Onan NHC Series Operator's/Service Manual And Parts Catalog page 277

If the duct length exceeds 5 feet, increase duct size 30
percent. Use no more than two 90 degree radius-type
(not square-type) elbows if it is necessary to change
air flow direction. When a duct is used between the
scroll discharge and the outlet vent, its unobstructed
airflow area must be at least as large as the scroll
discharge. The cross-sectional area of theduct must
be increased if air flow is restricted by bends, long
runs, screens or the exhaust pipe. Exhaust pipes
running inside Vacu-Flo ducts should be covered with
asbestos tape.
W A R N I N G
| The safety screen used to cover vents must be
J 1/
1/4-inch mesh or larger to permit sufficient air
flow, and must be commensurate with safety standards for
hazardous moving parts to avoid personal contact. Provide a short
canvas section between the engine air outlet and the external duct
or opening, to absorb vibration. If operation in cold weather is
likely, installing a shutter in the airoutlet is advisable. Cold weather
can cause overcooling if air flow is not regulated.
EXHAUST
WARNING
Exhaust gases are toxic. Provide an adequate
exhaust system to properly expel exhaust
gases. Check exhaust system regularly for leaks. Ensure that
exhaust manifolds are secure and not warped. Be sure the unit is
well ventilated.
FIGURE 2. VACU-FLO SCROLL
POSITIONS
Should a vacu-flo engine chronically overheat, the
most likely sources of the problem are:
1. Air inlet is obstructed or too small to allow proper
ventilation.
2. Air discharge opening is partially blocked by
external ducts or exhaust systems.
3. Recirculation of heated air into fresh air inlet.
The vacu-flo installation permits greater freedom in
choice of locating the air inlet and outlet openings,
and permits the use of a compartment only slightly
larger than the engine itself.
The area of the air inlet must be at least 140 square
inches. If a filter, grille, or louvers are used, the inlet
opening must be increased accordingly. The air
outlet opening should be located as close to the
engine as possible.
Use a length of flexible tubing between the engine
exhaust outlet and any rigid piping to absorb engine
vibration. Shield the line if it passes through a
combustible wall or partition. If turns are necessary,
use sweeping type (long radius) elbows. Increase one
pipe size (from manifold outlet size) for each ad-
ditional ten feet in length. Locate theoutlet away from
the air intake.
CARBURETOR AIR INTAKE
Proper engine efficiency depends upon a supply of
fresh air to the carburetor. Under special conditions,
it may be necessary to move the air cleaner off the
engine, using a longerconnection hose as necessary.
For extremely dusty or dirty conditions, install a
special heavy duty air cleaner.
FUEL SYSTEM
The engine uses either a diaphragm or pulsating
diaphragm type fuel pump. The fuel pump has a 1/8-
inch pipe threaded inlet, fitted with a 1/4-inch inverted
flare tube fitting. If a solid wall lineis used, form a loop
in the line, or install a section of flexible fuel hose, to
absorb vibration. If a line with a pipe thread end is
used, remove the flare type fitting from the fuel pump
inlet.
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