User guide

171020 – DIAPHRAGM BYPASS (w/ idle bleed jet)
171021 – DIAPHRAGM BYPASS (w/out idle bleed jet)
INSTALLATION INSTRUCTIONS
The diaphragm bypass design reacts faster than traditional poppet bypass valves, which maintain the set pressure with
more consistency. Gasoline applications use the standard bypass #171021 for high pressure mechanical or electric fuel
pumps and #171020 with jet for belt driven fuel pumps. Alcohol applications use #171020 bypass with jet. The regulator
pressure is adjustable between 4.5 and 9 PSI.
Demon™ recommends mounting the diaphragm bypass as close as possible to the carburetor.
As with any fuel system product, proper installation is mandatory for optimal performance. All plumbing instructions hold
true for both the #171020 and #171021. READ THE INSTRUCTIONS THOROUGHLY.
The bypass should be mounted as close as possible to the carburetor (Figures 1 & 2). The return line should be plumbed
with a single #10 line. The return line should expel the bypassed fuel at the bottom of the fuel cell and away from the
outlet. All fuel tanks and cells should have fuel cell foam installed to prevent fuel slosh.
WARNING: DO NOT BLOCK THE RETURN, VENT, OR PUMP FEED. MAKE SURE THAT THE SYSTEM IS LEAK
FREE AT ALL CONNECTIONS TO ENSURE ADEQUATE SAFETY.
To set the bypass pressure, loosen the lock nut and turn the pressure adjustment stud. Turning the stud clockwise will
increase fuel pressure. Turning the stud counterclockwise will decrease pressure. Always remember to tighten the lock
nut after the desired pressure has been set. Most gasoline engines will operate best between 5 psi and 8 psi. Alcohol
engines will require 3 - 5 psi at idle. Additional pressure adjustments may be necessary at the track to tune your system
for optimal performance.
When tuning the idle pressure on the #171020, remember that the larger the bleed jet, the lower the pressure - the
smaller the bleed jet, the higher the idle pressure.
If a repair or rebuild is required, the bypass may be serviced using Demon part #171025 rebuild kit.
Figure 1