THE VeloSoleX S
a Gurtner 6,5 LI Carburettor
with a Carburettor Venturi Diameter of 6.5 mm.
the engine to achieve speeds over 4000 RPM,
the Carburettor Venturi Diameter can
be increased up
a maximum of 8 mm and a larger Main Jet of up to 0.6
mm diameter (equivalent to a 44
Main Jet) so that the mixture
is rich enough at high RPM (the standard 28
Main Jet has
a flow-rate of only 28
cc [cm³] per minute with a 0.48 mm diameter hole).
The need to have the correct size
Main Jet (or Variable Main Jet setting) is very
important at high speeds, otherwise there is a possibility
that the mixture will lean out (weaken) causing overheating
with the risk of Piston Crown and Spark Plug
damage. A useful tool when drilling out the Main Jet
is the Jet Micrometer
In a basic carburettor such as the Gurtner 6,5
increasing the Carburettor Venturi Diameter will
cause a loss of Venturi
Signal at the Venturi (less depression) with
a loss of fuel
metering accuracy across the whole RPM range resulting
in poorer fuel economy and a reduction in low-RPM torque
needed for climbing hills.
Since the VeloSoleX S
has a limited low-RPM torque for climbing hills it is
to increase the Carburettor Venturi Diameter.
For those who are keen to experiment, an increase of Carburettor Venturi Diameter
from 6.5 mm to 7 mm is described below: