Tuning SU carbs

[pjbowman]

After a nice drive this past weekend in my Aceca (AC Engine), I pulled the plugs to check the condition. It appears the back 2 cylinders are running rich. This may sound like a basic question, but how do I lean out just the rear carburetor? Do I need to replace the needle with a different size, replace the jet, or what? There's no mixture screw like on the SUs on my motorcycles, so not sure how to go about this.
   
   Peter

[pjbowman]

As a follow-on question, has anyone used the Gunson's Colortune system? Any feedback on it?
   
   Peter

[aex125]

Peter,
   Not sure about the Ace SUs, but on other ones I have had the mixture is adjusted by turning a knob on the bottom of the carb body. If you reach under the carb, you should feel a knob that when you turn it, it move a sleeve that the needle is in. Clockwise rotation (when viewed from the top) will lower the sleeve and richen the mixture. There is also a a small rod that if you press it upwards, it will disable that carb by raising the piston so you can pin point which carb is not quite right. This is easy to do on a 2 carb system,but I am not sure how well it will work on a 3 carb system. I'll try and find an online link as I seem to remember there is a web site that talks about the mixture adjustment and probably will explain it much better than I have.
   Jay

[pjbowman]

Thanks Jay, I'm traveling right now, will give it a closer look this weekend.
   
   Peter

[aex125]

Peter,
   Here is a link with pics. The Jet adjusting nut is the the one you want.
   
   http://www.mgexperience.net/article/su-tune.html
   
   Jay
   
   PS I have always used a Unisyn for the adjustments and it works fine.

[pjbowman]

Excellent, thanks Greg. I look forward to getting home and seeing what I can do. I maye give the colortune a shot and report back to the group to see how it performs. Can never have to manny doo-dads int he toolbox,right?
   
   Thanks again.