Zenith afflictions, longish

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Honolulu

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Today I got back into the CS after several months not driving it. The plan was to get the carbs tweaked, because the car idles a little roughly, and softly pops out the exhaust pipe at idle. To me this suggested the idle was way rich.

Once the flow was equalized at idle, I found that manually tweaking the accelerator pump lever on the front carb sped up the engine, while doing it to the rear carb made it bog. Condition filed for reference, since I wanted to get at the richness first.

I went to the idle mix screws. Running in (leaning) the idle screw on the front carb eliminated the soft popping out the exhaust, but all the way in, and the speed didn't change. Also, the screw on the front carb didn't run it to a stop, it just got tighter and tighter. Running in the idle mix screw on the back carb had the expected result, speed dropped and engine died.

Removed idle mix screws completely and noted they are hollow on their long axis, end to end. There is a hole all the way down the long axis you can see through. They also have two radial drillings to the side of the jet.

Took a pair off my "other" set of carbs, and they were not drilled completely through the long axis. WHICH SET IS CORRECT? I can't imagine why one set is drilled all the way through...? I don't think "outside" air is intended to be mixed/metered into the idle circuit by these screws, and thus the "spare" set are proper.

Any thoughts welcomed. The suspect idle screws were from a Karzunparts kit for Zeniths, sourced long ago.
 
Honolulu - It's been a long time since I had a Zenith meter screw out but I generally recall them as being solid so I think the spare set is correct. I've got 4 Zeniths on the bench and I'll take a look. I also cannot imagine that the screws should mix in any air - strictly fuel should be metered. If they are hollow it might explain why running to the stop on the one screw didn't bog the engine down' no explanation for the other one unless the float level is too high and you are getting fuel sucked through the venturis at idle (a common Zenith issue) - if you look straight down the throats at idle you can usually see single drops every second or so when this is happening.
 
Thanks JJ

I will pull off the front carb and remove the lowest casting, which carries the idle mix screw and throttle plate, to inspect the tapped hole carrying the mix screw.

Meantime, have ditched the screws with lengthwise drillings and am running the ones with two holes in the sides and one at the needle tip. Runs like crap but that's mostly since I haven't done a proper adjust, and likely can't until this little mystery of the front carb is addressed properly.

In the end, I can swap in the lowest casting from one of the "spare" carbs. Although it originally came from a 2800, according to the fiche it's the same for both 2.8 and 3.0.

More news as it happens, the the roofers just arrived, blocked the driveway with two pallets of plywood and a 20-cubic-yard trash can. The coupe is out on the street, and the oil pressure idiot light, fuel gage, temp gage and tach are out of commission. Methinks the second to last fuse is bad, will have to crawl in for a look. Oh the joys of old cars.
 
I checked all four of my Zeniths and all four of my rebuild kits - solid idle screws every time.
I did find a nice exploded view of the carbs while searching (I did a scan from a rebuild kit):
Probably best to right click and do a "copy image" and then save the JPG file for printing!
Sorry for the size but I figure in this case resolution is everything! 8)

Link to high res down-loadable image:
http://i198.photobucket.com/albums/aa208/jhjacobs/Antar/Zenith.jpg

cc0ea549e735eeecaae063989a69a893.jpg
 
jhjacobs said:
I checked all four of my Zeniths and all four of my rebuild kits - solid idle screws every time.
I did find a nice exploded view of the carbs while searching (I did a scan from a rebuild kit):
Probably best to right click and do a "copy image" and then save the JPG file for printing!
Sorry for the size but I figure in this case resolution is everything! 8)
Click to expand...

You know I don't think this picture is big enough I can still read some of the text....:)

How about next time resizing it before uploading.
 
How about next time resizing it before uploading.
Click to expand...
I guess you missed my point - if it is small (low resolution) you cannot read it well due to missing detail. If it is big (high resolution) at least you can copy it and print it out.

I have fixed it - I've place a link to the high resolution image and posted a poor quality, low resolution version to make you happy.
 
Honolulu, sounds like the front carb has some air leaks. Could be bottom plate warpage, could be throttle shaft wear. X2 on the idle screws not being drilled, they should be solid.
 
I just did my carbs last weekend. My screws are hollow, and the screws that came with the rebuild kit have a much coarser thread, but are the correct shape, but solid. I put the originals back in because I simply did not have the correct new part. My soft "puff" in the exhaust note turned out to be timing, even though I thought I had covered that previously.
 
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