Idle screw would imo not make any difference if you can obtain base idle without iscv connected.

Its almost as if the iscv is getting too much voltage and is staying open.

Which brings me back to the amal valve which is on the same power feed? Did you disconnect and try?

Did you try the ignition feed as this is a sensing feed for the iscv when load is applied iscv will increase idle.

Just ideas James knows far more than me and may be able to assist you further.