from the air box (such as a leaking end plate gasket), or
a clogged blower air inlet screen.
Lack of power or black or grey exhaust smoke are
indications of low air box pressure.
To check the air box pressure, connect a manometer to
an air box drain tube.
Check the readings obtained at various speeds with the
specifications in Table 5.
Air Inlet Restriction
Excessive restriction of the air inlet will affect the flow of
air to the cylinders and result in poor combustion and
lack of power. Consequently, the restriction must be
kept to a minimum considering the size and capacity of
the air cleaner. An obstruction in the air inlet system or
dirty or damaged air cleaners will result in a high blower
The air inlet restriction may be checked with a
manometer connected to a fitting in the air intake
ducting located 2" above the air inlet housing. When
practicability prevents the insertion of a fitting at this
point, the manometer may be connected to the engine
air inlet housing. The restriction at this point should be
*Engines with four valve cylinder heads.
checked at a specific engine speed. Then the air
cleaner and ducting should be removed from the air,
inlet housing and the engine again operated at the same
Air Box Pressure
speed while noting the manometer reading.
Proper air box pressure is required to maintain sufficient
The difference between the two readings, with and
air for combustion and scavenging of the burned gases.
without the air cleaner and ducting, is the actual
Low air box pressure is caused by a high air inlet
restriction caused by the air cleaner and ducting.
restriction, damaged blower rotors, an air leak
Check the normal air intake vacuum at various speeds
(at no-load) and compare the results with Table 6.