Lazy aperture on a M42 Carl Zeiss Jena Biotar f/2.0

[drop_m]

Hello there, after a while of reading this forum I've decided to finally sign in to ask a question.
I just bought a Biotar f / 2.0 in M42 mount with some issues.
The main problem is that the aperture blades struggle to close properly to low f values.
They move freely from f/2 to f/4, with a bit lag from f/4 to f/11 and they rarely close to f/16.

I can't find a way to disassembly the aperture block, but I've completely disassembled the optical elements and cleaned the blades on both sides with lighter fluid, but the problem - even if improved - persists.

Any suggestion?

[tomasg]

The problem may be in the aperture mechanism, post few pictures so we can see which variant you have.