Re: [Enfield] Re: ALTITUDE CARB ADJUSTMENT
shall I lower the needle as well as use lower jets? Or do one at a
time alternatively as the bike gets crazier.
It won’t get crazier, as you climb it will come to run richer due
to the relatively lower air pressure. Richer will mean possibly
fouling plugs (although probably not as you’re pulling fairly hard up
hill, I’d think) On the way back down, though, it will run leaner,
Carburetor Jetting
Carburetion adjustment affects the proportion of fuel to air
enteringthe engine. Too rich a mixture can result in poor fuel
economy,carbon-fouling of spark plugs, and wooly or sputtery running
The Float Circuit
The gasoline is mixed with the airflow through the carburetor by
The Pilot Jet
The pilot jet affects idle mixture. Setting your pilot jet involves
getting the machine warm, and then adjusting the idle stop screw for
slowest possible clean tick-over, or idle speed. First, be sure
your throttle cable has at least 1/8-3mm slack at full closed.
Before startingthe engine, ensure that shutting the throttle full off
from 1/2 or so results in an audible clack as the slide hits the
stop. If not, look to your cable routing and condition. Later, with
engine warmed and running, adjust the pilot jet screw for fastest
engine speed. Reduce the speed with the stop screw once again, and
repeat the pilot jet adjustment. Do these until you get the best
possible idle adjustment. Sudden increases of throttle should not
cause the engine to go flator die. The pilot jet is an air bleed
– tightening it makes for richness, loosening leans it out.
Since it is the pilot circuit which provides the fuel during
Plug Checks
By a fortunate coincidence, the insulator for the center electrode
tip, at the business end of the spark plug, changes according to the
mixture. You will need good light and likely a magnifying glass to
see it. At the very bottom of the insulator tip (the top when the
plug is in place in the engine) will be found a soot ring – just
where the insulator begins to project into the well at the plug
bottom. This soot ring will vary in width according to mixture at any
given throttle range. The wider the ring, the richer the mixture. You
want to richen until such a ring is just discernable. For more
specific information, see the
appropriate section of the
Jennings Article, elsewhere in this manual.
Mid-Range Carburetion Check – The Plug Chop
This is a road-test exercise. Put a plug wrench in your pocket or
toolbox. Put on your hat and gloves, start the bike, and take it for
a warm-up cruise to your test site. This will be a place where you
can ride at half throttle for 30 seconds or more – at the proper
throttle opening for the jet circuit you’re checking – before
suddenly cutting off the ignition as you pull in the clutch and come
to a full stop at the roadside, in a place safe to do a little
work.(You will likely need a slight uphill in order to be able to
maintain half throttle for 30 seconds, and even steeper grade for the
full throttle check.) The last thing you should do before commencing
your test runs is to switch to a new plug – a new one will give you
the best readings, although you may find your present one
satisfactory. Be sure to use the proper gap and heat range.
IMPORTANT SAFETY NOTE!
Be VERY Careful! If any other motorists are present, there is heavy
danger they won’t understand your reason for the sudden stop, and
will run over you. Also, you must be in a safe place for working on
your engine when stopped. Be well off the shoulder and stay aware of
traffic coming toward you at all times!
Then, pull the plug and look for the soot ring. If it’s barely there,
you’re good for that jet range. If it’s more than a mm or so wide,
you’re heading into the territory of richness, and if it’s not there
at all, you’re too lean, and need desperately to raise your needle,
In times of leaded gasoline, it was also possible to read the plug
insulator by the colour of the entire cone. If it’s white, you’re too
lean, and the needle must be raised a notch (the clip lowered.) If
it’s black, you’re too rich, and the the needle must be lowered (the
However, to reiterate: with modern fuels, you may not find the
chocolate brown plug to be attainable with some modern fuels,
especially if they’ve been adulterated with such things as ethanol.
In these cases, you’ll be dealing with varying shades of grey, and in
some cases the best you’ll get will be a thin ring of carbon around
the tip of the insulator. If this is you, you’re looking for a thin
ring of carbon just above the tip (from which projects the electrode.)
The Midrange-Highrange Circuit:1/4 to Full
The needle and main jet circuits – 1/4 to 3/4 and 3/4 to
FullThrottle. Raising the needle relative to the throttle slide
richens the mid-range; a larger mainjet richens the top range.
To adjust the needle setting, you must unscrew the carburetor top
cap, remove the complete top cap/spring/slide assembly, pull the
needle clip, move the needle, replace the clip, and replace the top
cap assembly. With some models, this may require removing the fuel
tank and/or loosening the spigot clamp and turning the carburetor to the right.
The fuel flow is from the bottom of the floatbowl and into the jet
block, through the main jet upward into the needlejet, where it is
regulated by the tapered needle. At full throttle, the needle has
little effect; the actual size of the preceeding main jet is what
determines mixture ratio at the upper end – starting about 3/4
throttle. So the size of the main jet determines where the fuel flow
peaks out at wide open. If you don’t ever ride wide open, you’ll
never have to worry about this one – although too small a mainjet can
restrict needlejet flow.
For full throttle tuning, do the plug chop, although you may need a
slightly steeper hill. Run at full throttle for about 30
seconds,pulling in the clutch and turning off the ignition quickly,
then coast to a safe stop in a safe spot. If there’s no soot ring at
all, you’ll need a larger main jet, if it’s too wide, a smaller.
After main jet tuning, check the mid-range again, although only very
large mainjet changes generally affect mid-range. The mainjet is
screwed into the bottom of the jetblock, which may be removed on some
models by unscrewing the carburetor base plug, and reaching up in
there with a socket of the proper size. On others, or if you’re
unfamiliar with the territory, best to remove the entire float bowl.
It will be held in place byeither a spring clip, or screws running
from the bottom through the bowl flange into the carb body, or vice-versa.
A Note On Plug Colours
In earlier times, the entire insulator tip of the plug could be
Cleaning Specific Circuits
If a problem occurs which can be traced to a specific circuit, eg
poor starting pointing to possibly plugged starting enrichment
circuit, or poor but undadjustable idling pointing to possibly
plugged pilot circuit, the carburetor should be removed, stripped,
and cleaned, using carburetor cleaner, or even just washing solvent.
Jetting
The Indian Bullet is shipped, as are most vehicles in these
EPA-troubled times, with the carburetion set on the side of the EPA
angels, that is to say, very lean. Consequently, it is very common
for owners to find that white spark plug insulators are the order of
the day in the all-important mid range, even with the needle set as
high as possible(clip in bottom groove.) If you have fitted a less
restrictive air cleaner and/or silencer, this will be particularly
apparent; in fact your machine will be dangerously lean in this
instance with the stock jetting. This means installing a larger
needle jet – for the 500, for example, the original will most likely
be an O-8, and you will likelydo better with a P-8.
While in there, check your pilot jet – if a 30 is not already in
there, you’ll likely find it best, too. Most likely main jet has been
Float height/level is measured with the float bowl removed, and the
carb held upside down. Spec for distance from top (bottom) of float
to the top of the float chamber (on the carb body) is 22 – 24mm.
A Word On Idling
It seems to be a recurring problem with new riders that their
motorcyclewon’t idle. Although it is simply to be expected that you
can release the throttle on an automobile, and the engine will drop
down toa happy burble, it is not always the case with motorcycles,
particularlywith lo-tech big singles. And it has been my experience
that making my motorcycles do this is often just more trouble than
it’s worth. In fact, it is much more important to me that the power
drops off suddenly and completely when the throttle is shut, with no
run-over, as this is important to proper engine braking. Even if it
is sometimes possible to get a big single to idle, or tick over
when the throttle is rolled off, it can sometimes be very difficult
to get it to do both. Consequently, I have always considered the
primary reason for proper pilot jet adjustment to be to effect a
smooth transition up onto the cutaway and the needle, rather than as
a means of bringing the machine to a point where it will happily putt
away all by itself when I let go of the throttle.
It is, I think, to be preferred to learn to keep the engine running
withthe throttle while simultaneously using the front brake. The
process is easily learned, and like proper gear changing or other
at-first-new things to the beginning motorcyclist, soon becomes
second nature. The thumb and first and second fingers of the right
hand are for controlling the front brake lever, and the rest of the
hand, with the 3rd and 4th fingers, will be found sufficient to
control the throttle to keep the engine running when pulling up to a
stop, or holding the vehicle on a hill with the front brake. This
must be difficult for those circus chimps on the mini-bikes, but for
The Induction Pipe
A common problem with the Indian Enfield is the Induction Pipe
between the carburetor and its spigot on the cylinder head. North
American and European Fuel appears to have additives not common in
India, and the genuine rubber spigot pipe (Indian Rubber!) soon
cracks in most places outside the home turf – rotting, cracking, and
leaking are a common problem. The best fix for this one is to use a
straight piece of radiator hose – you will find you can buy a length
of the proper diameter at your local auto supply house, enough to
make a number of induction pipes, for less than the price of one new
one from the factory – and this one will likely last the life of the
machine. (The induction pipe is sometimes called the manifold,
although this is obviously a misnomer.)
When the pipe is leaking air, a lean mixture is the result. This can
damage the engine at worst, and at minimum will bring about uneven
running, especially at slow speeds, and hard starting, sometimes also
resulting in inlet blowback sufficiently violent actually to blow the
carburetor off the motor! If this occurs, replace the pipe when you
replace the carburetor.
Dirt in Carburetor: No Fuel Flow – Or Bad Flooding
Dirt from the fuel entering the carburetor can cause problems,
rangingfrom no fuel flow at all to excessively-enhanced fuel flow due
to the float valve being stuck open or closed. Either of these
conditions can be brought about by dirt fouling the float needle, or
an excessively worn needle valve orseat. First, remove the float bowl
and carefully take out the float lever hinge pin,(DON’T let it fall
into the dirt!) and then remove the float and lever. With a suitable
container, such as an empty tuna can, under the carburetor, open the
fuel tap and allow some fuel to run through the now-unimpeded float
valve. This should wash out any dirt particles. If dirt is obviously
a problem, the fuel tank may have to be removed and cleaned. Test the
float valve closure after reassembling the float and lever into the
carburetor, by turning on the fuel (tuna can still under the carb)
and gently raising the float asssembly – use a screwdriver of similar
tool to avoid getting gasoline on your skin – to ensure that the fuel
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