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The Keihin VD carburetor

Here some details about the carburetors mounted on the Honda NTV 650 Revere. They are two Keihin VD carburetors marked as G4AACDt. They are constant velocity (CV) carburetors, that uses engine vacuum to operate a diaphragm-controlled slide, rather than a direct cable connection.

Nomenclature

Jet needle
Main jet
Needle jet
Slow jet
Pilot screw
Float valve
Choke valve
Air cut off valve

NOTICE: The terms slow, idle, and pilot are used more or less interchangeably in Honda manuals and Keihin carburetor documentation.

Side view
- Air intake from the airbox.
- Choke valve seat.
- Pilot screw seat.
- Duct for the high intake vacuum that acts on the air cut-off valve diaphragm.
- Air cut-off valve: air in (normally open).
- Air cut-off valve: air out.
Float chamber
- Main jet
- Slow jet
- Starter jet (choke/enrichment jet)
- Float valve
- Breath hole
Vacuum chamber
- Air intake from the airbox.
- Starter (choke enrichment) air supply passage.
- Air intake for the air cut-off valve: normally open. It closes on deceleration (high rpm + throttle closed).
- Air intake for the slow jet.
- Air intake for the choke.
- Small vent that connects the chambers above and below the diaphragm: it acts as a vacuum/pressure control and damping system.
Airbox view
- Air intake for the pilot and the air cut-off valve circuits.
Engine view, bottom
- Primary pilot outlet port. This is the main idle fuel outlet, controlled by slow jet and pilot screw.
- Pilot progression ports. They provide staged extra fuel during throttle opening.
Engine view, top
- Starter (enricher) outlet port and vacuum pickup port that operates the air cut-off valve.
Slide, diaphragm, needle
- Slide lift holes: vacuum bleed holes that control how quickly pressure changes reach the above diaphragm chamber.
Float bowl
- Drain screw.
- Drain conduct.

Functioning

Fuel Supply System (Figure 4-15)

Fuel from the fuel tank passes through the inlet vale into the float chamber. The fuel entering the chamber causes th float to rise until it shuts off the fuel valve, stopping flow at a level pre-determined by float level setting.

The float chamber is vented to atmosphere through an air passage opening in the air cleaner mounting flange.

Starter System (Figure 4-16)

The starting circuit consists of a cable actuated starter valve and converging fuel and air passages in the carburetor body.

Fuel metered through the enrichener jet is directed upward through a passage to the valve chamber. The sarter valve opens the fuel passage to the carburetor venturi (vacuum side) when the enrichener knob is pulled outward. Air fom an opening in the carburetor inlet is directed to the valve chamber, where it mixes with incoming fuel.

Low pressure, (vacuum) created by the downward stroke of the engine pistons, causes the higher pressure in the float chamber to force the fuel/air mixture through the fuel/air outlet passage in the carburetor venturi.

Idle and Low Speed Circuit (Figure 4-17)

At idle, with the throttle plate closed and the air stream cut off, idle speed is maintaned by fuel metered through the slow jet. Air from the slow air jet mixes with the fuel and is delivered to the idle port at the vacuum side of the throttle plate. At low speed as the throttle plate is cracked open the transfer ports are exposed to the vacuum side of the throttle plate and additional fuel is directed to the barrel of the carburetor. With the throttle plate cracked open a quantity of fuel also enters the air stream fom the needle jet. The idle and transfer ports supply additional fuel to the carburetor barrel to assist during the transition period from idle to mid range.

The venturi opening is reduced by the low positon of the vacuum piston. This enables initial air stream velocities to be higher than normaly attainable with fixed venturi carburetors. The higher air stream velocities provide greater quantities of fuel necessary for good acceleration.

MidRange Slide Position and Fuel Discharge (Figure 4-18)

As te throttle plate is opened, air flow increases through the carburetor and the pressure drop in the venturi near the needle jet increases.

The low pressure in the vonturi travels through the vacuum port in the vacuum piston to the chamber above the diaphragm. The chamber beneath the diaphragm is vented to atmospheric pressure by a passage from the chamber to the carburetor inlet. The higher pressure at the underside of the diaphragm overcomes spring pressure and moves the vacuum piston upward in proportion to the pessure difference between chambers.

The tapered needle moves upward with the vacuum piston, opening the needle jet. The higher pressure in the float chamber forces fuel into the needle jet passage. Air at atmospheric pressure from the main air jet is forced through the main bleed tube opanings and mixes with the fuel. The fuel/air mixture is then delivered through the needle jet ino the air stream.

High Speed Circuit Slide Position and Fuel Discharge (Figure 4-19)

Accelerator Pump System (Figure 4-19)

The pilot screw

The Pilot screw is on the engine side of the carb, located after the pilot jet, it is a fuel screw.

Turning it IN (tightening clockwise) reduces fuel flow, leaner mixture
- Effects:
  - Harder cold start.
  - Lower idle speed.
  - Possible hesitation off idle.
  - Lean deceleration popping.
Turning it OUT (loosening counterclockwise) increases fuel flow, richer mixture
- Effects:
  - Smoother idle.
  - Better throttle pickup.
  - Easier starting.
  - Reduced decel popping.

rigacci.org

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