A 240D engine from '74-'76 would be a OM616.916, and indeed it looks like that's what you have there. And yeah, when there's lack of vacuum signal going to the IP you get way too much fuel injected. Busted diaphragm does the same thing.
As for the idler pulley, it does mount to the compressor bracket on a swivel arm. I have the AC compressor and bracket out of my car. Here's a shot of the idler on its swivel, seen from the rear and upside down:
If viewing this in the car the idler pulley mounts almost straight down from the compressor, on a lower corner of the compressor bracket.
Hope this helps.