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Zenith Carbs--will not drop to idle

MadMax

Diamond Level Sponsor
Hello Sunbeam Guru's!!! I am new to the cause, wife and I just purchased our first Sunbeam (1960 Alpine, Series I). Love it, and car was meticulously restored to the 97% level, leaving mechanical issues. First is carbs. Has dual Zenith's in great shape. Car runs great, idles well at 700RPM. But...it will not drop to idle. Get the engine up above 2200 rpm, and will not drop down below 2000-2200. It is a linkage/butterfly closing issue. So I have taken the linkage apart from pedal to the connection to the carbs and there are no hang-ups or issues. I added a couple springs (old Porsche trick) and that did not work. Note, I can move the linkage by hand and get the butterfly valve to close and get smooth idle. But even with springs, can't get the linkage to retract that last little bit. So what do you think the issue is? To tight connections in the carbs holding up the butterfly valve? Pull the carbs and see (really rather not, but in for a pound..) Thanks!!!
 

bernd_st

Bronze Level Sponsor
First of all welcome here. A Series 1 is a very good choice ;).

Regarding your Zenith issue several things to be checked:

1) Check whether there are springs on the outer carb throttle/accelerator pump levers/mechanism and whether they are properly engaged.

2) Slightly oil the mechanisms at the connecting/ bearing points. A smear of grease on the throttle lever ball heads is helpful.

3) Check the geometry of the interconnecting rod between throttle pedal lever and throttle lever. It can be installed upside down and create an unwanted binding. If in doubt, check the WSM for proper orientation

Question: Do the carbs return to proper idle when the interconnection rod is disconnected ?

P.S. A few pictures would help fault analysis...
 
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MadMax

Diamond Level Sponsor
Thanks for the help!!!
Work is getting in way of important stuff. Will take these steps and get pix Doug
 

greenbeam

Platinum Level Sponsor
One thing to check:
1. Disconnect the linkage to the carbs and loosen the screw on the rod between the two carbs.
2. Increase revs on each carb slowly and let it fall back - check the arm on the accelerator pump (little plastic wheel) isn't binding, stuck, broken.

On my car, the front arm was slightly bent and would bind slightly. If you gave it a bit of pressure it would seat again, but even with an extra spring it would bind. Took a while to find the culprit!

BTW - don't add springs that exert a side load on the throttle shafts or you'll increase wear on them.

Good luck!
Cheers,
Paul.
 

MadMax

Diamond Level Sponsor
Gents
Thanks for all the help--it paid off. So I followed your ideas and found
--geometry of the linkage was good
--return springs on carbs good
--many of the connections in throttle linkages were sticky or loose
--spring connected to main throttle connecting rod for return was really weak.
I took it all apart, cleaned the tight stuff, tightened the loose connections and put on better spring and...
It Works!!! Returns to nice idle.
Now, on to the alignment...
Thanks again
Doug
 

bernd_st

Bronze Level Sponsor
Excellent! Once properly adjusted the Zenith's will perform great. If you encounter further issues let us know...
 
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