- Inspect the intake valve clearances:
Excessive valve clearance can cause rough running, high vibrations, and an off-idle hesitation or stumble.
- Adjust valve clearances as per specs provided in the service manual.
2. Vacuum Leak:
- A vacuum leak can lean out the air-fuel mixture, leading to rough running and high vibrations.
- Check for leaks around the intake boot and carburetor base using carburetor cleaner spray or the propane enrichment method.
- Ensure all vacuum lines are properly connected and not cracked.
3. Carburetor Tuning:
- Verify the carburetor settings, including fuel screw adjustment and float height:
- An incorrect fuel screw adjustment or float height can affect the air-fuel mixture and cause the engine to run rough.
- Adjust carburetor settings as per the service manual specifications.
4. Spark Plug:
- Check the spark plug condition. A worn or fouled spark plug can cause rough running and vibrations:
- Replace the spark plug if needed, ensuring the correct plug gap.
5. Compression Check:
- Perform a compression test. Low compression can cause rough running and vibrations.
- Compare compression readings with the manufacturer's recommended values for your 400ex.
- Low readings might indicate a problem with the piston rings, valves, or cylinder head gasket.
6. Timing Check:
- Verify the ignition timing. Incorrect ignition timing can cause rough running and high vibrations.
- Adjust ignition timing as per specifications provided in the service manual.
7. Air Cleaner Element:
- Inspect the air cleaner element. A dirty or restricted air filter can cause the engine to run lean, leading to rough running and vibrations:
- Clean or replace the air filter if needed.
8. Engine Mounts:
- Check if any of the engine mounts are loose or damaged.
- Loose or damaged engine mounts can cause excessive vibration.
- Tighten or replace engine mounts as necessary.
9. Cam Chain Tension:
- Inspect and adjust the cam chain tension if the engine has high mileage or has not had a recent cam chain adjustment.
- Refer to the service manual for the correct procedure.
By addressing these potential issues, you can resolve the rough running and high vibration at medium to high RPM in your 400ex and restore smooth performance.
