What Happened To The 73 Z's ?
U.S. Emissions and Safety Regulations covering the auto industry changed significantly for the 1973 Model Year. These new emissions standards caused Datsun (NISSAN MOTORS USA) to alter the design of the engine fuel system, the design of the cylinder head combustion chambers and other related tuning spec.'s. The new safety standards also resulted in changes in the materials used on the interiors of the cars and the seat belt systems.
While these emissions and safety regulations were believed to be necessary by the Federal Government, they caused many problems for both Datsun and its Consumers.
Here we will cover in a general or informational manor, the "field repairs or modifications " performed at the Datsun Dealerships. Many of these modifications were incorporated into the production process for the cars produced later in the production year, in an attempt to correct the running or operational problems the drive train related design changes caused.
Magnifying this problem was the fact that Datsun Customers usually purchased their Z's in large part because of the great reputation that they had established for dependability!!
Some Common Misconceptions About This Problem:
Many people refer to this problem as a "Vapor locking" problem. While it was indeed that, it was also a "percolation" and "vacuum break" problem.
Lets Define These Terms At This Point, as Datsun Did In Their Service Bulletins of the time:
2. Float bowl evaporation may lower the float level while there is residual pressure in the fuel line. This pressure then forces an excess amount of fuel into the float bowl.
3. Fuel evaporates in the float bowl, and the resulting vapor pressure may not vent quickly enough. The high pressure developed in the float bowl forces liquid fuel into the intake manifold.
Vacuum Break When the choke is closed [except for a small air gap] and the engine starts, the vacuum created by the engine starting will try to completely shut the choke valve. This vacuum is broken by the vacuum break diaphragm, which holds the choke open a small amount. The vacuum break diaphragm operates by manifold vacuum and is connected to the choke valve by linkage.
Modification Kits: When a customer returned his car to the Dealership with complaints of stalling, hard starting when hot and poor performance. The problem was addressed in an orderly progression of repair procedures (adjustments) and/or modifications. These are referred to as "Modification Kits - Version 1, 2, 3.... or the V1, V2, V3 Mod.'s".
The procedures and modifications are too extensive and too technical to list here completely. You can find them in the " 1973 240-Z and 1974 260-Z Fuel System Modification Manual". However, here are a few of the main items involved: