my 71 250c is up and running. .. but way to fast. It's got dual zenith carbs. I went through the books. I tried adjusting the idle speed adjusting screw that did nothing. I adjusted the throttle damper adjusting screw, that too did nothing. Any ideas on how to slow the idle speed down????

Disconnect the linkage rod(s) that go from carb to carb and see if that returns the idle to spec. If it does, you know your rods need to be adjusted to the right length. If it doesn't affect the idle, you need to sync the carbs properly and then adjust the linkage rods.

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I did have the linkage disconnected. How do I adjust them?? I tried all the basic stuff.

I did all this but the idle speed never dropped.



1) Attach a tachometer to measure engine RPM. Use the low range as you will be looking for 25 to 50 RPM differences here.


2) Find the idle mixture screws (removing the air cleaner helps). These puppies are at the middle base of the carburetor and face you if you stand on the passenger side. If you have trouble, look in the Service Manual, page 01-3/24, find screw number 20. Most of these idle screws we will be talking about (screw numbers 20, 9, 19) are gnarled. You might want to take a minute and read the next page of the service manual on idle adjustment. It is the abridged version of what comes next here.


2a) If you are starting from scratch, say after a carb rebuild, lightly snug the idle mixture screws on each carb all the way in and back out 2 turns. They turn just like a faucet; CLOCK-WISE is in (lean) and COUNTER CLOCK-WISE is out (rich). You can also try this idle mixture screw setting as a starting point if your car is unable to idle without help from the throttle linkage (see next step).


3) Make sure that the throttle linkage is free and adjusted. Find the idle stop screw, number 9 in the service manual on page 01-3/24. It is on the linkage on the front carb. It may still have a spring around it. If it is pressing on the linkage, make it quit. If your car stops running go find the screws in next step 4 and make the car idle with those. Then come back and make sure that this screw does not press on the linkage.

4) Find the two idle adjustment screws (number 19 in the manual on page 01-3/24). They are near the top of the carb, left hand side and each one has a spring around it. Back them off as much as possible but keep the engine running. The three screws in steps 3 and 4 press against the linkage. When you are finished you should be able to get a low idle without these screws pressing against the linkage. If you can't, back the idle adjustment screws off the linkage as you balance the carbs. You should be able to back the idle adjustment screws off completely and still get a slow 600 to 800 RPM idle. Again, the idle stop screw must be free of the linkage
By Gordon Lewis

If you are 100% certain that the carbs are correctly syncronized and that he linkage rods are not pulling on the carbs at idle, including the linkage from the accelerator pedal, then your problem is elsewhere.
1) Vacuum leak....spray around the bases of the carbs and where the manifold bolts to the head with carb cleaner with the engine running. Any place you spray that changes the idle is a vacuum leak
2) Vacuum leak from brake booster....remove booster hose at intake manifold, plug fitting on manifold and see if the idle improves.
3) Ignition timing....put a timing light on it and see what the timing is set at. If it's set too advanced, or if it has a vacuum retard distributor, the engine will idle too fast.

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