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MG MGB Technical - Colour tune results

Today I tried a colour tune in cylinders 3 and 4. Test 1 (in the instructions) requires one to enrich the mixture until the colour is yellow and then weaken the mixture until blue is obtained. I found that no matter how much I enriched the mixture I couldn't obtain yellow.

Test 2 requires a blue colour from idle to 3000 revs. I found I got blue then yellow and couldn't get rid of the yellow by weakening the mixture.

Suggestions welcome on what these results indicate.

DW Staniforth

To me, it indicates that the Colourtune isn't a very reliable instrument! ;-) I've got one but have given up using it. I get much better results with the SU Toolkit as sold by Moss and others (two tubes that go into the dashpot pistons with wires that go into the tubes). With it, you can balance the carbs at idle, verify that they stay in balance through the entire rev range, and set the mixture strength (the wires give you great visual reference for using the lift pins). It's great!
Rob Edwards

DW. As Rob notes, the Colourtune does not seem to work very well with modern fuels. It was developed many years ago, back when all of the fuels were only petroleum based and the color charts and instructions are based on that fuel.

With modern fuels, I have never been able to use the Colourtune to effectively set the mixture on an engine. Rob's suggestion of the German made SU tool kit, sold by Moss Motors and others, is a good one. I purchased my first one in 1968 and have been using it every since with excellent results.

Les
Les Bengtson

Thanks for the advice guys. I've had a surf trying to find someone who sells the kit you refer to but no luck, does anyone know where they can be bought?
Dave (DW)

Dave. The Moss (US) part number is 386-300 and there is a photo of the kit next to the description. "SU Tool Kit. An assortment of handy tools for the home mechanic in a vinl pouch, including a jet wrench. Instructons included. Suitable for all cars with SU carbs." You might go to the Moss (US) website and see if they have a photo of the kit, then, check if it is available through Moss (UK) or other motor factors.

Les
Les Bengtson

My colortune works well, Shell Optimax with castrol
valvemaster last time I used it, since the head rebuild and restetting the timing I do not need the valve master and have not needed yto use the Colortune either. It did the yellow back to blue, then you could blip the throttle and see it turn to yellow as the mixture richened up ready to go to work.
Stan Best

The colours might not match the graduations on the chart exactly with modern fuels, but yellow still shouts rich and white lean. I've doubted mine in the past only to realise it was showing a mixture problem I was in denial of! What needles are you using? Inability to get a rich setting at idle usually indicates air leaks past worn throttle spindles. Spray some carb cleaner on the shafts and listen for the rpm's to rise to confirm.
Steve Postins

midget Tune-up Tool kit is available from Brown & Gammons for
Mike BGT

I remember trying Colortune years ago and found it pretty difficult to use as it needed quite coarse adjustments to get it to change colour. Since then I have always used the way the idle speed changes with the mixture nut/screw for coarse adjustment, and the lifting pin for fine. The lifting pin is *very* sensitive to small mixture changes, one flat or less being enough to make a change. It's also true the lifting pin only makes a very small change to the idle, particularly on HIFs. But if you start with larger swings between rich and weak you can 'tune' your ear, which will then allow you to detect smaller changes. And if you note how many flats or half flats it needs to change it from detectable weak to detectably rich and back again, then the correct position is mid-way between the two. Of course, everything else must be spot-on before you start setting up the carbs, or it is much more difficult if not impossible. A good way to check for carb problems is to wind the jets until they are flush with the bridge, then down two full turns to the starting position. If one carb then needs significantly more coarse adjustment to get to the point of highest idle revs, then it indicates something is wrong with one of the carbs.
Paul Hunt 2

I tend to use a vacuum gauge to dial in a mixture and the Colortune to simply verify what should be true. Altho current American fuels differ in composition from what they were 40 yrs ago, the big difference I have seen in Colortune results is that the "Bunsen Blue" of current fuels tends to be a lighter, almost verging on lavender color, whereas years ago, it was a stronger and purer Bunsen Blue. That may have something to do with a stronger oxegenation of the fuels today to meet EPA requirements? Where's an expert here??
Bob Muenchausen

This thread was discussed between 27/08/2006 and 28/08/2006

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