Quick Self-Check (Essential for Beginners)
Before proceeding with a deep disassembly, take 30 seconds to complete these three checks—they resolve 80% of “false faults”:
1. Fuel Tank Switch: Ensure it's in the “ON” or “RES” position, and the tank contains fuel.
2. Choke lever: When starting a cold engine, pull the choke lever fully (to close the choke and enrich the fuel mixture). Push it back after the engine warms up.
3. Kickstart method: For kickstart models, ensure you deliver a strong, quick kick—not a weak, half-hearted one.
If all checks are correct and the issue persists, proceed to our core diagnostic process.
Systematic Diagnostic Process (Core Method)
Follow the sequence “Electrical first, fuel second, compression third” for the most efficient troubleshooting path.
Step 1: Check “Electrical” — Is the spark strong?
This is the most common source of issues.
1. Remove the spark plug:
· Use a spark plug socket to remove the spark plug.
· Inspect electrode condition:
· Normal: Electrodes appear brownish.
· Heavy carbon buildup: Black, dry carbon powder indicates a rich mixture or poor combustion.
· Oil contamination: Wet appearance indicates oil entering the combustion chamber (oil burning).
· Initial treatment: Clean electrode carbon deposits with a brass brush or dry oil residue by heating.
2. Ground Spark Test:
· Replace the spark plug cap on the cleaned spark plug.
· Press the metal threaded portion of the spark plug firmly against any clean, unpainted metal surface on the engine (e.g., cylinder cooling fins).
· Turn the ignition switch ON and quickly kick the starter pedal.
· Observe: Does a bright, blue-white spark jump across the electrode gap accompanied by a crisp “pop” sound?
· Result Interpretation:
· No spark or weak spark (red): Electrical system fault. Possible causes: Damaged spark plug, distributor cap (high-voltage coil), ignition coil, distributor (platinum), capacitor (for older models), or wiring.
· Strong spark: Circuitry is generally functional. Proceed to next step.
Step 2: Check “Fuel” — Is the mixture correct?
3. Inspect fuel supply:
· Disconnect the fuel line at the carburetor base. Observe if fuel flows freely. If no fuel flows, check the fuel tank switch, fuel filter, and fuel lines for blockages.
4. Inspect the carburetor: (Primary suspect)
· Float Chamber: Loosen the drain screw at the carburetor's base. If no fuel flows or flow is restricted, the float needle valve or fuel passages are clogged.
· Cleaning: Carburetors are precision components. Prolonged disuse often causes fuel gumming that blocks orifices. Thoroughly disassemble the carburetor. Spray carburetor cleaner into the main jet, idle jet, and fuel passages, then dry with compressed air.
· Mixture Ratio: Verify the mixture screw has not been tampered with. The initial setting is typically fully tightened then back-off 1.5 turns.
Quick fix: Inject a small amount (approx. 1-2 ml) of fresh gasoline into the cylinder through the spark plug hole. Reinstall the spark plug and attempt to start.
· If it starts briefly then stalls: This indicates ignition and compression are generally functional, confirming the issue lies with carburetor fuel delivery.
· Still no response: The problem is more likely related to compression or ignition timing.
Step 3: Check “Compression” — Is cylinder pressure sufficient?
5. Finger Test Method:
· Firmly plug the spark plug hole with your finger.
· Kick the starter pedal and feel the impact force. If your finger is forcefully pushed away, compression is adequate; if the pressure feels weak, compression is insufficient.
6. Cylinder Pressure Gauge Measurement (Most Accurate):
· Screw the pressure gauge into the spark plug hole. Fully open the throttle (to avoid throttle body interference). Rapidly kick the starter pedal several times until the gauge reading stops rising.
· Healthy Pressure: Typically between 9-12 kgf/cm² (approx. 125-170 psi).
· Low pressure indicates internal engine wear or malfunction, such as:
· Improper valve clearance or valve leakage (most common; resolvable by adjusting valve clearance).
· Cylinder, piston, or piston ring wear.
· Damaged cylinder head gasket (often accompanied by oil emulsification or external oil leakage).
Ultimate Solution and Troubleshooting Checklist
If the above steps fail to resolve the issue, conduct an in-depth troubleshoot using this checklist:
Category 1: No Spark at All
· Possible Causes:
This condition is typically caused by electrical system faults. Common problem areas include: damaged or failed spark plugs; burnt contacts or improper gap adjustment in traditional contact breakers (platinum); capacitor failure in the protective contact; or internal damage to the ignition coil.
· Recommended Solutions:
First, replace the spark plug with a new one. For platinum ignition systems, thoroughly clean the distributor contacts and adjust the gap to the standard range of 0.3-0.4 mm. Replace the capacitor if damaged. Finally, inspect the ignition coil and replace it if necessary.
Category 2: Weak Spark
· Possible Causes:
A spark present but lacking intensity typically indicates aging of the high-voltage coil (ignition coil) due to prolonged use, resulting in diminished high-voltage output capability. Another possibility is poor electrical contact within the circuit system, such as oxidized plugs, loose wiring connections, or excessive contact resistance in the grounding wire.
· Recommended Solutions:
Prioritize replacing the high-voltage coil. Simultaneously, systematically inspect all circuit connection points from the ignition switch to the distributor, ensuring secure contact. Focus on cleaning the vehicle's grounding points to guarantee proper grounding.
Category 3: Fuel and Spark Present, but Engine Won't Start
· Possible Cause Analysis:
This is the most perplexing scenario, indicating normal basic supply but coordination issues. The most common cause is severe clogging of the carburetor's internal fuel passages or orifices by contaminants. For older models, ignition timing drift may occur. Additionally, valve seal leakage causing compression loss is a potential factor.
· Recommended Solutions:
Thoroughly disassemble and clean the carburetor to ensure all fuel passages are clear. For platinum ignition systems, use a timing light to precisely calibrate ignition timing. Finally, inspect and adjust valve clearances; if ineffective, check valve seal integrity.
Category 4: Cold/Warm Start Differences
· Cold Start Difficulty:
· Primary Cause: Choke mechanism malfunction or non-use, resulting in excessively lean cold-start mixture. Carburetor idle fuel passages may also be clogged.
· Remedy: Ensure proper choke operation by fully engaging it during cold starts. Simultaneously clean the carburetor, focusing on clearing idle fuel passages.
· Hot Engine Difficulty Starting:
· Primary Cause: Excessively rich fuel mixture. An overly high fuel level in the carburetor float chamber may cause “flooding,” where the spark plug becomes saturated with fuel and fails to ignite.
· Solution: Check the air filter for blockages and readjust the carburetor mixture ratio. If flooding has occurred, remove the spark plug, wipe it dry, and fully open the throttle while cranking the starter several times to expel excess fuel from the cylinder before attempting to start again.
Summary
By following the systematic troubleshooting steps outlined above, you can address CG125 startup issues more effectively. We recommend starting with the simplest circuit checks and gradually progressing to more complex diagnostics. This approach resolves most startup problems efficiently.
