Onan MJA Series Service Manual page 37

35
FUEL SYSTEM
adjusting needle. As the throttle is opened to increase power, it grad­
ually exposes the idle transfer port and fuel is drawn out through this
port also.
As the engine governor opens the throttle farther, under increased load,
the increased air flow through the carburetor produces a low pressure
at the venturi (narrow section of the carburetor throat) _ This pressure
drop draws fuel up the main nozzle to be mixed with air at the nozzle
opening. This is the load circuit, Fig 3.2. I;
th~
main adjust needle con­
trols its fuel delivery.
At the same time, because the throttle is open, the manifold vacuum
decreases so the idle circuit becomes less effective. In a certain
range, the two circuits blend, both delivering fuel, but as load is in ­
creased, the load circuit takes over.
Whenever load is increased, with the load circuit in operation, the gov ­
ernor opens the throttle to deliver more fuel. The main nozzle won't
immediately deliver this increased fuel because of the jet controlled by
the adjusting needle. To prevent a power lag when load is increased, a
metering well around the outside of the nozzle delivers fuel until the
main jet can catch up with the increased demand.
3.2.5.1 Adjustment, Gasoline Carburetor. - Adjusting the carburetor
means obtaining the correct fuel-to-air mixture for smooth, efficient
operation. The carburetor should be adjusted in 2 steps - first the
idle adjustment, and then the load adjustment.
NOTE: If the carburetor is completely out of adjustment so the engine
won't run, open both needle valves 1 to 1-1/2 turns off their
seats to permit starting. Don't force the needle valves against
their seats. This will bend the needle.
Before adjusting the carburetor, be sure the ignition system is working
properly and the governor is adjusted. Then allow the engine to warm
up.
MAIN FUEL
THROTTLE STOP
SC~EW
ADJUSTMENT'
. A: -®
FUEL IDLE
ADJUSTMENT
FIG 3.2. J - GASOLINE CARBURETOR ADJUSTMENTS