And more from an MG article ( with SU’s)
“High altitude running:
I drive in mountains often enough to notice this effect fairly often. When driving up Pikes Peak in Colorado (USA), the entrance gate is close to 9000 feet, and the peak is slightly above 14,000 feet altitude. At 10,000 feet you have 30% less air than at seal level, and at 40,000 feet about half of the air that you would have at seal level. The most noticeable affect is like having an engine only half the size with very low compression ratio, so power output will be dramatically lower. The engine can still run okay, and it will still get you there, but you may need to catch a lower gear and stand on the throttle with a little more patience.
Driving the car to high altitude can cause the carburetors to run richer. As air pressure drops the air will be less dense, and it will require wider throttle opening to get the power you need to climb a hill. For a given mass air flow, the flow velocity will be higher with less dense air, and this will draw slightly excess fuel flow from the fuel jets, making the fuel mixture richer. To compensate for this effect you can turn both carburetor mixture nuts as much as two turns leaner when running to these altitudes, and reset the mixture leaner when returning to lower altitude.
This may be an acceptable adjustment if you regularly run at high altitude, but re-adjusting fuel mixture for frequent changes in altitude can be a pain. The car will usually run okay with richer mixture while you are on throttle, but will likely not want to idle in thin air. My easy fix for this is just to reset idle speed at higher altitude, and then set the idle speed back to normal once I get back down to lower altitude again. If you will commonly be running at high altitude, and not going down to sea level very often, it may be appropriate to install leaner (thicker) needles in your SU carburetors. “
Looks like you need to lean the mixture up to 2 turns, (12 flats). Start with one turn and see if you have any improvement.
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