Carb Idle Speed

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1eugene445's picture


I ain't no complete idiot, but I must be when it comes to adjusting the idle speed on my 1970 250C..

I have the M130 engine with the dual 35/40 Inat carburetors, and although I have three lrepair manuals, all the photos are muddy and do not show clearly where the idle screw is located.

It sure must not be obvious from looking at the carbs.

Can anybody describe it to me, or send a really clear photo on this forum or off-line to my direct email :

Gene in NC

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Jim Hansz's picture
Kalispell, MT
Joined: August 31st, 1998


1969 230
1990 420SEL
Re: Carb Idle Speed


Welcome to the club of flummoxed Zenith is a link to the JaimeKop site that contains the factory Zenith manual. You will find it helpful, and I strongly recommend you spend time poring over the parts views to see what they are and how they relate to each other. Hit the carb manual link on the left...

Here is something I have prepared and shared many times to assist others in setting up their Zeniths, hope it helps.


Most manuals, like my Chilton and the Haynes, are pretty specific on how to set idle speed with these cars. But the information is not always crystal clear. Setting idle speed is almost always a two-step process that begins with properly balancing the carbs.

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 carburetor linkage. The dashpot has an idle stop screw and spring arrangement that work together to manage the idle speed. As idle speed drops when you put the car in gear, or engage the AC, 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 carburetors are correctly balanced and the initial curb idle speed is set. Carefully study the photos in the carb manual before you start work.

Step one: using the proper balancing tools, i.e. air horn adaptor and UniSyn and an open end wrench, set the carburetor balance and initial curb idle speed using the separate adjustment linkage on each carburetor. You are shooting to obtain the correct front-rear balance and the basic initial 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 carburetors balanced and have set the initial curb idle speed you can begin to adjust the dashpot and the final idle speed. You should notice that as you have accomplished the balancing and idle speed adjustments the dashpot will have most likely retracted itself drawing the idle stop screw even further away from the carburetor 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, screw in the idle stop screw to once more obtain clearance from the linkage and then recheck the balance and initial idle speed.

Assuming you finished step one in one try and there are balanced carburetors 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 now need to set a reference clearance between the screw and the pad. Try screwing out the stop screw on the end of the dashpot plunger toward the pad until there is about .004" clearance. Disconnect the vacuum hose to the dashpot and the plunger should extend to force the stop screw against the linkage pad raising the idle speed a lot. If this happens then re-attach the vacuum hose and try putting the car in gear to check the idle-in-gear speed. If it is not near specification for your car, place the transmission in neutral and adjust the dashpot spring retaining nut so the dashpot further retracts the idle stop screw away from the pad on the linkage. Then re-adjust the idle stop screw again to re-set the .004" clearance. Next, 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, carburetor balance and idle speed.

If you have more questions you can use the search feature to dig out loads of information. Good luck,


W.L.SOON's picture

Re: Carb Idle Speed

Hi guys! I don't know about Zenith carbs nor have I ever worked on them,but I do admire Jim Hansz's effort in explaining the idle tuning procedure in such detail. And we thank GOD for people like him on the forum, :D !

All the best,BYE and GOD BLESS!!! :) :) :)


W.L.SOON , West Malaysia.
1971 Mazda 1000,a dear old companion,gives me 40-mpg.
1972 Mercedes 230 (W114.015),but fitted with a 4-cyl 200 gasoline engine(M115.938).

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