Carburettor Diagnosis

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Revision as of 13:57, 26 December 2012

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 Ignition (spark) problems. Click Engine Electrical System Diagnoses to troubleshoot
• Incorrect Carburetor Adjustment. If you recently changed carburetors or had it apart, this is a likely cause. Otherwise don't mess with this.
• Broken or cracked vacuum devices. See Carburetor Adjustment#Check for Vacuum Leaks
• Anti-diesel valve not working (1973 and newer carburetors)

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

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

• Idle irregular
• Acceleration includes hesitation
• Won't run right at low RPM or high RPM but will do the other OK

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

1. Change the fuel filter

2. With engine cold, remove the main jet plugs from the bottom of the carburetor

3. Spray down the air bleed hole to back-flush the main jet

4. Refasten the main jet plugs

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).

Also See:

• Carburetor Adjustment
• Carburetor Repair (major service)