Colortune carb tuning

[chuck1006]

I tuned my HD6 carbs on my 100-6 BN6 per instructions in the manual but thought they might be running somewhat rich since I see black smoke when starting and racing the engine. I know float levels are OK and needles look OK and shoulders are set properly. I used a Colortune last weekend and the color was very yellow at idle. When I adjusted to try to get close to cobolt blue, engine would idle rough and backfire when letting up after racing engine. It doesn't backfire when set by the piston lift method but still seems to run too rich. Maybe I should try new needles. I did the basic rebuilt kit when rebuilding the car.
Any ideas? Other things to check?
Thanks,
Chuck Keeler

 

[Dave Russell]

Hi Chuck,
Click here for a pretty good discussion on the subject;
https://www.britishcarforum.com/ubbthread...=true#Post66542
D

 

[piman]

Hello Chuck,
there is very little variation in S.U. needle dimensions at the idle setting. So it is not a needle issue when you are setting the idle mixture. What can cause a conflict if the engine is drawing in oil at idle, this will also burn and give a misleading result. How many cylinders are you checking?, the ideal would be all six, expensive in colourtune kits. Really the minimum you need are equal to the number of carburettors.
The other guide is the colour of the plugs after an on load run.
Basically there is no point going for the blue set up if the car runs rough. What about when you open the throttle, what colour do you get then? I would expect yellow to begin then a medium blue.
Alec