Engine Idles Too Fast

Does your engine idle too fast? 

Aisan Secondary (2nd) Shutoff Solenoid — What It Actually Does

Short answer:

The Aisan secondary shutoff solenoid blocks fuel to the secondary circuit when it’s not supposed to be used. It’s there for emissions control and engine shutdown behavior—not performance.

What the 2nd Shutoff Solenoid Controls

  • It cuts off fuel to the secondary side of the carburetor when the solenoid is de-energized

  • It only opens when the engine is running and conditions allow secondary operation

  • When the key is turned OFF, it closes immediately and prevents run-on (dieseling)

Think of it as a fuel gate, not a throttle control.

When It’s Energized vs. Not

Energized (Key ON, engine running)

  • Solenoid opens

  • Secondary fuel circuit is allowed to operate

  • Engine can transition into the secondary normally under load

De-energized (Key OFF, failure, or emissions lockout)

  • Solenoid closes

  • Secondary fuel is shut off completely

  • Prevents:

    • Engine run-on

    • Rich overrun

    • Emissions violations during decel or shutdown

What Happens When It Fails

Failed CLOSED (very common)

  • Engine runs fine at idle and light throttle

  • No power when you floor it

  • Feels like:

    • Bogging

    • Flat acceleration

    • “Won’t rev out”

  • Secondary simply never gets fuel

Failed OPEN (less common)

  • Possible rich condition

  • Can contribute to run-on after shutdown

  • Emissions issues

Important Clarifications (No Myths)

  •  It does not control air flow

  •  It does not replace jetting

  • It does not “add power”

  •  It only allows or blocks fuel to the secondary system

Diagnostic Tip (Simple and Direct)

  • With key ON, you should hear or feel a distinct click

  • No click = no secondary fuel

  • If you apply 12V directly and it doesn’t move ? solenoid is bad

  • If it clicks but you still have no power ? blockage, vacuum issue, or secondary throttle not opening

Bottom Line

The Aisan 2nd shutoff solenoid is an emissions-era fuel cutoff, not a tuning device. When it works, you never notice it. When it doesn’t, the engine feels gutless under load. Ignore it during troubleshooting and you’ll chase your tail.

How to Test an Aisan Secondary Shutoff Solenoid with a Multimeter


This is a simple electrical device. Either it gets power and moves, or it doesn’t. Here’s how to tell which—without guessing.

What You’ll Need

  • Digital multimeter (DMM)

  • Basic hand tools

  • 12-volt source (vehicle battery is fine)

Test 1: Coil Resistance (Solenoid Health Check)

This tells you if the solenoid coil is open, shorted, or alive.

  1. Key OFF

  2. Unplug the solenoid connector

  3. Set multimeter to Ohms (O)

  4. Place:

    • One probe on the solenoid terminal

    • One probe on the solenoid body (ground)

Expected Result

  • Typically 20–50 ohms (exact value isn’t critical)

  • What matters is that it’s not open or dead short

What the Reading Means

  • OL / infinite resistance ? coil is open ? bad solenoid

  • 0–1 ohm ? shorted coil ? bad solenoid

  • Normal resistance ? coil is electrically alive ? move to next test

Test 2: Power Supply (Vehicle Wiring Check)

This tells you if the car is feeding it power.

  1. Leave solenoid unplugged

  2. Key ON (engine not required)

  3. Set meter to DC Volts

  4. Black probe to good engine ground

  5. Red probe to the harness side of the solenoid connector

Expected Result

  • ~12 volts

What the Reading Means

  • 12V present ? wiring and ignition feed are OK

  • 0V or low voltage ? wiring fault, blown fuse, emissions control issue

If there’s no power here, replacing the solenoid will do absolutely nothing.

Test 3: Actuation Test (Does It Actually Move?)

This confirms the solenoid isn’t stuck mechanically.

Method A: Installed

  1. Plug solenoid back in

  2. Turn key ON

  3. Listen and feel for a distinct click

No click = no movement.

Method B: Bench Test (Best)

  1. Remove solenoid from carb

  2. Ground solenoid body to battery negative

  3. Touch 12V positive to solenoid terminal

Expected Result

  • Sharp click

  • Plunger visibly retracts

Failure Signs

  • No movement ? bad solenoid

  • Weak / sticky movement ? unreliable under load

  • Clicks but plunger doesn’t retract fully ? secondary fuel still blocked

Test 4: Ground Integrity (Often Overlooked)

Many Aisan solenoids ground through the carb body.

  • Dirty threads = bad ground

  • Paint, corrosion, or sealant = bad ground

Quick check

  • Measure resistance from solenoid body to engine block

  • Should be near 0 ohms

Common Mistakes (That Waste Time)

  • Assuming “it clicks” means it works
    ? It may not open far enough

  • Replacing the solenoid without checking for 12V
    ? Dead wiring = same symptoms

  • Ignoring ground path
    ? Very common failure on older engines

Bottom Line

If:

  • Resistance is good

  • 12V is present

  • It clicks and fully retracts

Then the solenoid is not your problem—look at secondary throttle opening, vacuum signals, or fuel delivery next.

Throttle plates
Did you remove the throttle plates during your rebuild? If they were installed upside down the plates will stay slightly cracked all the time and the engine will act like it's under throttle.


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