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Hmm, where to start?...
When I first took ownership of my T3 it was getting time for a rebuild.
At the time I didn't want to replace the fuel injectors at $30 each and I
wanted a more powerful engine. Being young and impatient, I had it rebuilt
to 1641cc with dual 36mm Dellorto carbs -- to Hell with FI! Many years
later I ended up with my Porsche which came stock with the Bosch L-jetronic
FI. She, too, was having engine problems (I must be sucker for failing air
cooled engines :). Since Porsches tend to cost a tad bit more on parts and
repair I decided to learn more about my FI so I could more wisely fix my engine
before paying gobs of money to a mechanic. I got some manuals, joined a
914 email list and asked some questions. I came to understand the system
and was able to successfully fix my problems, all of them being FI related
(the engine itself is still quite strong).
It will take some time and patience to learn about the particular FI for
your vehicle (any one know of some good books?). But I found out that FI
is quite simple once you understand how it *functions* -- you don't really
need to know how it *works* as that will come later. Understand the primary
reason (function) for each component (air temp sensor tells computer if it's
cold, this allows the computer to squirt more fuel) because how it works could
be confusing (air temp sensor shows 350-500 Ohms @ 32F but 1-200 Ohms at temps >
100F, thus changing the voltage from -5v to -1.3v, which activates this
relay...). You'll learn how things work as you test them when the time comes.
For me FI is really simple now that I understand it. I finished a book about
Dellorto carbs (and carbs in general) and it turns out that they are very
complex devices that rely heavily on the physics of fluid dynamics (of air and
fuel). It's amazing how the idle circuit slopes off as the main circuit slopes
in; how the emulsion process works in the emulsion tubes to mix air and fuel
together; how small changes in jet and venturi sizes can really affect the
performance of a carb; the list goes on. There is more to carbs than meets the
eye!
Carbs are good if you are regularly changing something on/about your engine --
one reason why racers use 'em. It's easy to change a jet here or a venturi
there or adjust a screw on the spot. FI pretty much has to keep the engine the
way it is. However, I do know that there are programmable FI systems that use a
lap top computer and an oxygen sensor so you can have the FI work on whatever
motor it's on. FI is superior over carbs because it meters the fuel to exactly
what the engine needs. It takes in to consideration environmental factors such
as humidity, air pressure (elevation) and air and engine temperature. Carbs
cannot do this on the spot; it takes a human to change the components for the
necessary conditions.
Here's the number one reason why FI has problems, aside from people not
understanding it (like I once did): air leaks. Replacing the vacuum hoses is
easy, cheap and generally can solve several problems at once. In a pinch you
can use hose clamps at the hose ends to keep 'em on tight.
If you're staying stock I say keep the FI. If you really would rather have
carbs that's cool too, but you won't be solving any problems and you will gain
some more. This is one reason why I want to switch to FI when I can afford it.
Toby Erkson
air_cooled_nut@pobox.com
custom '72 VW Squareback 1.6L modified to 2.0L
stock '75 Porsche 914 1.8L