Gunson Colortune

Thought I’d get this out to check the mixture is ok from the carbs.

They’ve been about for years. I think they are still around but are a bit difficult to find. Hitchcocks (the excellent Royal Enfield experts) usually have them in stock.
It is essentially a see-through spark plug that allows you to see the colour of the explosion in the barrel above the piston as it fires.
The colour in the combustion chamber changes depending on the air/fuel mixture being sucked through from the carb. The aimed for mixture is 14.7 to 1. This ensures that all the fuel is burnt and there’s less carbon monoxide and more carbon dioxide and water (or something like that).
Whatever the chemistry - a lean mixture (too much air) will run hot and a too rich mixture (too much fuel) will foul everything up with carbon.

This is the little spark plug. There’s only one so each cylinder is tested individually. (Or you buy more kits). Make sure the correct diameter kit is ordered - bikes are normally 14mm.

It has a ceramic/plastic centre so it can be seen through.

Run the bike up as normal getting it warm so there’s no choke involved and it’s ticking over well.
Remove the normal sparkplug and thread in the little tester sparkplug. The head will be hot and the weenie tester sparkplug is a bit fiddly - you don’t want to thread the plugs, make sure they go in easily, no need for a spanner. Just nip up onto the copper washer.
Screw on the extension cable that came in the kit…

…and connect the cap onto the extension - it just pushes on.
Start the bike up and watch the flashes in the test sparkplug window. Orange is too rich, blue is too lean. The air mixture screw can be turned to watch the clour change. The aim is to move from orange and just into blue.

See how the window is dark then lights up blue around the core.

Some people swear by these testers. But it will flash both orange and blue, it’ll go bright orange when opening the throttle as more fuel is sucked through the carb. And settle down to blue at idle.

This bike has the correct float levels the correct sized new jets and needles, I have the needle set on it’s leanest slot but still runs rich and deposits loads of carbon on the plugs.
I run hotter sparkplugs to burn the carbon (they don’t) and cane the motor when conditions allow it. Nothing has helped so I just pootle around now but will try smaller jets and needle when I get round to it.

The kit is easy to use but the results are a bit trickier than the instructions would let you believe.


Mine is still on the garage shelf. I got it in the 80’s for my Mini 850, then used it on my BMW r100s and a few others. I’ve only just received my 12mm adaptor because the T300 triumphs use that size. I need to be ready after I bugger up the carbs on the 1200 because the engine is still stumbling and I’ve decided it’s time to clean out those jets… or at least have a go… and then hand them over to someone who knows…


Takes me back. :slightly_smiling_face: :+1:

Don’t forget the Gunson Sparktune meter too, set that dwell angle. Although a feeler gauge and a light bulb works too.

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Got one and use it to, great thing for making your best guess more reassuring.

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Always a good idea to NEVER look directly into the glass window , use the chromium plated swivel cover that is supplied with the kit , it slides easily over the HT wire . Adjust the chromium " Mirror " with your line of vision . Why I hear you ask ? . . . . if the glass blows out under the engines running compression then it’s goodbye to your eyes .

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I had one of these back in the 80’s - very useful, doubt I have it now though.
Also had a Gunsens Carb balancer I seem to recall.

Never owned one but seen people use them and impressed