The 3mm key only adjusts the idle mixture. The off-idle mixture is adjusted via the control pressure. If you have a warm-up regulator, that sets the baseline pressure and the lambda valve bleeds some off to trim it. Lower control pressure gives a richer mixture.

I haven't had a K-jet system for a while and that was on a SAAB, but the systems are all quite similar. I recall something like 70psi for the line pressure (which can be measured at the filter) and about 40psi control pressure for a warmed-up engine (which can be measured at the top center connection on the fuel distributor). You will need to buy, borrow or make the adapters for the pressure gauge. I made some.