Weber questions

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

I bought my 79 spider four years ago. It was fitted with a Shankle Weber
conversion, Shankle headers and test pipe and a Shankle Sport Marelliplex
ignition. The carburetors are 40DCOE 139 with 32 mm venturis. The
carburation was always tricky when the engine was cold and it sneezed back
through the carbs when cruising at about 3500 rpm. I got a steady 22 mpg.  I
recently rebuilt the engine with 10:1 pistons, 10548 cams and a Euro intake
manifold and air box. I also replaced the high pressure fuel pump/pressure
regulator with a low pressure pump. I am running with 39 degrees of ignition
advance at 5000 rpm. The carburation is much better, there is no sign of
detonation on 93 octane gas and I am getting 27 mpg. The NGK BP6ES spark
plugs are a clean tan color. I have made only minor adjustments to the
mixture screws and throttle coupling screw. But the Weber settings, as
supplied by Shankle, seem unconventional.

1.    The idle mixture screws are 3 turns out from fully closed. This is the
leanest setting that works. The books say that if the screws are more than
1.5 turns open, then the idle jet is too small. The idle jets are 59F21
2.    The auxiliary venturis are 6.0  As far as I can tell, 4.5 was the
standard for almost all Alfas. 6.0 has the effect of delaying the main
circuits until higher rpms.

The combination of 1 & 2 seems to be that the under sized idle jet is
influencing the mixture over a wide rpm range, which may be getting me
excellent gas mileage.

The main jets are 138 and the air correction jets are 160. These seem to be
a bit richer than standard.

My questions are: Should I go to bigger idle jets? Am I likely to be giving
up much mid range performance with the smaller jets? How much change in gas
mileage would I likely see if I changed?

Thanks,

Ed Prytherch, Columbia SC
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