Zenith choke operation: Why heated only above 63 degrees? (Jerry you out there...?)

Luis A.

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As part of my engine restoration I've taken apart my Zeniths and in doing so realized the coolant temperature sensor under the front manifold is not working (WTB ad in parts section) but I can't figure out why it works the way it does.

The choke heater circuit is set up so that the electric heater coils in the chokes only work at temperatures above 17C, or 63 degrees F. Why heat only above 63 and why is the electric coil in there at all if the coolant will soon warm up to above 63 and perform the same function as the heater coil, which is have the bimetallic spring unwind and start opening the choke flap?
 

61porsche

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The temperature

switch was a limited very early attempt for emissions to control the rear carb choke only. Interesting fact is that it was also on some European cars. That choke was different ( looser spring) as well and supposedly the cover is marked. It basically limited enrichment until the temperature swiitch reached 63 degrees. You're right. It's counter intuitive to what you want for cold starting.

Mercedes shop manuals explain this in detail.

http://www.benzworld.org/forums/attachments/vintage-mercedes-benz/532482d1376351071-online-resources-zenith-carbs-5.pdf

I suggest everyone with Zeniths go find the Benzworld and search for Zeniths as the manual explains the operations of the carb clearly better than BMW did.

Needless to say, you shouldn't use it. Chokes should be powered on all the time
via the ignition.

VDO makes the switch but it operates at 106 degrees and costs about $23.

Always glad to help.
 

Luis A.

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Thanks for the explanation and the MB source Jerry.

You think running with no switch at all, or even the 45C, would feel any different?
 
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