First, a little background. I'm running a CB750 engine in a vintage racing car. This is a small formula car about the size and weight of a mini-sprint. The engine in the car is basically stock, but I've also acquired a second, race-tuned engine that I plan to put in the car later, after a rebuild. I'm experiencing intermittent clutch slippage on the track, after upshifts. (It seems to depend on my shift technique. With a really clean shift it's fine, but if my shift is less than perfect it's likely to slip. I haven't yet tried upshifting without the clutch.)
The springs in the two clutches are quite different. The springs from the clutch that's slipping are 36 mm (1.42") long, made of 2.6 mm (0.103") wire, with six coils. The other springs are 32 mm (1.26") long, made of 3.16 mm (0.125") wire, with five turns. The second set is obviously much stiffer but, being shorter, it's not clear to me that they necessarily will have more pre-load than the first set. To further complicate the picture, the longer, softer springs were installed with 1.6 mm thick washers underneath them, whereas the stiffer springs had no washers.
Which, if either, of these springs is stock? Does anyone have any idea what kind the other ones are? If forced to guess I would think that the shorter, stiffer springs would allow the clutch to transmit more torque without slipping. But I'm concerned about the lower pre-load. Should I perhaps install the shorter, stiffer springs but also use the washers to shim them a bit, for more pre-load?
