re: ' ... wouldn't the vacuum in fact be less, because it wouldn't be sucking
from the engine, well it still would but
it would be pushing air through as well, and maybe a push doesn't suck the gas
up as well as regular manifold vacuum,
therefore...'
Remember the jet of an SU is located upstream of the throttle plates, so there
is no engine or manifold vacuum to speak
of around the jet. The vacuum--or 'depression' as the Brits call it--is caused
by Venturi Effect of air flowing between
the bottom of the vacuum piston and the jet bridge. As the airflow increases,
the 'port vacuum' formed as air passes
over the bridges and under the pistons--similar to the port vacuum that changes
distributor advance--causes the pistons
to rise, in turn mediating the lower pressure in this area. This is why SUs
are referred to as 'constant depression' or
'variable venturi' carburettors, as opposed to 'American' style carbs which are
'fixed venturi' type (that's why they
need several jets instead of just one and usually need other mixture
augmentation schemes).
Note that there are two holes in the bottom of an SU vacuum piston,
and--usually, I think some carbs don't--there is a
machined channel downstream of these ports so as to avoid any extra turbulence
or pressurization in this area.
I suspect--only speculation--that increasing pressure and hence airflow in this
region disrupts this balance, quite
possibly resulting in the lean-running Chris D. describes.
Bob
On 1/21/2012 7:11 PM, Greg Mandas wrote:
> Ah. That makes sense.
>
> Greg
> 65BJ8
>
> "You brought me here to fix this thing and people are key." Steve Jobs
>
> Sent from my iPhone.
>
>
> On Jan 21, 2012, at 12:33 AM, "Greg Lemon"<glemon@neb.rr.com> wrote:
>
>> Greg, I am not a scientist or engineer, but have a rudimentary or slightly
> more understanding of how the carbs work, you are right more air flow should
> raise the piston. Normally a higher piston means less cross section of the
> needle blocking the gas flow from the jet, so more gas more air more power....
> However you said more vacuum, wouldn't the vacuum in fact be less, because it
> wouldn't be sucking from the engine, well it still would but it would be
> pushing air through as well, and maybe a push doesn't suck the gas up as well
> as regular manifold vacuum, therefore (someone who knows more may confirm
> this??? or tell me I am koo koo) a lean condition results instead of more
> power, along with the possibility of holed pistons and such nastiness as has
> been mentioned as the result of rm air to an SU by other listers.
>> Greg Lemon
>
--
*******************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
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