Ted Palmer's comments on Colortuning

You have to be a bit creative when using one to find the right needle. Of course, a chassis dyno is the best way to find the right needle, but if you are having a problem at a particular rev range, a Colortune can tell you if it is running rich or weak.

What I've done in the past is to put the front end up on stands and run the motor against the brakes. Two people can do my "backyard dyno" trick, one to operate the throttle and brake and maybe watch the carby inlet through the removed centre speedo, and the other to watch the colour of the burn and maybe take carby piston height measurement.

No doubt Gunson's will emphatically denounce my technique :-O I still have my original Colortune plug, BTW. It's still in one piece, but the viewer extension had melted at the plug end long ago :-/

You need to keep the power runs short as the Colortune isn't really designed for this use. It helps a lot if you can see into the mouth of the SU to see the piston height, else you can wedge a _lightweight_ stick of some sort onto the bore of the damper cylinder so that you can directly measure the height of the piston. Having no damping is no problem at steady state. Once you know the piston height where your problem is, you could find the exact point on the needle that needs to be altered, and you can consult the tables for something that matches your estimated required diameter. A few thou in diameter will make a discernable difference in mixture.

For full-power runs on a big bore, a chassis dyno would be the safest option. I imagine you could melt the transparent insulator in extended use, but I haven't blown one (yet). Doing the runs at night is the best way, making it easiest to see the colour of the burn from a distance.

On a smallbore, you could see if the carby piston is getting near full travel on a _short_ full power run, and then pick a spring accordingly.

A bit of inspired and meticulous use can get you a needle that is in ballpark, which would be a good starting point when you take the car in for a dyno run eventually. Just be careful and keep in mind the limitations of the plug. For the person working on the engine side during the backyard runs, wearing an industrial face shield would be a good idea, but me being young and immortal at the time I didn't ;-) Having a tacho in the car helps with the precision, too.