HG is considered vacuum. All those springs for tuning have a HG after the number.
METERING RODS:
The metering rods are accessible through top of the carb by loosening and rotating their retainer plates. The metering rods use a
small vacuum piston to pull the rod down and a spring to push the rod up in relationship with motor vacuum. High vacuum pulls
the piston down, low vacuum allows the spring to push the piston up. The high/low vacuum relationship corresponds with motor
demands, no/low motor load conditions (cruise) will have high vacuum, high motor load conditions (power) will have low vacuum.
The metering rods are machined with two different diameter steps, smaller at the bottom (power step) and larger above (cruise
step). When the motor is operating at cruise speeds there is no load on the motor and it produces high vacuum keeping the
metering rods down in its cruise mode. In the cruise mode the larger diameter part of the rod is in the primary jet restricting the
jet’s area allowing less fuel through the jet into the primary boosters (lean). When the motor is under power (pulling/load) the
vacuum is low allowing the spring to push the metering rod up into its power mode. In the power mode the smaller diameter part of
the rod is in the primary jet allowing more fuel through the jet into the primary boosters (rich).
Different sized rods change primary overall jet sizing, changing a rod size on either step by one diameter size (.002) is equal to
one half (1/2) of a jet size, making it easy to fine tune the jet mixture. When a jet change is slightly too rich or lean, a rod size
change will usually get you where you need to be.
METERING STEP UP SPRINGS:
The metering springs are rated by the amount of vacuum (HG) required to compress the spring and hold the metering rod in the
down position (they are not rated by weight or length). When vacuum decreases as the throttle opens the metering rod spring
pushes the metering rod up out of its cruise position and into the power position increasing fuel into the motor. In the power mode
the piston/rod is up (low vacuum spring pushes rod up), in the cruise mode the piston/rod is down (high vacuum compresses
spring). The higher the HG rating of the spring in relationship with the vacuum present, the faster the rod will move up into the
power mode and visa / versa. If air/fuel ratios (jetting) are correct, a higher HG spring in most cases will cure a stumble. 8hg
(silver) is the heaviest spring, 3hg (blue) is the lightest. 5hg is normal (orange).