Mercedes stalls in hot weather

1979 Mercedes. Idle runs great in cool months. But in summer it stalls, have to set the idle very high to keep it from stalling when stopped. Once the weather gets cool again, I can set the idle back down to a reasonable setting. Why would it stall when it’s hot outdoors?

First off, I’m not a Benz expert but I think this particular car uses CIS injection. My condolences; VW and SAAB also used this system.
Great when it’s working; pain in the ahem when it’s not.

You really shouldn’t mess around with the idle speed screws on CIS unless you’re knowledgeable of the system and have specialized tools and equipment.

Vacuum leaks (injector seals are common), poor injector spray pattern (also common), and fuel pressure oddities could be behind this problem.
CIS is very nitpicky about vacuum leaks. The fuel pressure test requires a special pressure tester and the chart showing fuel pressures at certain temps, testing procedures, etc.

The best way to check for vacuum leaks is with a vacuum gauge.
Injector spray patterns can be checked without the special bench unit but it requires a procedure.
Fuel pressure tests require a specialized fuel pressure gauge. Either a European import shop, buy one (ouch), or make your own. The latter is what I did a long time ago as the gauge at the time was about 400 dollars.

That’s probably not much help but it’s where you have to start.