Zenith's Idle Suddenly Too High.

bavbob

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My Zeniths were running perfectly, but all of a sudden, the idle speed will not kick down when it warms up. Choke setup seems to be working as butterfly position is where it is suppose to be when cold and then opening up when warm. I assume something is hanging up, float or needle but before I tinker, maybe this rings a bell to someone.
 

stphers

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Check your choke linkage rod that it is not hanging up. Not the one that attaches to the choke flap but the one behind the choke housing assembly that links to the linkage. The fast idle speed comes from the stepped lever inside the choke housing which is attached to the linkage rod , it sometimes rubs on corresponding linkage You can also disconnect each carb separately to see which one is hanging up Is the choke flap is completely open once the engine is warmed up? Can you flick the flap and does it flip right back to full open ? There are also a number of other areas where a carb could be hanging up but this is a good starting point
 

bavbob

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Yes, both flaps pop back up when pushed down when the car is warm. I will check for hangup, it is something as simple as that given one day it was fine, the next it wasn't.
 

Cornishman

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With an injection engine an air leak will cause the idle to increase, i assume it is the same with carbs.
Manifold or carb gaskets, lose bolts, cracked manifold etc. Worth a look?
 

bavbob

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Will explore all of these possibilities tomorrow and hopefully get to the bottom of this.
 

bavbob

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About 2200 and its an automatic so once in gear, it drops to about 1400. Use to be around 900 and then dropping to about 5-600 in gear.
 

lloyd

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Aside from the choke plate operation, what about the throttle plates returning to the fully "closed" idle position? The fast idle linkage would obviously affect this, but it is possible that something else is interfering with or preventing the throttle plates (on even one carburetor) to be cracked open. This might possibly include the automatic "kick down" linkage or a weakened return spring. As strange as it may seem, I have also observed mixture and throttle screws that were installed without springs that typically tension each screw to keep them in place. Remove a spring and at some point, the screw may move and settings are changed.

Perhaps a more likely scenario involves a small vacuum leak. This could of course include a perished vacuum hose, but it could also be as simple as a disconnected vacuum hose used to activate the distributor's vacuum fittings that control advance and/or retard settings. Likewise, bumping a slightly loose distributor could have a mild impact on both ignition timing and idle speed. I hasten to add that if you have emission controls, any loose or leaky vacuum fittings could cause or contribute to a fast idle.
 

HB Chris

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I would disconnect both carbs from the throttle rod to make sure they return against their stops. I have also seen the single stop screw on this rod become loose where the vertical rod comes up and affect the throttle rod.
 

bavbob

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Although disconnected, the dashpot plunger from the thermal system somehow migrated downward and displaced the entire linkage. All is well. Thanks you all.
 
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