It's been a while since I've started the 280SE I rescued because I've been doing other work on it. But today when I started the engine, it ran rough at first, then raced to about 4000 RPM. Kind of alarming! The rough idle on cold starts needs to be addressed, but it was like that before. The racing is new. I shut the engine down right away when that happened, but tried a couple more times and it did the same thing. The throttle plate appears to be fully closed, the linkage isn't binding, and I don't see any disconnected vacuum hoses. What could be causing this?
D or K jetronic?
K jet
Look at the linkage that goes down from the air flap to the actual throttle plate below.
The air flap is up top where you can see it, but the throttle plate is actually below where you cannot see it.
The other thing to look at is the cold start valve possibly stuck open?
I'm thinking some kind of throttle linkage issue too. The thottle linkage functioning and adjustment is described here on page 2. Pictures are terrible but you can probably puzzle it through when looking at the actual car.
https://cdn.w116.org/tech/K-Jet_Diagnosis_Adjustment.pdf (on this site).
When this happened to me, it was the linkage caught on something that I'd previously fiddled with. Double check!
A good opportunity to lube all linkage and pivot points.