How to adjust/tune the carburettor slide needle for improved riding…

⚙ What does this do?

This modification allows the bike to reach better cruising speeds via slide needle adjustments. The throttle slide needle controls main jet fuelling as revs change. This also eliminates flat spots between 4k–7k rpm, common on 125s. For example, if the bike struggles at near full throttle, this mod helps it pull with less twist. Your bike should run better from 1st to 5th gear.

👇 Before we start, this tutorial assumes:

• You will do this at your own risk and without rushing.
• You have good spark plugs (e.g., Platinums) or older IX plugs.
• Your ignition coils & HT caps are sparking well.
• Your carb has already been upjetted (tap here…).
• You have a performance filter (K&N, BMC, etc.), or if using a stock cotton one, cut its snorkel.

Tools needed:

1. Go to the right side of your bike

Spend most of your time here. Work on one pot at a time and avoid distractions.

2. Unscrew the 1st diaphragm cover

Remove the 4 bolts holding the diaphragm cover.

⚠ Disconnect throttle cable!

3. Remove the spring & diaphragm

Gently remove piston slide & spring.

4. Inspect & clean parts carefully

Use Wynns Carb Cleaner for all parts including diaphragm cover. This prevents sticking between revs/gears & scoring internal carb walls.

5. Unscrew plastic plug holding needle

Use long flat blade screwdriver, gentle ⚠.

6. Remember the order of re-fitting

Keep the image in mind when refitting small parts.

7. Remove the E-Ring (Circlip) slowly

Use a small watchmaker screwdriver. ⚠ Handle gently.

8A. Needle grooves

• Pointy end of needle faces down
• Push down black spacer slightly
• Put E-Ring in 3rd groove
• Push black spacer back against E-Ring

8B. Another view

Ensure E-Ring is in 3rd notch with black spacer pushed back. (Lowest groove, pointy end down. Factory usually 2nd or top near flat end.)

9A. Check throttle slide hole

9B. Grab needle & follow image

9C. Slot needle & mini-spring

⚠ Review step #6 and 9A/B

10. Check needle for free movement

10B. Coat throttle slide ring with oil

Prevents leaks & particle trapping.

11. Refit slide, spring & cover

• Reconnect throttle cable
• Ensure 4 screws are finger tight
• Finished? Move to next cover.

12. Repeat on rear carb

13. Final check 🏁

• Airbox clamps tight
• Slide cover bolts finger tight (x8)
• Throttle & choke cables checked
• Air lines secure (prevent vacuum leaks)

Done!

Straightforward process. If unsure, comment. Otherwise, go ride! 🏁

FAQ / Troubleshooting 🎳

• Bike runs worse after mod? Check covers & airbox for leaks.
• No air leaks but still worse? Inspect spark plugs; ensure better sparking than stock.
• New plugs but still worse? Check HT Cap & ignition coils. HT Cap removed ≈ 6kΩ; Coil ≤6kΩ to frame.
• HT cap/coils high resistance? Replace both; stock ignition parts can’t be repaired.
• Changed coils/HT caps but still worse? Ensure main & pilot jets are clean. Tap here to remove carbs.
• Can this be done on stock 125cc? Mod will slightly enrich mid-range; jetting & better electrics improve results.

💡 Tips:

Replace the O-rings every time you remove manifolds to prevent air leaks. Always work in a clean environment as intake holes are exposed. We sell the O-rings in the shop!

For less stress, use stainless steel socket cap (Allen key head) bolts with a good CRV Allen key and tighten properly. Form A M6 stainless steel washers are recommended. In the UK, these are widely available at car shops including Halfords.