BMW E21 323i dropping on rews

[Atle_323i]

Hi,

I wonder if anyone can help me to troubleshoot why my engine is dropping below 500 rpm after being rewed up, and let down.
I've included a link to youtube showing what I mean.

https://www.youtube.com/watch?v=L4TfobN_Xjc

When the engine get to normal operating temp, the engine starts to idle very rough, going from low 300 rpms to 800-900 rpms.
This is very anoying the coming to a stop in traffic lights.

What could be wrong?

[Jeroen]

Most likely a vacuum leak. Can you get an exhaust gas analysis done at a local garage? Could be just too lean with CO too low, CO and O2 readings will tell you more.

[Atle_323i]

Most likely a vacuum leak. Can you get an exhaust gas analysis done at a local garage? Could be just too lean with CO too low, CO and O2 readings will tell you more.

The engine can't be started at the moment, waiting for "new" overhauled gas tanks, as my old ones where done, and is now taken out.
And I'm replacing rubber fuel lines aswell, as they are done.

I'm going to take out the engine later this year, and will check all vacuum hoses, and probably replace them to avoid any more problems. There is alot of vaccum hoses in my 79 year model, I wish I had the newer version without all those vacuum hoses.