Dell'Orto PHBG 21 DS carburettor with oil mixer inlet for 2-stroke scooters and motorcycles

The Dell'Orto PHBG 21 DS carburettor is the replacement part for 50 cc two-stroke scooters and motorcycles with separate lubrication: 21 mm bore, an inlet for the mixer oil, a cylindrical throttle slide and an independent starting circuit. It connects to the manifold with a flexible fitting and uses the manual cable choke.

It is intended for two-stroke engines and some four-strokes, and is widely used both to replace a worn or dirty original carburettor and for tuning motorcycles and scooters. Dell'Orto is the benchmark name in carburettors in Italian motorcycling.

What the carburettor does and how it is built

The carburettor prepares the mixture of air and fuel sent to the combustion chamber. On the PHBG 21 DS the air is metered by a cylindrical throttle slide; idle is handled by a jet with a mixture or air adjusting screw, the main circuit by a tapered needle with mechanical mixture adjustment, and cold starting goes through an independent circuit operated by the choke.

Technical features

• Dell'Orto PHBG 21 DS, 21 mm bore
• Oil inlet for the mixer (separate lubrication)
• Cylindrical throttle slide
• Flexible fitting to the manifold
• Manual cable choke (cable not supplied)
• Independent starting circuit
• Idle with jet and mixture or air screw
• Main system with tapered needle and mechanical mixture adjustment
• For two-stroke and some four-stroke engines

Which scooters and motorcycles

It is used on 50 cc two-stroke motorcycles and scooters from many makes, including:

• Derbi: Senda, GPR 50, GP1 50, DRD Pro
• Aprilia: RS 50, Rally, Sonic, SR 50, Tuono 50, Area 51
• Beta: Ark, RR 50
• Gilera: DNA 50, Runner SP, Stalker, Typhoon
• Malaguti: F10, F12, F15 Firefox, Centro 50
• MBK: Booster, Nitro, Stunt, Ovetto, Mach G
• Piaggio: Zip, Liberty, NRG, Beverly 50
• Yamaha: Aerox, BW's, Neo's, TZR 50

Before buying, check that your model uses a carburettor with a 21 mm bore, mixer inlet and flexible mounting.

Tuning and setting up

If you fit it after changing the exhaust, air filter or cylinder kit, check the mixture and jets: the setting depends on the engine and the modifications. Set the idle first with the screw, then check behaviour at mid and top revs and change the jets if the mixture is too rich or too lean.

When to replace the carburettor

Replace or overhaul the carburettor when the engine is hard to start from cold, idle is unstable, fuel consumption rises or power drops and cleaning the jets and float bowl does not help. A worn carburettor or a scored throttle slide upsets the mixture and can damage the engine.

Before you buy

Check that your engine is a two-stroke with an oil mixer and compare the bore, manifold fitting and oil inlet with the fitted carburettor. The choke cable is not included: buy it separately if needed.

Frequently asked questions

What bore does the PHBG 21 DS have?

21 mm (PHBG 21 series).

Does it have the mixer inlet?

Yes, it has the oil inlet for separate lubrication.

Is the choke automatic?

No, it is a manual cable choke and the cable is not included.

Does it suit four-stroke engines?

It is intended for two-stroke and some four-stroke engines: check your model.

Can I use it for tuning?

Yes, it is also widely used on tuned motorcycles and scooters; the mixture must be set after fitting.