Testing Time
( Grace P., 2012, The Royal Enfield, Issue 2)
- We test a nifty gadget for checking carburetter settings
Tingkering and fettling, they
are lovely words aren't they? They imply a sense of competent non-threatening
servicing, to be enjoyed with a mug of strong coffee and dulcet tones of Radio
4 serenading in the background, within a snug and purposeful shed
=================================
Hold that thought for a moment. It’s wonderful when your
motorcycle is working perfectly and requires no major maintenance work. One’s
leisure time can then be spent perfecting its running and keeping its cosmetics
in shiny good order. Bliss. No rush to get that engine back together from an
exploding big-end, or that clutch replaced before the annual club summer rally
and ‘drinkathon’; instead, some quality fettling time, with little risk of
failure and ridicule from your chums.
One such fettling task is adjusting the carburetter on the
old faithful, a nice tweaking job if you have the right tools.
Many people
suggest using a ‘plug chops’ technique to set up the fuelling on old
motorcycles. A technique that involves riding the motorcycle under load at four
different speeds between idle and full throttle, then, killing the ignition
abruptly and coasting – hopefully – quite elegantly to halt. Next, scalding
your fingers by quickly pulling the hot spark plug and examining its burnt
colour, and finally refitting a new clean spark plug for the next run. So
that’s 4 runs with 4 different throttle openings and 4 new spark plugs, one for
each run. Are you still with me?
Not an easy or safe task, and to be honest, not that useful
these days with our unleaded petrol and all its additives.
Alternatively, for a relatively small sum of money it is
possible to purchase a Colortune kit to actually look inside the combustion
chamber, gasp, and thus see the actual colour of the combustion flame front. It
becomes easy to understand and adjust the fuelling, noting the visual effect
that adjustments have on the colour (and therefore fuel/air ratio) of the
combustion.
I obtained my Colortune from www.frost.co.uk
who also stock low cost adaptors, so it can be used on other vehicles too. The
14mm size is usually the most useful one. Before using the Colortune kit and
making adjustments, firstly warm up the motorcycle so it can idle without using
choke, and then remove the spark plug. As you can see from the example in the
photo, this motorcycle seems to be set up fairly well anyway, with the plug
electrode displaying a nice beige tone.
If the plug is dry and sooty then you may have a fault with
excessive fuelling; if it is oily then it’s possible that engine wear may make
adjustments difficult.
In place of the normal spark plug you screw in the special
Colortune Transparent plug. It only needs to be just a fraction more than
finger tight to take care not to over-tighten it! Next, screw the HT extension
lead into the Colortune plug whilst the other end connects to the spark plug
cap. The insulator on the Colortune plug is glass rather than ceramic – the area
around the white centre – so you can look down into the centre of the plug and
actually see the combustion taking place.
With the engine running just above idle speed you can
clearly see and orange glow on the inside of the spark plug.
Just a fraction of a turn anti-clockwise on the air-screw on
the side of the Amal Concentric carburetter enables you to trim the colour of
the combustion to the correct blue hue.
Too far and the colour will turn white, indicating and
unwanted lean burn situation.
You can use the Colortune to check the combustion at other
engine speed too, rather than just at idle. The instructions that come with the
kit describe what to expect at various throttle opening, to perhaps diagnose an
incorrect main jet or slide, or even a defective accelerator pump on such
equipped carburetters.
I tend to prefer using my experience and ‘feel’ to set these
up, or if it’s a very complex problem by using a Dyno rolling road with an
air/fuel ratio print out at a local specialist.
However, for most simple classic motorcycles the Colortune should be more than adequate. It’s especially useful when faced with twin carb
equipped engines. Each air-screw can be set spot on using the Colortune, then,
the idling balanced using an air gauge.
When you are finished, simply give the Colortune plug a
clean with some brake cleaner using the small brush provided in the kit, then
pack everything safely away for the next time it is needed. I use and old
wooden wine box to ensure all the various parts are kept together.
“Look after your tools and they will last a lifetime” as my
old dad used to say.
Using the Colortune kit is a rewarding yet simple bit of
fettling, when carried out in the comfort of a well-ventilated shed.
Go on, treat yourself to another cuppa with the feeling of the job well done!
Go on, treat yourself to another cuppa with the feeling of the job well done!
Paul Grace
(Full Magazine is available at: http://www.magcloud.com/browse/issue/402366)