The
Standard Carburettor fitted to 350 and 500 Royal Enfield India Bullets is a
Mikcarb, a copy, made under licence, of the popular Japanese Mikuni. The 350
Bullet uses a flange-mounted 24mm bore version and the 500 Bullet uses a 28mm
bore, mounted
on a rubber stub. The other details are similar for both.
The
way Mikcarbs are originally jetted means that any modifications to the air
filter or exhaust system, especially in the colder UK climate, normally make it necessary to alter the
carburettor settings.
Before
blaming the carburettor for poor performance, hard starting, back-firing etc. a
few simple checks should be done. Engines need a supply of clean, fresh petrol.
Pull the fuel supply pipe off the carb, turn on the fuel tap and run off a
small amount of petrol into a clean glass jar. Stale petrol has an unmistakable
smell; rust or water will quickly settle to the bottom of the jar.
A
common fault is an air leak due to a perished intake rubber or over-tightened
flange. Leaks will cause the exhaust to smoke and engine speed to rise.
Next
stage is to check on the ignition. After a short run, remove the plug and check
the colour and condition of the tip. A sooty plug shows an over-rich mixture, a
‘hot’ nearly white plug tip shows a weak mixture and a hot running engine.
The
firing-ends shown in the photograph above are all considered ‘good’ by NGK
technicians.
Our
experience is that on Bullets ideally the plug tip should be slightly sooty,
indicating a slightly rich mixture.
Now
test the spark, which should be blue, snappy and strong. A low battery, dirty
or worn points or a weak condenser will all give a weak, yellow spark and
produce similar symptoms to carburetion problems. A new spark plug (short reach
for a 350, long reach for the 500) might be all you need. When fitting the
spark plug, lightly smear the threads with copper grease, replace the plug with
your fingers, screwing in the plug with your fingers until it is ‘seated’,
tighten no more than a quarter turn with a plug spanner. Take care not to
over-tighten the plug—overtightening spark plugs is a common cause of thread
damage, which can be costly to repair.
If
none of these steps cure your bike’s problems then turn off the fuel, and
unscrew the carb top from the body. Carefully lift out the slide.
Remove
the carb — but beware as petrol will spill from the float chamber, so be
certain that there is nothing to cause a fire. Working on a clean surface, turn
the carb over and remove the four screws which hold the float bowl.
Ease
the float bowl away from the main carb body (a quick tap with the handle of
your
screwdriver
will release the gasket’s hold). Tap the float spindle out, lift the float assembly
away and tip the float needle out. Check the float for leaks – a quick shake
will show if there is petrol inside the float. Clean the float needle with a
spray of carb cleaner. The spring-loaded plunger must be free moving. Replace
the float needle, float assembly and spindle. With the carb held upside down
and the float chamber gasket removed measure from the carb body surface to the top
of the float, as shown below. The standard measurement is between 24-26mm (see
picture).
If
necessary the float height can be adjusted by gently bending the tangs located
between the two brass floats.
With
the float bowl removed the jets will be seen in the centre of the carburettor
(see the picture below). Now is the time to remove the pilot and main jets, to
clean them, check their sizes or change them.
With
a suitable screwdriver carefully unscrew and remove the main jet with its
washer – the washer may need to be carefully prised out of its housing. The
main jet screws into the base of the needle jet. The needle jet cannot be
removed from this end of the carb.
To
remove the needle jet, detach the carb top and remove slide, unscrew the main
jet (as just described) and tap the needle jet up into the carb body. Refitting
is the reverse of this procedure. Be careful on refitting that the slot in the
bottom end of the needle jet locates in the spigot in the base of the jet
housing (see picture below) – it may help to use long-nose pliers. Take care
not to cross-thread the main jet in the needle jet and don’t over-tighten.
NOTE:
In certain cases the needle jet may not screw fully into the main jet. There
are two types of jets, each with a different thread pattern, to prevent a jet
of one type being substituted for a jet of the other type.
The
pilot jet is the small jet located in the deep drilling next to the main jet
(see the picture). To remove the pilot jet unscrew it carefully with an
appropriate flat blade screwdriver and gently shake it out of its housing.
If
you have made changes to the air filter or exhaust system now is the time to
fit new jets. Different air filters or silencers affect the amount of air the
engine is able to ‘breathe’. As more air is drawn into the engine the ratio of
petrol to air decreases, the result is a mixture weaker than the ideal of about
14 parts air to 1 part petrol. If the mixture is too weak the engine performance
will be adversely affected—the engine may over-heat, with potentially serious damage
to crucial engine components. The remedy may be to adjust the air screw and/or
move the throttle needle clip but in most cases it will be necessary to replace
the jets—the higher numbered jets let more petrol into the carburettor, giving
a richer mixture.
There
are three jets in the Mikcarb: the pilot jet (used at small throttle openings),
the needle jet (used at throttle openings of about ¼ to ¾ - this jet is only
available in a standard size for 350 engines) and the main jet (used from ¾ to
full throttle).
The
mixture at small throttle openings is also affected by the size of the cutaway
in the throttle slide. The larger the cutaway (the higher the throttle slide
number) the weaker the mixture, as more air is allowed into the carburettor. A
smaller cutaway will, of course, give a richer mixture.
Fine
adjustment to the mid-range may be made by raising or lowering the throttle
needle. Release the throttle cable from the alloy block inside the slide. There
is a small plate that
prevents
the cable from accidentally detaching itself from the slide, note how the tag
and slots are fitted. (The picture below illustrates the method used for fitting
the throttle cable.) Unscrew the two screws, lift out the block and the needle
will drop out. By refitting the clip in a lower groove you will richen the
mixture (this raises the needle and so allows more fuel through the needle
jet). Placing the clip in a higher groove weakens the mixture.
When
reassembling be sure the slide locates in the peg projecting from the side of
the carb (see the picture below). Take care that the throttle cable is routed
without sharp kinks and has enough free-play for the slide to drop fully – even
on full lock with the petrol tank in place. Replace the fuel pipe and check for
leaks.
Setting
the tick-over requires patient sequential adjustment of the pilot air screw and
the throttle stop screw (see picture below). Turn the pilot air screw fully in
and then out one and a half complete turns. Start the engine and allow it to
warm-up thoroughly. When the engine will run cleanly without the enrichment of
the mixture provided by the use of the cold start plunger, close the starting
jet by lifting the cold starter lever to its horizontal position.
• Adjust the throttle stop screw so the engine runs
at a fast tick-over, say about 20%
above
the idle speed you wish to achieve
• Unscrew the pilot air screw to increase the
engine speed to its fastest possible
• Gradually unscrew the throttle stop screw to reduce
engine speed to the target idle
speed
• Again unscrew the pilot air screw to increase engine
to the highest tick-over possible
• Finally, gradually unscrew the throttle stop screw to
reduce engine speed to the correct idle speed. (Usually this process
carried out twice – as described – will be sufficient to achieve a correct
tick-over.)
