David Massey wrote:
> The level of vacuum at this port is the depression at the venturi. This is
> the vacuum that draws fuel through the metering jet into the air stream.
I am afraid this is very much NOT true.
It is not the intake vacuum that pulls fuel through
the jet. The intake vacuum pulls air over the top of the
jet, and the venturi effect pulls fuel in from the
jet. (Bernouilli's principle)
The intake manifold depression is created by the
pistons and held between them and the throttle
butterfly. The jet is on the OUTSIDE of this
area, before the throttle. Other than the restriction
caused by the air filter and air trunking, the
air pressure at the jet is atmospheric.
Therefore, the "vacuum" at the jet is only there
because of the air flowing over it, not from any
vacuum caused by the mechanical action of the engine
itself.
Case in point, the intake manifold depression is
highest at idle, and lowest at full throttle. Obviously
the fuel flow is LOWEST at idle and highest at full
power, which would not be the case if fuel was pulled
by this vacuum.
Fuel flow is kept roughly proportional to air
flow by the venturi, with two exceptions. The oil
in the dashpot piston causes the venturi to stay
smaller-than-usual during acceleration, which raises
the venturi effect and increases fuel flow.
As well, the taper of the needle compensates for
non-linearities in the venturi effect over different
airflow rates by effectively increasing the jet size
for higher airflow rates.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
The stoplight in front of my house is currently: AMBER
/// triumphs@autox.team.net mailing list
/// To unsubscribe send a plain text message to majordomo@autox.team.net
/// with nothing in it but
///
/// unsubscribe triumphs
///
/// or try http://www.team.net/cgi-bin/majorcool
|