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[Datsun 1200 encyclopedia]

Carburettor Diagnosis

From Datsun 1200 Club

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Categories: Hitachi 306 Carburetor | Fuel System | Diagnoses and Corrections

Before replacing your carburetor, check to see if it really is bad. Perhaps it just needs correct Adjustment or the problems you have experienced are related to ignition or other causes.


How Can I Tell if I Need to Repair or Check my Carburetor?

Carb-related problems include:

  • won't idle under 800 rpm
  • non-steady idle speed
  • engine keeps running when ignition turned off (dieseling)
  • poor mileage
  • stumbling when accelerating
  • misses or lacks power or at high speed
  • black smoke from tailpipe. Definitely a carb problem, probably the Choke or Power Valve
  • engine won't start. More likely this is not a carb problem, but some other problem
  • etc.

Now, some of these symptoms can also be caused by electrical or other causes. So see the Carburetor Adjustment page to rule this out.

Engine Won't Idle

If your engine will start, and will run if you hold the pedal down but will not idle on its own:

Not a cause:

  • Plugged fuel filter. If this was the case, it would get worse as you rev the engine

Potential Causes:

Engine Runs On

Symptom: Engine runs on (diesels) after cutting the ignition.

Cause: Idle speed too high. Solution: Adjust to spec.

Cause: Open chamber cylinder head. Solution: use idle-cutoff valve.

The Anti-diesel Valve started with 1973 models and is also known as the idle cutoff solenoid. This is an electrical wire hooked up to the Idle circuit of the carburetor.

Anti-Diesel (idle-cut) solenoid valve
1006.jpgAlbum 1006

If your carburetor has one of these, the carburetor can run well, but it won't idle at regular speed unless properly connected to wiring.

See Anti-diesel valve Troubleshooting

Multiple Symptoms

Symptoms include one or more of these (not necesarilly all):

  • Idle irregular/intermittent
  • Hesitation during Acceleration
  • Won't run right at low RPM or high RPM but will do the other OK

Cause: crud/water droplet in the fuel bowl, partially blocking the main jet. Solution: drain the fuel bowl.

Drain the Fuel Bowl

  1. Change the fuel filter
  2. Clean the carburetor fuel inlet filter (if equipped)
    25023.jpgAlbum 25023
    25022.jpgAlbum 25022
  3. With engine cold, remove the main jet plugs from the bottom of the carburetor (first pop the retainer off with a small screwdriver)
    9815.jpgAlbum 9815
  4. Unscrew the primary main jet. When it is out, spray with carb cleaner. Screw the main jet back in.
  5. Refasten the main jet plugs and snap the lock strap back over the nuts

NOTE: This may need to be repeated two or three times until the problem totally clears up.

Alternatively, you can remove the carburetor and clean the fuel bowl out, backflush the main jets and rinse the fuel bowl clean. See Main Jet Removal.

High Idle Speed

The A12 engine is very smooth and quiet. High idling is usually caused by a vacuum leak (unless your carburetor is shot).

Note that the cold idle spec is 1800-2200 RPM.

Also See:

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