i've always been adjusting carbs with the rpm trick(moving up and down to 750rpm etc) but now i've got a friends car over and the plugs turn black using this method. My question is which way does it become lean/rich by turning the mixture screw in and out. Yes i have searched but came up empty
thanx in advance

Was covered in this forum 2 week ago:

https://www.fiatforum.com/showthread.php?t=26993

lifted from "Porter".

The mixture screw only affects the mixture at or near idle speed. If your friends car is getting black plugs after a run, you need to look elsewhere (starting with the airfilter and cold start mechanism)

Providing the idle screw is on the fuel circuit, turning clockwise leans off the mixture. If it's the air circuit, clockwise richens the mixture.

What car and year is it?

It's a 1990 999cc fire with the weber 32 tlf, i've tested the compression and it checks out fine. The only problem is that after every run the plugs get fouled and it starts to misfire(and the screw is all the way in!!!(CW)), i've also tried to unsrew it until it just about idles(CCW) and it does the same thing.

BTW the carb has been replaced last year by Fiat for some unknown reason

I though we'd done this one to death last year:

https://www.fiatforum.com/showthread.php?t=13390

If the carb is in good nick, check out how it is plumbed up:-

Is the fuel return blocked?
Are the vents blocked?