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When fitting
the Mk2 Amal Concentric Carburetter to your engine, check the following
points:
1 The rubber flange adapter fits correctly with no air leaks. Do
not overtighten the clips, which could split the rubber.
2 The throttle cables are of adequate length and there is adequate
movement of the inner wire for the throttle valve to close completely.
3 Petrol pipes must be connected tightly to the banjo to prevent
leaks and in such a way that the carburetter is not prevented from
moving on its rubber mounting.
4 There must be a good connection to the air cleaner if one is used.
| Starting
from cold |
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| Depress
lever, or if cable operated, open the handlebar lever. This
will introduce the cold start jet system which is completely
separate from the main jet system. It will introduce over-rich
mixture on the engine side of the throttle valve. Do not open
the throttle valve more than one quarter or the cold start system
will not work so well. The jet normally fitted is number 50.
This can be changed for alternative sizes if extreme conditions
are experienced. |
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| Tuning
sequence |
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To
obtain correct carburation for any stated fuel, assuming that
the correct size carburetter is fitted, the sequence is as follows:
1 Main jet for power at full throttle 2 Pilot air adjuster for
idling speed 3 Cutaway for take-off from the pilot jet
4 Needle jet and needle position for clean carburation between
one quarter and three quarters. |
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| 1st:
Main jet |
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| If
at full throttle the engine runs heavily and does not pull at
full power, the main jet is generally too large. If the engine
fades and perhaps deteriorates with possible detonation, then
the jet is too small. With the correct size the engine should
run freely at maximum rpm giving good power. When testing for
the correct main jet, give careful attention to spark plug readings;
the correct colour will be light brown or chocolate colour surrounding
the central electrode. |
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| 2nd:
Pilot jet |
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| The
pilot system governs engine tickover and can affect pickup and
transmission to the main system. Choose a jet that when the
engine is set for steady idling, the pilot screw is one and
a half turns from its seat. If the adjuster is further on its
seat, a larger jet is needed, conversely if the adjuster is
screwed out a long way a smaller jet is needed. There are two
alternative pilot jet locations; the one in the float bowl is
generally considered best for four stroke engines, that located
in the body beneath the air tube usually best for two stroke
engines. This is not a hard and fast rule and the reverse may
sometimes be the case. Irrespective of engine type, when the
carburetter is mounted in a down draught attitude, the pilot
jet should be placed in the mixing chamber body. |
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| Throttle
valve cutaway |
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| The
throttle valve governs transition from idling to the main system
and also influences response at small throttle openings. When
opening the throttle, if the engine fades or spits back through
weakness, a smaller cutaway is needed. If the engine runs unevenly
and heavily, a larger cutaway is needed. The main influence
of the throttle valve takes effect up to one third throttle
opening. |
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| Needle
jet and needle position |
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| These
influence the range from one quarter to three quarters throttle:
the needle jet, mainly the lower half of the range and the needle
the upper part of this range. It is important to get the right
combination. Usually the needle jet supplied with the carburetter
is the correct one for that instrument. Raising the needle produces
a richer mixture and lowering, conversely, a weaker mixture.
If it is necessary to go to the extreme of the adjustment in
either direction, then probably the next sized needle jet, up
or down, could be utilised usefully with some further re-adjustment
of the needle position. There are two different systems for
four stroke and two stroke engines; refer to the parts list
to ensure that the correct type is used. Always use the correct
combination of needle and needle jet as a set. |
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| Air
jet |
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This
controls the amount of air which pre-atomises the fuel before
it enters the mixing chamber body. Normally the air jet fitted
as standard for the particular size of carburetter should
be correct, but it is a component that can be changed should
the depression on the main jet need to be influenced. Fitting
a smaller jet will increase the depression while a larger
jet will reduce the depression.
All
of these adjustments overlap to some extent and in order to
obtain the optimum setting it may be necessary to go through
this procedure more than once to get the final degree of accuracy.
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| Air
filters |
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these
may affect the jet setting, so if one is fitted afterwards
to the carburetter the main jet may have to be smaller. If
a carburetter is set with an air filter and the engine is
run without it, take care not to overheat the engine due to
too weak a mixture.
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| Effect
of altitude on carburetter |
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Increased
altitude tends to produce a rich mixture. The greater the
altitude, the smaller the main jet required. Carburetters
ex-works are set for altitudes up to 3,000 to 6,000 feet should
have a reduction in main jet size of 5 % and thereafter, for
every 3,000 feet in excess of 6,000 feet altitude, further
reductions of 4% should be made.
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| Tuning
twin engines with twin carburetters |
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(where
each cylinder has its own carburetter). First slacken the
throttle stop screws and put the twist grip into the shut
position to allow the throttles to close: there should be
a slight backlash in the cables which backlash can be obtained,
if necessary, by screwing in the cable adjusting screws on
the top of the carburetter. Then, with the handlebars in the
normal position, and with the throttles closed. adjust the
cables adjusting screws so that on the slightest opening of
the twist grip both throttles begin to open simultaneously.
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For
slow running, set the twist grip to make the engine run slowly but
just faster than tickover: then gently screw in the throttle stops
to just hold the throttles in that position, and return the twist
grip into the shut position, leaving the engine running on the throttle
stops.
Next
set each carburetter, according to the instructions already given
for pilot jet settings, to obtain the idling by screwing down the
throttle stop screws and adjusting the pilot air screws accordingly.
For
setting the pilot, a fairly satisfactory method is to detach one
spark plug lead, and set the pilot adjusting screw on the other
cylinder as a single unit, and then reversing the process to the
other cylinder. When both leads are connected to the spark plugs,
the engine may run slightly quicker than desirable; a slight adjustment
of the throttle stop screws will put this right. It is essential
that the speed of the idling on both cylinders is approximately
the same, as this will either make or mar the smoothness of the
get-away on the initial opening of the throttle.
It
is essential with twin carburetters that the throttle slides are
a good fit in the bodies, and also that there is no suspicion of
air leaks at either of the flange attachments to the cylinder.
For
the lower end of the throttle range, which is always the more
difficult to set, one can only take great pains to ensure that
the control cables are perfectly adjusted without any excessive
backlash, or difference in the amount of backlash between one
carburetter and another: otherwise one throttle slide will be
out of phase and so result in lumpy running.
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