Triumph Repair: spitfire clucth, clutch disc, thrust washer

Question
I have 1972 spitfire when the car is at idle speed  and I disengage the clutch I lose about 4oo rpm  there is no noise that I can hear but the engine labors when the clutch is disengaged I can not figure out why this would happen.

Answer
So when you press in the pedal, the engine speed drops 400 rpm.

When you press in the clutch, you are causing the transmission input shaft (and clutch disc) to be separated from the flywheel/crank/pressure plate.

This symptom would suggest that there is increased friction when this occurs.  I would first look to whether you have "dropped" a thrust washer.  Check for forward/aft motion of the crankshaft when the clutch is depressed, or by using a prybar on the front pulley when the engine isn't running.

If you can noticeably move the crank forward/aft (in relation to the block), then that is liekly the problem.  The crankshaft end float is supposed to be about 0.008", which would be virtually unnoticeable.