Actually "hotter" cams, or cams with more overlap than stock (those that
cause lumpy or uneven idles) let a portion of the exhaust gas back into the
intake tract because the intake valve is open early enough in the exhaust
stroke to allow for this (for other performance reasons which would be
lengthy to explain, and I'm trying to keep it short). It's not unlike
exhaust gas recirculation (at idle). It can, depending upon the overlap
and cam timing, also lower the available vacuum that one normally has at
idle (usually around 18-21 hg) or very low throttle conditions. Radical
racing cams can even cause the intake pulse to actually reverse direction
in the intake track blowing backwards out of the carb, and one of the
reasons you see those long velocity stacks is to contain the "standoff" or
small cloud of fuel caused during this condition. My cam has about 16-17
hg at idle and this normal for this cam but this condition required
recalibration of the computer for my fuel injected application. Remember,
the carb (except C/D'types but I'll get into that later) really doesn't
care which way the air flows through it. It will mix fuel and air in the
Venturi no matter which way the air flow's. In these cases the idle
mixture may actually have to be set a little leaner to compensate for the
"double dose" of fuel when the air finally flows back the normal way in the
intake at idle and picks up an additional shot of fuel. Cams of this
nature can wreak havoc with idle emissions and no amount of carb tuning may
be able to bring it within (emission) specs. Sorry but you don't get
something for nothing. If the vehicle manufacturers at that time could
have gotten away with using a hotter cam and still stay within allowable
emission specs I would think they would have used them. They didn't like
the HP losses either :-) The problem with using constant depression carbs
with a "hot" cam is that with a reversal of flow the Venturi would work,
but the air valve wasn't designed to operate "backwards" as it were, and
therein lies some of the problem. This is where Webers, or other more
conventional type carbs might be a better choice. These can be more easily
tuned to run under the adverse (to the carb anyway, remember were talking
idle and part throttle not WOT) conditions. In racing situations, where
idling isn't a major concern, and vehicles don't sit idling for very long
it isn't a big problem. Many times with cars of this nature you'll hear
them blip the throttle during extended idle periods. It isn't because it
sounds cool (although it may to some) it's because the plugs are starting
to foul because of the overly rich mixture and they need to clean them out.
For a street driven vehicle which spends a whole lot of time
,comparatively speaking idling and having to pass emission standards, it
can be a real headache-
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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