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Honda CB125 K4

Introduction#

I did restore an old Honda CB125 K4 some time ago and I'm sharing some apsects of the restoration here. Most valuable is probably the documentation sections where I store all the files I found on internet over the time, including the Workshop manual and the parts list.

Fork seal assembly#

Assembling back the front fork may include changing the oil seals between the fork bottom and the pipe. the manual shop (see documentation) refers to the fork seal driver 07947-3550000 tool to this.

I used a 40mm PVC pipe meant for wastewater. You will need a tube longer than the fork pipe, about 1m. Drill a hole at one end of the pipe. This will be used to pull a ratchet strap trough. With the strap go though the fork bottom hole meant for the wheel axle. Adjust everything and use the ratchet to put the seal in position. Be sure to oil all the parts before.

Hole in the pipe for the strap Everything adjusted with the seal in the middle The fork seal assembly setup

Piston ring compressor#

In order to put back the pistons in the cylinders the manual shop (see documentation) stipulates the use of a tool named piston ring compressor 07954-2000000. As a remplacement to this tool that I don't have I've designed an equivalent object which is 3d printed. It makes it very convinient to put the pistons back as you have both hands free.

Doumentation showing the tool 3D printed replacement in use

You can find the 3d files on my Makerworld profile.

Ignition timing oil cover#

If you want to precisely adjust the ignition timing you will need to use a stroboscopic timing light and you need to see the F mark and Advance mark of the generator rotor while the engine is running.

You will likely have oil everywhere as the generator rotor is partially immersed in the engine oil. To avoid a mess while doing the adjustment I have designed a 3d printed cover plate.

Dcumentation showing the tool

You can find the 3d files on my Makerworld profile.

Carburetor adjustments#

Adjusting the carburetors on this model can be quite complicated if you don’t have the right methodology. I spent hours trying to find the proper settings and never really got close until I discovered the methodology explained on this website: https://cb125k.lebonforum.com/t7-methode-bitza-le-detail.

That methodology shows you how to adjust each carburetor so that it has the correct air‑fuel mixture, but it does not explain how to synchronize both carburetors without a specific tool, which I don’t have. I adapted a friend’s method to do that.

Before starting, make sure both carburetors are clean and that the floats are set up properly. You can find information about this in the service manual (see documentation).

I have extracted photos from the documentation to clarify the names of each part we will need to adjust.

  • Figure 2‑8 (1) Lock nut
  • Figure 2‑8 (2) Grip‑play adjuster
  • Figure 2‑7 (1) Throttle‑stop screw
  • Figure 2‑7 (2) Air screw
  • (5) Throttle‑cable adjustment nut on carburetor
  • Grounded spark plug

(1) Lock nut<br>(2) Grip play adjuster (1) Throttle stop screw<br>(2) Air screw (5) Throttle cable adjustment nut on carburator Grounded spark plug

Note

It is called an air screw because it is on the side of the air intake. The more you unscrew it, the more air enters the mixture.

Air‑screw adjustments#

  1. Make sure there is a little play in the throttle cable, both on the grip‑play adjuster and on the throttle‑cable adjustment nut on the carburetors.
  2. Screw the air screw in to its maximum (without forcing), then back it out 1 ½ turns.
  3. Start the engine and turn the throttle‑stop screws of both carbs until the idle speed is around 2000 rpm.
  4. Let the engine warm up for about 10 minutes.
  5. Stop the engine and cut the ignition.
  6. Remove one spark plug and ground it to the frame so it still sparks (otherwise you might damage the ignition coil).
  7. Start the engine on a single cylinder and start a 10‑minute timer to avoid overheating.
  8. Adjust the throttle‑stop screw to bring the engine back to 2000 rpm.
  9. Turn the air screw ¼ turn (either direction), wait 15–30 seconds, and note whether the rpm rises or falls.
    • If the rpm falls, turn the screw the opposite way ¼ turn.
    • If the rpm rises, readjust the throttle‑stop screw to return to 2000 rpm.
  10. Repeat step 9 until you reach a point where a further ¼‑turn change makes the rpm drop (that is the optimum setting).
  11. Perform step 9 and 10 again with ⅛‑turn adjustments for finer tuning.
  12. Adjust the throttle‑stop screw to obtain roughly 1200 rpm.
  13. Repeat steps 5–12 on the other cylinder.
  14. Both carburetors’ air screws are now set; do not touch them again.
  15. Run the engine on both cylinders and fine‑tune the throttle‑stop screws to hold about 1200 rpm.

Carburetor synchronization#

  1. Turn the handlebar fully to the left and keep it there for the entire procedure.
  2. Start the engine and let it warm up for about 10 minutes.
  3. Stop the engine and cut the ignition.
  4. Remove one spark plug and ground it to the frame so it still sparks.
  5. Start the engine on a single cylinder.
  6. Use the grip‑play adjuster to obtain about 2000 rpm, then lock it with the lock nut.
  7. Fine‑tune the throttle‑cable adjustment nut on the carburetor to hit exactly 2000 rpm.
  8. If a counter‑nut is present, lock the adjuster in place; this may shift the rpm slightly, so re‑adjust if necessary.
  9. If there is a “joint” over the nut, pull it back; this also changes the rpm slightly, so re‑adjust as needed.
  10. Wait 1 minute and verify that the engine remains stable at 2000 rpm.
  11. Repeat steps 3–11 on the other cylinder without doing step 6, do not touch the grip-play adjuster
  12. Both cylinders should now run at 2000 rpm with the same grip position, meaning they are synchronized.
  13. Use the grip‑play adjuster to introduce a small amount of play in the cable, then lock it with the lock nut.

Checking your adjustments#

There are two main ways to confirm that your adjustments are correct.

Spark‑plug color#

Both spark plugs should display the same color and fall within the acceptable range shown in the picture below. (Source: https://e85eml327.blogspot.com/2017/08/couleur-des-bougies-le85.html)

Spark plug colors

Temperature#

If you have an infrared thermometer, compare the exhaust temperatures on both sides. The difference should be less than 5 °C.

Documentation#

I don't remember the sources from where I got all thoses files, sorry for not quoting them.