The drive straps on your flywheel may not be parallel to the flywheel when assembled. If so they bend when you let the clutch up on changing down, as they are then in compression (not tension) but not straight.
This allows the backplate and the diaphragm to move off centre giving you a shudder.
This misalignment is caused by people machining the faces of flywheels and backing plates with no regard for the assembled clutch height.
see
http://www.minimania.com/web/DisplayID/ ... ticleV.cfm for the theory of setting a clutch up properly.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
