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
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
- 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.
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.
- 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.
- Change the fuel filter
- Clean the carburetor fuel inlet filter (if equipped)
- With engine cold, remove the main jet plugs from the bottom of the carburetor
- Unscrew the primary main jet. When it is out, spray with carb cleaner. Screw the main jet back in.
- Refasten the main jet plugs and snap the lock strap back over the nuts
This may need to be repeated two or three times if the problem does not totally clear up. Alternatively you can remove the carburetor and clean the fuel bowl out, backflushing the main jets and rinsing the fuel bowl clean.
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.
|Hitachi 306 Carburetor Articles|
|Diagram - Parts breakdown|
|Specifications - year by year, including Jet specifications|
|Jets & air bleeds (part numbers & locations)|
|Parts general parts|
|Adjustment (normal tuning)|
|Hitachi 306 Photo Index|
|Diagnosis & troubleshooting|
|Choke Operation and Adjustment|
|Float Level Adjustment|