EETC Carburetor Service and Repair 2 — Questions and Answers
Question 1: What tool is required to adjust the mixture screw on many EPA-compliant carburetors equipped with limiter caps?
- Standard flat-blade screwdriver
- Phillips #2 screwdriver
- Splined or D-shaped limited adjustment tool (Correct answer)
- Torque wrench with a socket adapter
Correct answer: Splined or D-shaped limited adjustment tool
EPA-compliant carburetors use tamper-resistant limiter caps that require a special splined tool to restrict how far the mixture can be enriched.
Question 2: What symptom most clearly indicates the idle mixture screw is set too lean?
- Black smoke emitting from the exhaust at idle
- Engine runs rough and tends to stall at idle speed (Correct answer)
- Excessive fuel consumption during normal operation
- Fouled, black-coated spark plug electrode
Correct answer: Engine runs rough and tends to stall at idle speed
A lean idle mixture provides insufficient fuel for stable combustion at low rpm, causing rough running or stalling.
Question 3: What is the correct initial starting position for a typical carburetor mixture screw before fine-tuning?
- Fully closed (seated) with zero turns out
- 1 to 1.5 turns out from lightly seated (Correct answer)
- 3 turns out from lightly seated
- Fully open to maximum travel
Correct answer: 1 to 1.5 turns out from lightly seated
Starting at 1–1.5 turns out from lightly seated provides a safe baseline mixture that allows the engine to start for final adjustment.
Question 4: Which carburetor adjustment directly sets the engine idle speed?
- Rotating the main jet to a larger orifice
- Turning the throttle stop screw in or out (Correct answer)
- Raising or lowering the float height
- Replacing the venturi insert with a different size
Correct answer: Turning the throttle stop screw in or out
The throttle stop screw mechanically limits how far the throttle plate closes, setting the minimum airflow and engine idle speed.
Question 5: What is the correct method for verifying carburetor adjustment after making changes?
- Rev the engine to full throttle briefly with no load attached
- Test performance at idle, mid-range, and full throttle under actual load (Correct answer)
- Run the engine at idle for 30 seconds and listen for smoothness
- Check the color of fuel remaining in the bowl after shutdown
Correct answer: Test performance at idle, mid-range, and full throttle under actual load
Each throttle range uses a different carburetor circuit, so adjustment must be verified across all operating conditions to confirm proper tuning.
Question 6: What color exhaust smoke on a four-stroke engine indicates a rich fuel mixture condition?
- White smoke indicating water vapor or coolant
- Blue smoke indicating oil burning
- Black smoke indicating unburned fuel (Correct answer)
- Gray smoke indicating normal combustion
Correct answer: Black smoke indicating unburned fuel
Black smoke results from excess unburned fuel (carbon soot) passing through the exhaust due to an overly rich mixture.
Question 7: What does float height measurement determine in a carburetor?
- The throttle plate angle at idle position
- The fuel level maintained in the float bowl (Correct answer)
- The diameter of the main jet orifice
- The size of the air bleed passage
Correct answer: The fuel level maintained in the float bowl
Float height determines how high the fuel rises in the bowl before the needle valve closes, directly affecting the fuel level and mixture richness.
EETC Carburetor Service and Repair 2
What tool is required to adjust the mixture screw on many EPA-compliant carburetors equipped with limiter caps?