If the throttle plates on an Autolite 2100 won’t shut completely, something is preventing them from returning fully to the idle (closed) position. This can be mechanical, adjustment-related, or due to wear.
Here’s a structured breakdown of common causes:
1. Idle Speed Screw Set Too High
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The curb idle screw (on the throttle lever) physically holds the plates open.
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If it’s turned in too far, the plates can’t close completely.
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Fix: Back off the screw until the plates just barely open for proper idle airflow.
2. Choke Fast Idle Cam Engaged
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When the choke is partially closed (cold start), the fast idle cam holds the throttle open slightly.
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If the choke linkage is sticking or improperly adjusted, the cam may stay engaged even when warm.
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Fix: Make sure the choke blade is fully vertical when warm and the fast idle cam rotates away from the stop.
3. Linkage Binding or Interference
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Throttle shaft linkage may be catching on the choke rod, return spring, or other carburetor parts.
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Cable or rod from the accelerator pedal may not be returning all the way.
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Fix: Check for smooth travel and no rubbing points; lubricate pivot points.
4. Throttle Shaft or Bushings Worn
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Excessive wear can cause the plates to misalign in the bore, preventing a full seal.
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The plates might scrape or wedge against the bore instead of seating evenly.
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Fix: Inspect for side play; rebush if needed.
5. Plate Misalignment on Shaft
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If the throttle plates were removed and not reinstalled perfectly centered, the edges may not fit the bore’s taper.
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Fix: Loosen the plate screws slightly, center the plates in the bores with the throttle fully closed, then retighten.
6. Carbon or Dirt Buildup in the Bores
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Deposits at the edges of the throttle bores can block full closure.
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Fix: Clean the bores and plate edges thoroughly with carburetor cleaner.
7. Return Spring Weak or Disconnected
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If the return spring doesn’t have enough tension, the plates may not fully close on their own.
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Fix: Replace or adjust spring tension.
Quick test: With the engine off, disconnect the throttle linkage and operate the carb lever by hand. If it closes fully when disconnected, the problem is in the linkage or pedal system; if not, it’s inside the carb.
