This article deals in detail with the setting and the general operating principle of the fuel mixture supply especially for the Italian Dellorto PHB & PHBH drum carburettors or rotary valve carburettors. In the meantime, YSN’s PHBH carburettors are manufactured in Taiwan in the same quality. The Dellorto PHBH BS 30 or the YSN PHBH BS 28 are such carburettors.

When adjusting the jetting, make sure that the engine is at operating temperature and that the jets are all clean. The function of the jets and carburettor components is often complex and confusing. This contribution should bring light into the darkness. Unfortunately, there is no accurate carburettor jetting for an engine. We do not prescribe you if you take this or that jet number, the engine runs perfectly. Too many influencing factors are relevant to this. For example:

  • Temperature
  • Air pressure
  • Oxygen content in the air
  • Tolerance fluctuations on components inside and outside the carburettor

 Basic tasks of the Dellorto & YSN PHB carburettor

The primary goal of the carburettor is to produce an ignitable gas mixture of air and gasoline. When the piston moves towards the top dead centre (TDC), the negative pressure in the crankcase draws in air that flows through the carburettor into the crankcase. In the intake manifold (Fig. 3, designation 7) of the PHB carburettor, the air passes the atomiser (Fig. 3, designation 4) and carries tiny droplets of gasoline. The result is a finely atomised gasoline-air mixture, which finally is ignited in the cylinder head.

Components and their importance for the mixture supply

First, let’s take a look at the schematic drawing of a Dellorto & YSN PHB carburettor. Subsequently, we will deal with the individual components, which are responsible for the mixture preparation.

Fig. 3:

  1. Air intake
  2. Throttle Slide
  3. Needle
  4. Atomiser
  5. Main jet
  6. Choke
  7. ?
  8. Idle mixture screw
  9. Fuel/air mixture screw
  10. Choke jet
  11. Idle jet
  12. Venting valve
  13. Fuel banjo
  14. Float valve
  15. Float
  16. Float chamber


To be continued.

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