Monday, 25 June 2012

Testing Time

( Grace P., 2012, The Royal Enfield, Issue 2)
- Running rich or feeling lean?
- 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 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! 

Paul Grace
(Full Magazine is available at:

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