Dealer Modifications To The 1973 240-Z

by Carl Beck, IZCC #260


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.

About The Modifications

What Was The Problem?
The main problem was that during hot weather the Z's would hesitate, stumble and stall or stop running. They were hard to start or re-start. This left owners stranded along the side of the road or worse caused their cars to stall in the middle of an intersection! Not to mention the fact that the 73 Z's just ran and performed poorly.

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:

Vapor Lock: Vapor lock causes a leaner than normal air-fuel mixture. It effects the suction side of the fuel pump. The gasoline vapor-liquid ratio in the fuel line becomes greater than the fuel pump is able to handle. The fuel pump sends less fuel to the carburetor, causing a lean fuel-air mixture.

Percolation: Percolation causes a richer than normal air-fuel mixture. It affects the carburetor float bowl and the high pressure fuel line [the outlet side of the fuel pump], and may cause hot start problems. Percolation can come about in several ways:

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.

Meanwhile Back At The Dealership @#%*+%#*!

Corrections and Modifications Made At The Dealership Level:
Datsun responding to their Customers great displeasure with this problem, rapidly developed several repair procedures and modifications intended to correct the poor running characteristics of the 73 240-Z's.

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:

Additional Measures Taken After All Modification Kits Were Installed and A Problem Was Still Present: