Colortune and CV carbs

[Skinner]

I have CV carbs on my 1981 650GL. I've read that the Morgan Colortune does not work very well with CV carbs. Can anyone explain the rationale behind this?

Thanks!

[Steve]

I can not explain the rationale, but I can relate my experience.

I used a borrowed ColorTune to work on a set of VM carbs and was VERY impressed on how well it worked. While using it, I could see the difference in the flame color and could hear the difference in the engine while I was making adjustments. It was a WONDERFUL experience.

Feeling good, I took the ColorTune home to check the carbs on my wife's bike. Started adjusting the mixture screw, saw NO change in color. As I turned the screw IN, the flame started to sputter occasionally, then more often, finally going out completely as the screw got turned IN farther. As I turned the screw back OUT, the flame started to sputter occasionally, then got more regular, finally becoming a smooth, steady combustion flame. IT NEVER CHANGED COLOR THE WHOLE TIME. I tried it on the other carbs on that bike and then on another bike, with the same results.

On the other hand, there are some who say that it has worked on their CV-type carbs quite well. Don't know what the difference is, but many will say that the ColorTune works well on the VM carbs, but probably NOT so well on the CV-types.

.

[koolaid_kid]

I have no idea either. I do know from personal experience that it did not work on my GS1100E. I had exactly the same experience as Steve.
Perhaps the Morgan company can shed some light on this phenomenon?

[Skinner]

That's what made me wonder what the problem was. I used it on my 650GL and the colour changed along with the idle and it seemed to work as advertised. Then I read a few threads about not working with CV carbs and I wondered if I was missing something.

[azr]

I use a colourtune on my 80' 1000G CV carbs and it works great, exactly like it does on my 78' 1000 VM carbs. I know a lot of people have had an experience like Steve and Koolaid_kid though, very strange stuff.

[koolaid_kid]

That is very odd. And my GS was 100% stock, absolutely no mods.

[bwringer]

One thing you have to understand about a Colortune is that it's not a fine-tuning tool -- it's a way to get close. You then fine-tune based on idle, idle drop, and off-idle throttle response.

On VM carbs, you have a wide range of adjustment available with the separate fuel and air screws. A Colortune really helps get these in the ballpark quickly, especially since it's so easy to get confused (the air and fuel screws work in opposite directions). The wide range of adjustment also means that it's very easy to overshoot, so a tool that lets you see the immediate effect on the mixture is very effective.

On CV carbs, the range of adjustment is much more narrow -- the idle/off-idle mixture is mostly controlled by the pilot jet, and the screw position is mostly fine-tuning. So you're not going to see dramatic effects. In addition, I think CV carbs also get some of their mixture via the needle, even at idle (the slides are open a bit at idle), so this will obscure the results quite a bit as well.

What we need is a clear plastic carburetor...

[Skinner]

Thanks bwringer for a great explanation. On my 1981 650GL the colour change is very evident, as is the change in idle speed as the colour changes to blue. I fine tune from there for throttle response.

Cheers!
K