How to Maintain the Carburetor and Fuel System (6)

 

Accelerator
Pump Lever Adjustment

Do
not forget about the accelerator pump lever adjustment. For oval track use, the
pump lever should be adjusted so there is no play in the pump linkage when he
throttle is closed. This will assure that there will be no lean stumble when
the carburetor comes off idle. Tuning an accelerator pump for maximum
performance off the corner often involves reducing, rather than increasing, the
pump volume and discharge rate.

 

Drag
racing calls for a slightly different approach. For the hardest starting line
launch with a foot brake, the pump lever override spring should be adjusted so
that fuel starts to discharge through the nozzle at an engine speed lower than
launch RPM. If a car leaves the starting line at 5000 RPM, the pump shot should
begin at 4700-4800; an 1800 RPM launch calls for the accelerator pump shot
starting at 1500. The key is to have no slop in the accelerator pump system at
starting line RPM, so that the pump shot is not “used up” below that
RPM. Although adjusting the accelerator pump as described will create a lot of
slop in the pump linkage at idle, (and may produce a stumble when driving in
the pits) a car will leave harder. Drag cars equipped with a stick shift or
transbrake, where starting line launch is accomplished with the carburetor wide
open, requires adjusting like an oval track application.

 

Float
Level Adjustment

When
assembling a carburetor or reinstalling the fuel bowls, you need to adjust the
float to be approximately 0.450″ from the top of the bowl (in line with
the bowl screw bosses when the bowl is upside down). This “dry” setting
is to get you in the ball park. Recheck the float level with the engine running
and the sight plugs out. Fuel should just wet the outside of the bowl as the
fuel seeps out. On the secondary side, where the sight plug is lower, fuel
level should be slightly higher. On Demon carburetors, the large, patented
sight glass windows allow float level setting without having fuel escape from
the float bowl. You can adjust the float level relative to the three cast-in
marks next to the float window.

 

Power
Valve Tuning

The
power valveýs function is to supply extra fuel for wide- open throttle, high
load conditions. When manifold vacuum falls below the vacuum level stamped on
the power valve, the power valve opens and enriches the main circuit by about
six to ten jet sizes. This occurs under high engine loads such as full
throttle. The power valve number should be at least 1.5-2.0 in/hg of vacuum under engine idle vacuum. If an
engine produces 8.0-9.0 in/hg
of vacuum, a 6.5 power valve would be a good first selection. Using a power
valve with a rating lower than this will delay the enrichment and can cause
hesitation. However, on an oval track car with restricted carburetor rules,
using a power valve with a lower rating can sometimes help performance coming
off corners. Any drag race carburetor with a secondary power valve must be
turned sideways to avoid fuel starvation. The power valve is higher than the
jets and is the first to be uncovered as fuel is pushed to the rear of the
float bowl. Since there is no way to put an extension on the power valve, the
carburetor must be turned sideways to eliminate fuel starvation.

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