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Old 05-11-2005, 12:17 PM
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Join Date: Jan 2005
Posts: 13
72 250c Zenith Carb Adjustment Help Needed.....

i have just rebuilt my carbs on my new 72 it runs ok but i think i need to fine tune them. i have read and reread many of the posts on this topic through this wonderful forum but can not seem to find one that i need to re read. if anyone can help point me in the right direction that would be great. the post had percise numbers on rpm drops of 25 or 50 when the mixture screws were turned in or out. these would be turned out 1/4 then 1/8 and back in untill the desired numbers were acheived. this post also had other carb sync directions. if you have this info or know of the post your help would be appreciated.

timing done
valves done
ignition done


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Old 05-11-2005, 02:39 PM
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Old 05-11-2005, 04:45 PM
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Join Date: Mar 2004
Posts: 758
1972 250C:

I suspect you are remembering the thread "1967 230S Idle Problems." Search it out and you will find a couple of posts I made dealing with the specific items you are mentioning. I would link it but cannot for the life of me figure out how it is done. So, I copied the salient points from my post in the hope it will be of help to you.

"Most manuals, my Chilton and the Haynes, are pretty specific on how to set idle speed with these cars. Assuming you have the original Zeniths and the original linkage, then you should be setting final idle speed using the vacuum dashpot and idle stop screw that actuates against a pad on the rear carb linkage. The dashpot has an idle stop screw and spring arrangement that work together to manage the idle speed. As mentioned in one post, as idle speed drops when you put it in gear, the vacuum does likewise, and in response to the lower vacuum the dashpot extends the idle stop screw to raise the idle speed back up. All this monkey-motion only works properly after the carbs are correctly balanced and the initial curb idle speed is set. Study the photos in the manual carefully before you try what I am going to suggest.

Step one: using the proper balancing tools, i.e. air horn adaptor and unisyn and open end wrench, set the carb balance and basic idle speed using the separate adjustment linkage on each carb. You are shooting to obtain the correct front-rear balance and the basic idle speed. This is fairly easy to do. You should begin this effort by screwing in the idle stop screw on the dashpot to get some clearance away from the pad on the linkage so that it does not interfere with your balance and speed adjustment effort. You will return to this part of the process later to get the dashpot properly adjusted.

Step two: once you have the carbs balanced and have set the initial curb idle speed you can begin to adjust the dashpot. You should notice that as you have accomplished the balancing and idle speed adjustment the dashpot will have most likely retracted itself drawing the idle stop screw even further away from the carb linkage. This happens because the vacuum normally increases as the idle speed and balance is corrected. If this has not happened and the screw is touching the linkage again, then screw in the idle stop screw again to obtain clearance from the linkage and then recheck the balance and basic idle speed.

Assuming you finished step one and there are balanced carbs and an engine idling at the proper basic speed, and there is some clearance between the idle stop screw and the stop pad on the linkage, you need to set a clearance between the screw and the pad. Try screwing out the stop screw, toward the pad, until there is about .004" clearance. Disconnect the vacuum hose to the dashpot and the plunger should extend forcing the stop screw against the pad and raising the idle speed a lot. If this happens then re-attach the vacuum hose and try putting the car in gear to see the idle-in-gear speed. If it is not near your specification, then place the transmission in neutral and adjust the dashpot spring retaining nut so the dashpot retracts the idle stop screw away from the pad on the linkage. Then adjust the idle stop screw again to re-set the .004" clearance. Then place the car in gear and observe the in-gear idle speed. Repeat as necessary to get the combination of curb idle speed and in-gear idle speed you need.

As you can see, this is a trial and error process to get these parts all working together, spring, screws, carb balance and idle speed.

Try it, it has worked for me for more than thirty years and I have the original Zeniths and they work perfectly. They always have since I figured out how to set them up."

a reply from Kilgore raised a question about setting idle mixture in gear vs out of gear...

"I have WEBER carbs... so this fix, although valid and would work, will not work for me.

Here is what I did after thinking about things for a little while.

I decided to try and adjust the mixture on the carbs after D was engaged. This smoothed out the idle and raised it a little. I then adjusted idle screws a tiny bit to get idle right.

NOW it idles almost perfect and the RPM drop from N to D is less extreme.

I was thinking that maybe the vacuum drop from the Auto tranny could cause a mixture problem. ??

I'll drive it around and see what the plugs look like.."

to which I replied,

"The small speed reduction from N to D should not cause any mixture problems unless there is something VERY wrong with your Webers. Bear in mind that virtually all cars with carburettors operate off the idle circuit for a significant portion of their lower speeds. Adjusting the idle needles properly, and carefully in modest increments of 1/8 to 1/16 turn, equally balanced between both carbs should get you a sweet running engine. Try the lean drop approach to setting them. Set both screws equally open, then turn in both in small increments allowing the engine to stabilize between each adjustment. When the idle drops 25-50 RPM, back out both again equally and slowly to the smoothest running position. Old advice was to continue backing out allowing the speed to increased until the speed dropped again, then back down slightly to gain back that highest speed. I perfer to set a happy medium so that there is not such a rich mixture. I get better fuel economy and usually smoother running. The important thing is to attain equality between both carbs. If you cannot achieve this, close is good, perfect is best but close within 1/8 turn is OK, then you should have a good setting. All things being equal, your engine may just be fussy and prefer a richer setting. The most common "problem? that we end up chasing is the uneven wear on multi-carb systems and their linkages. Keep in mind that any setting for the idle mixture will be with you quite a distance up the speed range, so set it with this in mind. Also, at idle, every other problem area, vacuum leaks, spark plugs, points, wires, etc., is magnified greatly.

Not much help, I know, but good luck."

The foregoing thread dealt with the issue of setting idle speed. And, as you can see, the response ends up dealing with balance and other Zenith related tuning.

Again, I believe this is what your are refering to, and like before, good luck in your effort.


p.s.: can anyone 'splain me how the linkage to a thread is accomplished? Keep it simple, please.
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Old 05-11-2005, 05:01 PM
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BMW carburators

It's a BMW website but they're still Zenith's

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