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Hitachi DCH306 was first used in the 1972 Datsun 1200 for North America. It is just like the original 1200 carburetor DCHG306, but the H version has an Automatic Choke. This series continued to be modified, with an anti-diesel valve added for 1973 and was futher developed well into the 1980s.
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Overview
The DCH306 carburetor is a very good carburetor. It only has 2 or 3 vacuum hoses on it.
1. Vacuum advance port 2. Choke Pull-off port & diaphragm 3. EGR port (added with 1974 North America models)
Additionally models which use the Throttle Opener system have the T.O. diaphragm attached to the carburetor which uses manifold vacuum. It has no carburetor vacuum connection.
California models eventually added 2 or 3 ports for Altitude Compensation. This added to the hose count, but technically they are not vacuum connections.
1972: Debut with automatic choke 1973: Anti-diesel valve added 1974: EGR port added 1975: 2 ports for an external Altitude Compensator added California models only 1981: 3rd port for Altitude Compensator fitted with a pipe 1981: Vacuum-controlled Secondary Throttle actuator added 1981: Replaceable fuel bowl sight-glass added 1981: Vented fuel bowl incorporated
In 1981 one last hose was added, not a vaccum hose but a large diameter vent hose to the top of the fuel bowl.
1972 Datsun 1200
The DCH306 was introduced for 1972 North America models in June 1971. Its only noticeable feature over the DCG306 was the addition of Automatic Choke. This is a fairly simple reliable system that it totally automatic.
DHC306 began with DCH306-1, for North America Datsun 1200 Manual Transmission.
Anti-Diesel
For 1973 North American models, and Anti-diesel valve was added to prevent engine run-on. This required a new casting, but no other changes were made. Change occured July 1972 for the 1973 model year.
Anti-diesel started with DCH306-4.
EGR
An EGR port was added to the DCH306 for North American models.
1981
New type sight-glass introduced. This type is replaceable unlike previous versions.
Fuel bowl is now externally vented. This goes to a special 5-hose Charcoal Canister. The canister has a Vent Switching Valve which is open when the engine is not running, so that vapors from the fuel bowl can go directly to the charcoall filter. When the engine starts, vacuum from the intake manifold closes the VSV.
Altitude Compensator ports marked 1,2,3
Vacuum-Secondary introduced
3rd A-C Port
California models: 3rd port for Altitude Compensator is finally fitted with a pipe.
See main article: Altitude Compensator
This port is also used for the Mixture Ratio Rich-Lean Exchange System.
Mixture Ratio Rich-Lean Exchange System
California models use the 3rd Altitude Compensator port on the carburetor to improve fuel economy by leaning the mixture at certain times. This occurs with the engine coolant is below 35F OR when it is above 35F and above 40mph. This only leans the Primary-side of the carburetor via the main air bleed port. At high altitudes the Altitude Compensator opens this line, so the MRRLES has no effect at high altitudes.
Fuel Shut-Off System
Although not requiring any special carburetor feature, 1981 saw the introduction of the Fuel Shut-Off System for increased economy. It cuts fuel flow while the engine is decelerating.
See main article: Fuel Shut-Off System