Demon Fuel Systems 1905 - Manual (Page 12)

12
On the other hand, when the engine is under a light load such as idle or steady state cruising, the engine does not need
extra fuel. Under light load, there is adequate vacuum to overcome the step-up spring tension, therefore the step-up
piston stays in the bottom of its bore which also positions the metering rod at the lowest position. When the metering rod
is down or at its lowest position, the large step or lean step of the metering rod is in the main jet orifice restricting fuel flow
or leaning out the air/fuel mixture so a desirable air/fuel ratio is maintained.
Figure 18 is a cutaway view of the enrichment circuit in a light load condition such as idle or steady state cruising. The
step-up piston is in the bottom of the bore and the lean step of the metering rod is in the jet, therefore restricting fuel flow
so the desired air/fuel ratio is maintained. Figure 19 represents the enrichment circuit in a heavy load condition such as
wide open throttle. The step-up piston is in the top of the bore and the rich step of the metering rod is in the jet.
Figure 18 Figure 19
Tuning
Tuning is best completed by using a wide band oxygen sensor to monitor air fuel ratios, however tuning can be performed
by reading spark plugs and taking vacuum readings. Manifold vacuum will increase the closer you get to an ideal
calibration; it will fall off once you get past this point. The ideal color for the spark plug porcelain is light brown or tan. A
color lighter than this indicates that the carburetor is lean; a darker color indicates that the carburetor is rich.
There are a few different areas that need to be looked at separately when it comes to tuning a Street Demon
™
carburetor. These areas are; steady state cruise, part throttle acceleration and wide open throttle.
NOTE: Before making any tuning changes, it is recommended that all test drives for steady state cruising and
part throttle acceleration are completed before making any changes to the metering rods or jets.
WARNING:
Always take a helper with you to watch and record vacuum or AFR readings. Never try to drive
and write at the same time as it could result in an accident.