Royal Enfield 500866 – Pilot Jet-15 (CV Carb BS26) for Classic 350 BS4, Bullet 350 ES Disc, Thunderbird X 350 Disc & Thunderbird 350 ABS (Final UCE)

The Royal Enfield 500866 Pilot Jet is the genuine slow jet (Pilot Jet-15) for BS26 CV carburettors used on select 346cc UCE 350 models and some earlier CI/AVL platforms. Multiple OEM-focused catalogs and dealer sites list this part as “PILOT JET-15 CV CARB BS-26” or “Pilot jet (Slow Jet) BS 26-15”, confirming the jet size and carb family.

500866 appears as “pilot jet” and is mapped directly to:

• Bullet 350 ES Disc – BS4 – VNCR42MS

• Classic 350 – BS4 – 1800366

• Thunderbird X 350 Disc – BS4 – 1800321 & 1800379

• Thunderbird 350 ABS – final UCE ABS model (portal internally labels this as “BSVI”, but this is still the last-gen UCE Thunderbird, not a separate BS6 platform).

This is the factory slow jet for those BS4 UCE carb bikes, ensuring correct fuelling at idle and just-off-idle.

PILOT JET-15 CV CARB BS-26 suitable for Machismo A350, Machismo A500 and Thunderbird 350 (2002–2012), all BS26 CV-carb models. 

Replacing a clogged or worn pilot jet can transform a rough, stalling 350 into a bike that starts easily, idles smoothly and pulls cleanly in traffic—with no guesswork jet sizes.

Key Features

• Genuine Royal Enfield pilot jet – Official OEM description “PILOT JET-15 CV CARB BS-26”, reference part number 500866.

• Correct slow jet for BS26 CV carb – Designed for BS26 CV carburettors used on specific 350cc UCE and older CI/AVL models.

• Confirmed BS4 UCE 350 coverage – RE portal maps 500866 to Classic 350 BS4, Bullet 350 ES Disc BS4, Thunderbird X 350 Disc BS4 (two codes) and Thunderbird 350 ABS (final UCE ABS model) as per your microfiche screenshot.

• Improves idle and low-rpm response – A fresh pilot jet fixes hard starting, lumpy idle, hesitation off-idle and frequent stalling caused by worn or blocked jets.

• OEM drilling and flow – Factory-spec orifice and length give the correct fuel flow; unlike non-OE jets which are often off-size and hard to tune.

• Ideal for carb/throttle body overhauls – Best replaced when servicing or rebuilding BS26 CV units, especially on high-mileage or long-stored bikes.

• Single-piece OE packing – Supplied as one pilot jet in sealed, traceable packaging.

Compatibility Table

*For your listing text, call it “Thunderbird 350 ABS (final UCE model)”, not “Thunderbird BS6”.

Not for: J-Series Classic/Bullet/Meteor/Hunter 350, Himalayan/Scram 411, Himalayan 450/Guerrilla 450, 650 Twins or any EFI-only model (these do not use BS26 CV jets).

CHECK PART NUMBER BEFORE PURCHASE 

Variants

• Part Number: 500866

• OE Description: Pilot Jet-15 CV Carb BS-26 / Pilot jet (Slow Jet) BS 26-15

• Jet Size: 15 (slow / pilot circuit)

• Quantity: 1 pc (single jet)

FAQs

Q1. What exactly does the 500866 pilot jet do?
It meters fuel at idle and very small throttle openings in the BS26 CV carb. When it’s blocked or worn, you’ll see hard starting, poor idle, and hesitation as you crack the throttle. Replacing it with a fresh 500866 brings the low-rpm mixture back to OE spec.

Q2. Which bikes are 100% safe for 500866 with no guesswork?
From the Royal Enfield spares microfiche (screenshot), it belongs on Classic 350 BS4 (1800366), Bullet 350 ES Disc BS4 (VNCR42MS), Thunderbird X 350 Disc BS4 (1800321/1800379) and Thunderbird 350 ABS – final UCE ABS model. These you can list as confirmed.

Q3. Why do some sellers show Machismo and old Thunderbird 350 for the same part?
CHECK PART NUMBER BEFORE PURCHASE 

Q4. Does this fit J-Series Classic 350 Reborn or Bullet 350 J?
No. All J-Series models use a different EFI system and do not reference 500866 in their parts microfiche. For them, this jet is not applicable.

Q5. Do I need re-jetting if I install 500866 on a stock BS4 bike?
If your bike is stock (airbox, exhaust) and you’re just replacing a clogged jet with the same OEM size (15), you do not need to change other jets—just clean the carb, fit 500866 and reset idle and pilot screw. Re-jetting is only required for heavy intake/exhaust mods or altitude changes.