Adjusting
the "HL" Tillotson Carburettor
The
"HL" series carburettor is a lightweight, aluminium
die cast carburettor composed of four basic parts: metering
body, main diaphragm cover plate, fuel pump body and strainer
cover. the diaphragm carburettor incorporates many of the
same type components found in float type carburettors: choke,
throttle, idle and main mixture adjustment screws, idle speed
screw and inlet needle and seat.
Two styles of main and idle adjustment screws are available.
"O" ring type and spring loaded packing type. Both
types are designed to perform the dual purpose of sealing
the metering chamber and providing adjustment screw friction.
A special
insert, housed in a brass cage, forms a seat for the inlet
needle. An inlet tension spring exerts a pre determined force
on the inlet control lever which holds the needle on it's
seat.
A metering
diaphragm is subjected to engine suction on the metering chamber
side and atmospheric pressure on the vented side. Atmospheric
pressure on the vented side pushes the diaphragm toward the
inlet control lever, opening the inlet needle to allow fuel
to enter the metering chamber, from which it is then delivered
into the mixing passages.
The vented
side of the metering diaphragm may be vented either directly
to the atmosphere, or in the case of a balanced carburettor,
may be balanced (internally vented) to the choke bore. The
balanced type can be recognised by a brass tube in the choke
bore which is connected internally to the vented side of the
diaphragm. The purpose of internal balance is to offset the
enriching or choking effect of a partially dirty air cleaner.
Some carburettor
metering systems include a ball check type main nozzle. These
can be identified by the brass cage located in the venturi
choke band of the body casting. The ball check valve allow
fuel to flow into the mixing passage and prevents air from
flowing into the metering chamber.
The movement
of the pump diaphragm draws fuel into the fuel chamber and
a reverse movement of the diaphragm forces fuel out of the
fuel chamber through the inlet needle and set into the metering
chamber. Movement is caused by pulsations from the engine,
acting on the diaphragm. A plastic turret type inlet connection
is the cover to the fuel strainer section of the carburettor
and can be rotated 360 degrees for any required fuel connection
location. the strainer consists of a fine mesh screen to insure
clean fuel supply to the metering section of the carburettor.
ADJUSTMENT
INSTRUCTIONS
To properly
adjust the carburettor for best performance, the engine must
be thoroughly warm.
INITIAL
ADJUSTMENTS:
Do not
force adjustment screws into seats!
To start
a cold engine, first carefully close, by turning clockwise,
both idle and main adjustment screws. Open main adjustment
screw counter-clockwise, approximately one and one quarter
(1 1/4)) turns. Open idle adjustment screw three quarters
(3/4) turn. Back idle speed regulating screw off it's contact,
with throttle stop lever, then turn it inward about one (1)
full turn, so as to slightly open throttle shutter.
Open fuel
line shut off valve, close choke shutter, partly open throttle
shutter and pull starting cord. When engine fires, open choke
shutter slightly and idle the engine. Do not race the engine.
Then as engine warms, open choke shutter. If as in the case
of Racing Karts, no choke is fitted then pat the carby mouth
or venturi opening with the palm of your hand until fuel is
drawn up. To start a warm engine it should only be necessary
to pull starting cord, or rotate wheels on a Kart, if the
carburettor is properly adjusted.
FINAL
ADJUSTMENTS:
Completely
close throttle shutter and readjust idle speed regulating
screw, so engine idle speed is approximately 1200 RPM for
lawn mowers, 2000 to 2500 RPM for chain saws, the same for
a non-gearbox kart whilst sitting on a stand, then slowly
readjust the idle adjustment screw to obtain smooth and even
engine performance. Poor acceleration may result from setting
the idle mixture too lean.
Warning:
If running your Kart engine while Kart is sitting on a stand,
it is advisable to have some one applying pressure on the
brake so as to place load on the engine.
Most Go-Kart
'two stroke' racing engines are controlled with a Tillotson
style carby, there may be variations in design but the method
of adjustment is basically the same. This type of carby is
normally used to operate Reed, Rotary and Piston Port engines.
The exception being the Yamaha 100SE or 100J, these use the
Walbro carby, or often refered to as the 'Bumble Bee' which
is a pure pain in the arse to get right.
There
is a very comprehensive article on the Walbro in the books
'Racing The Yamaha 100J To Win' and 'Rebuilding and Racing
The Yamaha KT100SE' See our Book Store.
