Demon Fuel Systems 5563020GC - Manual (Page 11)

11
Figure 14
Main Well Jets
Main jets, located along the bottom edge of the block on the side opposite the bleeds, control the
amount of fuel that enters the metering blocks, and for that matter, the amount of fuel moved by the
carburetor. If the engine appears to be running rich through the entire powerband, a reduction in jet
size will lean the fuel curve out. Jets are numbered, and the larger the number, the greater the
amount of fuel that will be able to enter the fuel circuits.
Tuning should be done by making jet changes of a number or two, in either direction. As with any
engine tuning, erring to the rich will produce less than optimal performance, whereas the same
mistake to the lean can result in severe engine damage. Err to the rich!
Demons produce a very linear fuel curve, however it differs from the curve of other carburetors. Due
to the improved atomization characteristics of the Demon, tuners with data acquisition equipment may
notice brake specific numbers lower than what has become known as the “normal.” A Demon with an
overly rich tune-up may act excessively sluggish compared to other style carburetors. In other words,
if you
attempt to match “normal” brake specific numbers, you may be left with a sluggish Demon that
is operating well below its potential.
Power Valves
The power valve, located in the primary metering block between the two main jets, offers a boost in
available fuel to the engine in high load conditions without creating an over-rich idle or part throttle.
Power valves are marked with a vacuum rating. This rating is the point at which the valve will open.
The lower the number, the more the vacuum has to drop for the valve to open. When the engine is
under a big load, it produces decreased manifold vacuum along with an increased demand for fuel.
When the manifold vacuum is less than the rating on the power valve, it opens and allows extra fuel
to enter the main circuit of the metering block. It is a good idea to have a power valve rating that is
lower in number than the vacuum produced by the engine at idle.