effectively done the exact opposite of what the road does to the car - it pushes from unserneath, you've pushed from above).
If you are driving the car then the slack shock absorber situation will allow the car to float or wallow over bumps. This is best felt by taking a long undulation in the road at speed. If the shocks are too loose it feels like a fair ground ride, the car lunges upwards, hangs for a second and then plunges down, perhaps hitting the bump stops. Tthe tight shock absorber situation gives a jiggly ride where you feel evey minor imperfection. Note that with soft springs the action is slow, with stiff springs the action is much faster but still discernable.
So how would I go about it? Well it is easy to make an initial setting of rebound damping. Go back to the sudden heavy lean onto the rear of the car. Adjust the rebound until the car only just goes beyond the static position on the first bounce back. That will do to start with. Then onto bump adjustment. I suggest you set the damper at nearly maximum and drive the car. Too jiggly? Back it off until the car becomes acceptably comfortable. Then go out and find that long undulation. Any wallowing? If so tighten up a tad. This process is only sort of scientific. At the end of the day you are looking for a compromise with which you are happy.
Once you have reached your intial compromise, then you can start using the car more and can judge whether you were right. Probably the best test of the rebound setting is driving through pot holes. If it is too slack you will tend to get a boing effect as the wheel accelerates enthusiastically into the hole. If you start to feel the car is too stiff or too sloppy I'd be inclined to adjust both bump and rebound together by the same amount from here on. Only adjust one alone if you are sure there is a specific problem.
Happy playing!
Chris
