BMW Zenith Carb Issues (NLC)

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111Robin
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Joined: Sun May 28, 2006 3:40 pm

BMW Zenith Carb Issues (NLC)

Post by 111Robin » Sat May 12, 2018 9:48 pm

Anyone familiar with the twin Zenith carb's as used on 70's BMW/Mercedes ?. My 2500 is running very poorly since the winter layup. Previously it started on the automatic chokes perfectly and soon settled to a smooth idle. Now it's running poorly, not on all cylinders. It starts on the choke then the rev's drop and it runs rough even when warmed up. It's a very complicated setup compared to the SU carb's I'm used to, especially the automatic choke with electric heating element, water heating, some kind of temperature switch in the inlet manifold all running through a relay. I've previously only had to set the mixture and idle speed and it's been fine. As far as I'm aware there are no vacuum leaks but there are two vacuum units on each carb' to do with the choke actuation and I have no idea about these or if they could be causing the issue. Give me an SU on a manual choke any day !!!. Any advice is welcome.

Image20180511_120604 by Robin Derrick, on Flickr

This is the wiring diagram showing the choke heating elements 24 & 25 connected to the relay 101 and temperature switch 26. Not sure if the problem is choke related though. One thing I noticed is with engine cold and ignition switched on the front carb choke Flap gradually opens in around 5 minutes but the rear carb flap doesn't move. There is a feed at the element so I'll need to remove the cover to see if there's an obvious fault. However the flap is opening as the engine warms, presumably due to the water temperature, therefore I don't think the electrical issue is causing the poor running.

Image20180511_161835 by Robin Derrick, on Flickr

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