Webers

["Ed & Marsha Prytherch" <mred2u@xxxxxxxxxxxxxx>]

Both my cars have Webers and very similar engine setups, but very different
low end and cold performance. The 73 GTV has 10:1 pistons, 10548 cams,
Shankle Sport Marelliplex, Euro intake manifold and airbox, low pressure
electric fuel pump, cast iron 2 piece headers and OEM front exhaust section.
The 79 spider is similar except for Shankle headers and "test tube" in place
of the cat.

The carbs on the GTV are 40DCOE28 and the ones on the spider are 40DCOE139.

The jetting of the GTV carbs is pretty much standard except that the idle
jets are 45F8 instead of the more normal 50F8. The idle screws are out about
3 turns from closed. The car starts first time from cold and only needs a
little choke when the outside temperature is below freezing. It idles at 800
rpm and has no flat spots. I get about 24-25 mpg average. The car had a
standard Marelli distributor and coil when I bought it and it ran fine then.
But as I started to get points wear I got a big flat spot just above idle
and it was temperamental when cold. I switched to 'plex and the problems
went away. I think that it probably runs lean below 3000 rpm, but the strong
spark makes it driveable. The max ignition advance is about 39 degrees. I
advanced it until it started to ping, then backed it off a little.

The spider runs fine when it is warm. It too idles at around 800 rpm and
picks up cleanly. But it won't idle when it is cold. The jetting in these
carbs is unusual, but it is as supplied by Shankle. The idle jets are 59F21.
Nobody lists these jets. The air hole is a lot bigger than F8 or F9. The
idle screws are out 4 turns. Until about a week ago, everything that I had
read about setting up Webers said that the idle screws should be out about
1.5 turns and anything over 2.5 means that the idle jets are too small. I
called a Weber dealer out West and they suggested that I try 55F9. I did and
they made things much worse. I experienced the uneven idle that other
listers have described. I switched back to 59F21 jets and got my smooth idle
back.

last week, I was reading the Alfa factory engine manual where it described
setting up Webers. The specific carbs were 40DCOE78. It said that the
mixture screws should be out 4 turns. I assume that my 40DCOE139's must be
similar to the 40DCOE78's and different from the 40DCOE28 and earlier carbs.

The other bit of information is that the spider had a SPICA manifold with
Shankle conversion parts when I bought it. The off-idle performance was no
where near as good as it is now with the Euro manifold, but other changes
were made at the same time - pistons, cams, head rebuild, so I can't be sure
that it was the manifold that made the difference. It could be as Fred
suggested, that Webers work better with Euro cams than with cams designed
for SPICA. The spider did about 22 mpg before the engine rebuild and now
does 26-27 mpg under similar driving conditions and it is a lot faster.

Can anyone shed any light on the idle mixture screw settings - why it is 1.5
turns in most literature but 4 turns in the Alfa manual? If it is a
difference in carburetor design, when were the changes made? Also, I would
like to know which jets to change to from 59F21 to go a little richer.

Ed Prytherch, Columbia SC
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