Rick,
The dilution of the exhaust gases by adding fresh air was an Italian
trick, if I remember. Perhaps Alfa pulled that one off. Best one,
done by Alfa, was to keep shipping vehicle's made before 1/1/68 when
the new emission and safety rules took place. Seems they kept selling
"made in 1967" Alfa's up until 1971 or '72.
For DATSUN, the injected air was to help burn any unburned fuel
(Hydrocarbons - HC).
Well, for it to work well you need to have LOTS of unburned fuel
in the form of an overly rich mixture. To keep the exhaust hotter, to
allow better burning, they also retarded the ignition timing at
idle on those '68-'70 models. Goal was to reduce emissions at idle.
Without the air injection, the car is really running too rich.
The retarded ignition timing also would lead to high under hood temps
and overheating. On the U20 motors, they added a temperature controlled
valve, so if the engine started to overheat, it would apply manifold
vacuum (high at idle) to the distributor's vacuum advance, causing
the engine to advance the timing, revving the engine higher and reducing
the heat build up. Alas those "Thermal modulator Valves" tended to fail
within a few years. Further, to keep the air moving under the hood came
the seven bladed fan, with a fan clutch. Alas the fan clutch control
was a wax pellet type, that also failed.
Sad part is a lot of the emission equipment failed within the first
five years.
We used to use small wire mesh in the hose leading from the air pump to
catch the carbon vane pieces. Kept the air gulp valves from being stuck
open (leaning the mixture, causing running and overheating problems).
Alas when the three vanes on the U20 air pump finally gave up the
ghost, the check valve on the air injection port would fail. Once
the check valve failed, corrosive exhaust gases would back flow
into the rest of the system, making the air injection pumps near
impossible to get apart and rebuild. At one time Datsun had small
rebuild kits with the bearings and new vanes.
So if you have a "desmogged" vehicle, doing things like changing to
earlier needles will help. Four bladed fan, electronic ignition,
and keeping the car in a good state of tune.
On the ignition systems: going to the electronic distributor that
Gary has available is a good solution. I also like using the original
points, but with the addition of a MSD 6A ignition system. Typically
a "excellent" ignition point setup will still have 1000ppm misfire
(ignition doesn't occur). That means out of 1,000,000 complete four
stroke cycles, that in 1000 cases the fuel was never ignited and sent
right out the tail pipe! The 1000 ppm is a excellent target for the
old mechanical spark system. The OEM's were trying to keep it to 100
ppm to reduce emissions, by using modern electronics. The FED's would
really like the number to be less than 2 ppm occurances! Whew, quite
a target.
I was lucky enough, while working for my previous employeer, to get to
see some of the test cells that are used for modern engine design. What
still leaves me shaking my head was a puddle of fuel inside a combustion
chamber that just "sat there" while the engine was running. The liquid
was never ignited, but at times would be added to the incoming A/F
mixture and other times would start building up again. Engine was up to
temp, and running at full throttle, yet that puddle just sat there. Even
with the flame front expanding at 3000K, there is was. The team of
engineers were working on a model to prodict the behaviour of the fuel
sitting there. They still have some years of work, as "it just can not
occur". Stuff like that made my old job very interesting.
SU Tuning Tips: Excellent collection by Julian Serles' can be found
at: http://www.jetlink.net/~okayfine/sutech.html Note the 510's
commonly run the 1.5" (38mm) carbs like the 1600 roadster, the 2000's
run the 1.35" (46mm) carbs like the 240Z. Helps to remember that when
reading the articles.
Cheers,
Tom Walter
Austin, TX.
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