Applicable Models

All CT90 Models

What is the recommended interval for setting the valve clearances?

The Honda Recommended Valve Adjustment Interval: 2,400 - 3,000 miles (4,000 - 4,500 km)

The CMC Recommended Valve Adjustment Interval: 1,000 - 1,200 miles (1,500 - 2,000 km)

We often refer to the "core tune-up procedure," which consists of three essential maintenance steps outlined in the Honda service manual for the CT90: cam chain tension adjustment, valve adjustment, and ignition timing. Performing these steps in the correct sequence is crucial for ensuring that the motorcycle starts reliably on the first kick and accelerates smoothly to redline. Since Trail 90s are designed to run hard—often at near full throttle—a properly tuned engine is essential for both performance and longevity. The final step, ignition timing, ensures that the spark plug fires at the precise moment in the compression stroke.

Preparation

Remove the points cover.
If you haven't done so before, clean and inspect the ignition advance mechanism before setting the ignition timing. A properly lubricated and smoothly operating advance mechanism is crucial. See our instructional video on how to do this.
Remove and clean the point. Lubricate the point rocker pivot.
The factory-installed JIS screws on the points plate can make adjustments difficult—consider replacing them with our Allen bolt kit.

Points Gap Adjustment

Remove the ignition advance bolt to expose the two tick marks on the cam.
Rotate the engine until the foot of the rocker arm aligns with the two tick marks on the ignition advance cam.
Set the points gap to 0.012"-0.016".
Loosen the two bolts securing the point to the points plate.
Insert a feeler gauge between the points pads.
Tighten the bolts just enough so the feeler gauge fits snugly but can still slide in and out smoothly.
Tighten completely then recheck the points gap.
Reinstall the ignition advance bolt.

Setting the Ignition Timing

Remove the spark plug.
Turn the engine over while covering the spark plug hole with your finger. When you feel air being forced out, the engine is on the compression stroke.
Once on the compression stroke, rotate the rotor until the "F" mark aligns with the stator coil alignment mark.

The crankshaft turns twice for each camshaft revolution. If the "F" mark aligns but you're not on the compression stroke, turn the engine 360 degrees to reach the correct position.

Ignition Timing Setup

In our video, we demonstrate using an external battery to set ignition timing. This is for two main reasons:

Preventing coil damage – Setting ignition timing can take a while, especially if you're unfamiliar with the process. When using the motorcycle’s battery, the coil remains live when the key is on, which can cause overheating and internal damage. Using an external battery eliminates this risk and allows you to take your time.
Isolating potential electrical issues – In our CT90 project, we have not yet verified the condition of the electrical system. By using an external battery, we can set the ignition timing without interference from other electrical components.

Connecting the External Battery for Timing

Use a jumper wire to connect the positive terminal of an external battery to the metal terminal on the small bolt at the bottom of the ignition point.
- Ensure the jumper wire only touches the terminal. If it touches the point spring or points plate, it will create a short and disrupt the process.
Connect the negative terminal of the battery to the test light’s jumper.
Touch the test light probe to an unpainted metal part of the engine.
- The test light should turn on and remain lit until the moment the point opens.

If you are setting timing using the motorcycle’s battery (connected through the main harness), the test light will remain off and only turn on when the point opens.

Adjusting the Timing

Slowly turn the engine over from just before the "F" mark to just after.
- If the test light turns off before the "F" mark, the timing is too advanced.
  - Loosen the points plate bolts and rotate the plate counterclockwise to retard the timing.
  - Recheck.
- If the test light turns off after the "F" mark, the timing is too retarded.
  - Loosen the points plate bolts and rotate the plate clockwise to advance the timing.
  - Recheck.
When the test light turns off exactly as the "F" mark aligns with the indicator notch, your ignition timing is correctly set.
Tighten the points plate bolts and recheck to ensure the timing hasn’t shifted.
Once confirmed, reinstall the inspection cover and points cover.

Troubleshooting

A common issue is the points chamber cover shorting out against the wire terminal or the point spring. If either of these components touches the cover, the circuit will ground out, preventing the spark plug from firing.

To check for this:

Start the motorcycle without the points chamber cover.
If it runs fine, reinstall the cover and test again.
If the bike suddenly stops firing after reinstalling the cover, a grounding issue is likely causing the problem.

Battery Ignition vs. Magneto

One last thing to note—the CT90 / Trail 90 uses a battery-based ignition system, not a magneto. This means that if your battery is low or dead, the motorcycle will not start. A fully charged battery is essential for starting and smooth operation.