I have a 78 242 with a B21a engine with a single SU carb.
It has about 200K mile (325k kms).
The car has been running well for the past 10 months. We have had some recent cold (-30c, -25f) weather, and the car started to not idle well, performance at highway speeds was reasonably OK.

I set the timing, checked plugs, installed new rotor cap, adjusted the fuel jet screw as outline in my Haynes manual and the idle returned to normal.

3 days later we had a blip in the temperature, it warmed up to just below melting, then plunged to -25c (-17f) again. The next morning the car acted like the carb was iced up.

After work, I bought a can of carb-clean, sprayed the carb clean as per instructions, poured 3 small cans of gasline antifreeze into the tank, added new fuel and went for a drive to get the "dried" fuel into the carb.

I just completed a 100 mile drive home - the car runs well above 2200 rpm.
Mated to a 4 spd manual transmission, it purrs along at 3700 rpm at 70 mph.

As soon as the revs drop below 2000 rpm there is little power, and the car won't idle.

QUESTIONS:

There is an adjusting screw on the top of the intake manifold, just after the carb gasket - what is it? and how should it be set???

The best way of setting the fuel jet screw - is the Haynes way OK???

Thanks in advance