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Carburettors
and control cables
Rich mixture?
General indications
of an ultra-rich mixture are heavy, thumpy running, regular
misfiring (eight stroking in the case of a four-stroke engine),
black smoke at the exhaust, and soot on the plug. A weak mixture is
apparent by spitting back through the carburettor, a tendency for
the engine to knock readily, and by the plug showing signs of heat.
Choke check
Before altering
carburettor settings, verify the correctness of the fuel feed, stop
air leaks, check over ignition and valve operation and timing. Then,
if at a particular throttle opening the choke is partially closed
and the engine goes better, weakness is indicated; if the running is
worse richness is indicated. It is not correct to cure a rich
mixture at half throttle by fitting a smaller main jet because the
main jet may be correct for power at full throttle; the proper thing
to do is to lower the needle.
Low speed richness
Too rich a mixture at
low engine speeds can point to a loose pilot jet (this is a taper
fit onto its seat). Any slackness will permit fuel to pass in
addition to that governed by the actual jet. Don’t force it though;
the jet should be tight, but no more than that.
Heavy fuel
consumption
The last thing to
suspect is the main jet because, if the correct jet was fitted
initially, no amount of petrol flow will wear it larger. A gradual
rise in consumption during average running would indicate a worn
needle jet, enlarged by a sloppy throttle needle clip. The throttle
needle is unlikely to wear though it may be bent. So, when replacing
a needle jet change the needle clip at the same time, and check the
needle for straightness by rolling it on a sheet of glass.
Slow tick-over
When adjusting the slow running mixture
remember to finish the job by resetting the throttle stop. Although
the setting for slow running affects the mixture strength only at
low speeds, so much mileage is covered in towns that unnecessary
pilot jet richness makes a big contribution to heavy fuel
consumption.
Heady mixture
At high altitudes an
engine will show signs of rich mixture, and the rarified air will
result in loss of power. For normal mountain pass storming there is
no need to make carburettor adjustments, and the power drop will not
be enough to worry about, but these figures will be of interest to
Alpine and Himalayan travellers
At 3000 ft the
mixture should be about 5 per cent weaker than at sea level; at 6000
ft, about 9 per cent; and at 9000 ft, about 13 per cent.
Power loss is about
10 per cent at 3000 ft; 20 per cent at 6000 ft; and 30 per cent at
9000ft.
For Triumph TRW
owners planning that lifetime trip to China, Her Majesty’s User’s
Handbook gives a 22 per cent power loss at 5000 ft and 40 per cent
at 10,000 ft. Hard luck!
Rule-of-thumb mixture
guide
Heavy lumpy running
with, usually, black smoke (not the blue of too much oil) from the
exhaust, indicates richness. When the mixture is weak the running is
erratic, and may be accompanied by spitting back through the
carburettor. Another indication is firing in the silencer with the
throttle closed and the engine on the over-run.
Spark plug test
When testing for
mixture strengths at high speeds by judging the spark plug colour,
don’t come to a standstill with the engine running slowly or allow
it to tick over. It is best to kill the engine on the ignition
cutout, get into neutral, and coast in.
Twin mixture
Plug reading should only be treated as a rough
guide as it can be misleading. For instance, by examining the plugs
of a parallel twin it might appear that the right hand cylinder is
running slightly richer than the left. On a twin carb engines this
may be so – but there is another factor to be considered. On most
twins the main oil feed to the crankshaft is from the timing side
and, consequently, that cylinder is likely to get a shade more oil
than the other. Therefore, the plug deposit is possibly the dark
carbon of burnt oil rather than the soot of richness.
 Amal
carburettors
Amal
Carburettors are straightforward, workmanlike instruments, which can
be readily adjusted to obtain fuel economy.
Instructions for
tuning the Type 276 also apply to the later Monobloc and Concentric.
The basic difference between these three designs is the position of
the float chamber. In normal circumstances it will be found that the
best all round settings for performance are those recommended by the
manufacturers of a machine, with the idling individually adjusted to
suit a particular engine by means of the pilot air screw and the
throttle stop screw.
Amal Type 276
carburettor
The Amal Type 276 was
a prewar design with a separate float chamber. The main jet screws
into the bottom of the needle jet. It is calibrated to indicate the
number of cubic centimetres of petrol that will flow through the
orifice under given conditions in one minute. A jet stamped with the
number 140 will pass 140 cc, and it follows that the smaller the
number the smaller the jet and vice versa.
Remote floats on Amal
carburettors
Remote
float Amal carburettors could be supplied with one of three
thicknesses of mixing chamber union nuts. They were usually supplied
with the middle one. The thinner one would have the effect of
raising the level of the float chamber, providing a greater reserve
of fuel for snap throttle openings or for hilly work, such as in
trials. Finer adjustments can be obtained by using varying
thicknesses of fibre washer inside the nut.
Stuck lids
Stubborn remote float chamber lids can be
removed by using a leather strap and a pair of pincers as a
makeshift gentle pipe wrench.
Amal Monobloc
carburettor
This exploded
diagram, of the Monobloc, introduced in 1955, clearly indicates its
constructional features. The float chamber is now part of the main
body of the carburettor; instead of being separate. The main and
needle jets screw into the ends of the jet holder. The pilot jet is
detachable, and the throttle slide is guided internally on the jet
block.
Amal mixture strength
It is important to
remember the influence that each variable has on the mixture. If one
has constantly in mind that (a) the pilot air screw controls the
mixture up to 1/8 throttle opening, (b) the throttle cutaway from ¾
to 1/4, (c) the throttle needle from 14 to ¾, and (d) the main jet
from ¾ to full throttle, the effect of changes from standard
settings can be anticipated fairly accurately. Further, it is a
simple matter to decide where to start in altering carburettor
settings.
To take an extreme
case, it is a waste of time fitting a smaller main jet if economy at
30 mph is required. At that speed the throttle is less than a
quarter open and the major influence on mixture strength comes from
the throttle cutaway. It should not be inferred from the Amal
instructions that mixture control by pilot air screw, throttle valve
cutaway, throttle needle position and main jet takes place in
clear-cut stages. It does not. There is a certain amount of overlap,
and it will be found, for instance, that an over-rich pilot
air-screw setting will cause a measure of heavy fuel consumption,
though the machine may be driven for long periods on half throttle.
However, these stages of mixture control are the key to the methods
used in tuning.
Satisfactory idling
on an Amal
At its business end
the pilot air screw is tapered. Turning the screw clockwise brings
the taper closer into the air orifice and gives a richer
slow-running mixture; turning the screw in the other direction
weakens the mixture. When making this adjustment the best results
are achieved by coordinating the throttle stop setting. The only
point to watch is that there is enough slack in the cable to allow
the throttle valve to seat on the head of the stop screw. Further,
it must be remembered that satisfactory idling will never be
obtained if there are air leaks between the carburettor and the
combustion chamber, or if there are ignition faults. Once the
desired tick over has been obtained, the slack in the cable can be
taken up by means of the adjuster in the outer casing or at the top
of the carburettor.
Amal throttle valve
markings
Throttle valves are
identified by markings such as 6/59,
5/5 and so on. The first figure identifies the
type of valve with its type of carburettor. The second figure gives
the depth of the cutaway in 1/16 ths of an
inch. Thus, a 6/5
valve has a 5/16 in. cutaway, and a
6/4 valve has a ¼ in. cutaway. The deeper the
cutaway the weaker will be the mixture.
Amal - weaker or
richer?
The final adjustment
in the sequence is that of the throttle needle position. Needles are
tapered and have five notches at the top end. The needle operates in
the needle jet through which the fuel passes up into the mixing
chamber. A spring clip located in the notch selected attaches the
needle to the throttle valve, and the needle thus moves up and down
with the throttle. Putting the clip in a notch nearer the top of the
needle lowers the needle relative to the throttle and results in a
weaker mixture. Raising the needle has the reverse effect.
Tuning twin engines
with twin carburettors - setting the valves:
First of all, slacken
the throttle stop screw valves and put the twistgrip in the shut-off
position to allow the throttle to shut off. There should be a slight
backlash in the cables, which can be obtained by screwing in the
cable adjusting screws on top of the carburettor. Then, with the
handlebars in the normal position, and with the throttles closed,
adjust the cable adjusting screws so that on the slightest opening
of the twistgrip, both throttle valves begin to open simultaneously.
To make an exact
check on the simultaneous throttle opening, wait until all other
adjustments have been made. Then, shut the twistgrip back so that
the throttles are resting on the throttle stop screws. Insert the
fingers into the air intakes and press them on the throttles; with
the other hand gently open the twistgrip and feel that the throttles
lift off their stops at the same time.
Tuning twin engines
with twin carburettors
—running
adjustments
Main jet sizes are
selected by checking the effect of the mixture on the spark plugs
after taking a run at full throttle over a straight piece of road,
preferably under load. You don’t need to be in top gear. The
smallest pair of jets that give the best maximum speed is usually
correct, provided that the plugs do not show any signs of excessive
heat. It might be that for really critical tuning one carburettor
will require a slightly different jet size from the other.
Slow running
For slow running, set
the twistgrip to make the engine run slowly but just faster than at
tickover. Then gently screw in the throttle stops to just hold the
valves in that position, and return the twistgrip into the shut
position, leaving the engine running on the throttle stops. The next
thing to do is to set each carburettor to obtain the idling by
screwing down the throttle stop screws and adjusting the pilot air
screws accordingly.
Pilot jet
Regarding the setting
of the pilot jets, a fairly satisfactory method is to detach one
spark plug lead, and set the pilot air adjusting screw on the other
cylinder as a single  unit,
reversing the process for the other cylinder. It may be found that
when both leads are connected to the spark plugs, the engine runs
slightly quicker than desirable, in which case a slight readjustment
of the throttle stop screws will put this right. It is important
that the speed of 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.
Regarding the lower
end of the throttle range, which is always the more difficult to
set, one can only take excessive pains to make quite sure that the
control cables are perfectly adjusted, without any excessive
backlash or difference in the amount of backlash between one
carburettor and another. Otherwise one throttle slide will be out of
phase with the other, resulting in lumpy running. It is essential
with twin carburettor 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.
Amal Concentric
The sketch shows the
Amal Mk. 1 concentric carburettor which is ideal for a twin set-up.
Earlier Monoblocs had their float chambers mounted on one side,
which either made them awkward to adjust when paired, or more costly
to manufacture when handed. The Concentric overcame these problems
by positioning the float chamber underneath the body of the
carburettor.
Twin carburettor
tuning – the Douglas way
A Douglas owner found
this method quite satisfactory for setting up his machine’s twin
carburettors in 1951.
The Douglas is put on
the center stand with the rear wheel clear of the ground. The engine
is started and allowed to warm up for a few minutes. One of the
gears is then engaged (usually 2nd gear), the throttle
opened a certain amount, and a note made of the speedometer reading
with one plug disconnected. Without altering the throttle position,
I replace the disconnected plug cap and remove the other one; the
speedometer reading should be the same. If not, the required
adjustments are made. This process can be repeated for various
throttle settings except for very slow running. A good tick-over can
be adjusted with both pots firing.
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