[Fot] SU Carb question

[Scott Janzen]

the pistons in the SUs on my recently acquired E production TR4 clatter wildly at idle and up to 2500 rpm, at least.  Springs are installed, and the damper oil just gets quickly pumped right out the vent hole in the plastic cap when the car is running.
It certainly smooths out at high speed, but the condition also makes it difficult to start - I need to stick a finger in each one to hold the damper up slightly, then get it running over idle speed.

Ideas on where to start?