scared of the terms and the electronics. If you worked on a program like Windows Excel, you
should be able to breeze through this.
On a carburetor you had a low pressure fuel pump which only job was to fill the bowl with the help
of a needle and seat. Then the fuel was drawn with suction through the jets into the intake
manifold. With fuel injection you need a high pressure pump around 3.5Bar which can deliver
enough flow to keep a constant pressure on the injectors. This is the job of the fuel pressure
regulator. It is normally set around 2.5Bar, but will vary the pressure with intake vacuum. This
means that if the vacuum drops to absolute the fuel pressure will also drop so that a pressure
difference over the injector is constant. Remember the tip of the injector is subjected to vacuum
which will suck more fuel through it at idle than at wide open throttle WOT. So the regulator must
have a vacuum line connected from its diaphragm to the intake after the throttle body. This is also
true for boost pressures with turbo’s and superchargers. So before you begin, power the fuel
pump and check the pressure and leaks. The ECU will not know if this system fails except maybe
a lean mixture indication on the air/fuel ratio sensor (lambda).
On the carburetor, fuel is metered by the main jet and ventury size. The jets are fixed and can be
varied by changing them. This jet must be large enough to fuel the engine on normal load
conditions. With the ECU Fuel is metered by opening the relevant injector for a specific time. The
more air is let into the cylinder, the more fuel is required and the longer the injector is opened. The
ECU uses the Map sensor signal to calculate how much air goes into the cylinder. Changing this
amount is done by adjusting the main jet bar on the fuel maps. This value is the main fuel
parameter and all other calculations are based around it. That is why we set it first. See Setting the
main Jet.
On the Weber you have idle jets which ensure that enough fuel and air is supplied for idling. The
ECU also has this Idle Jet. It is calculated with a portion of the main jet value and is faded off as
RPM’s increase. This is the second setting to be made and is done with the engine idling in neutral
with no load.
On automatic cars the air idle adjuster is made a bit larger to accommodate a richer mix if the car
has to idle in drive. With the ECU you simply raise the dots around the vacuum bar on the Vacuum
map under Fuel maps. The vacuum bar will settle on different values between neutral idle and
drive idle. This means 2 idle jet configurations.
Normal running on the Weber is only done by the main jet and ventury size. On the ECU you have
the main vacuum map and a cruise map to vary the fuel for different conditions. Higher gears will
decrease the vacuum and lower the RPM’s. That means you can vary fuel ratios and even
compensate for different gears.
For WOT the Weber sometimes had 2 tricks. One is the power valve which will enrich the mix at
lower RPM’s and some had a separate jet on top to add fuel at high RPM’s. The ECU has a RPM
Map just for WOT. Now you can adjust the mix through the whole of the RPM range.
Carburetors all have an accelerator pump. If you press the throttle too quickly, the manifold
vacuum falls away and the flow over the ventury is for a moment not enough to suck enough fuel
for the change. Also during the transition from idling to main jet create this problem. On the ECU
vacuum is measured very fast and as the injector sits right at the intake valve, the ECU react
almost instantly. There is however provision for extra fuel and two accelerator pumps is
incorporated. One is working with vacuum signal and one working with TPS signal. If you have a
slow vacuum signal it is better to use the TPS signal, as it is connected to your foot.
For cold starting the engine requires more air and richer fuel mixture. The Weber had a trick where
you press the throttle once. The choke butterfly will close completely and the accelerator pump will
squirt some fuel to prime the mix. The bottom butterfly will also be forced open slightly. If you start
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