Colortune question

[fizeus]

Hi all,

This is my first post to the forum, so, at first a big hello to all the members.

I'd like to buy a Gunson colortune to fix the carbonation of my 911 2.4T (Model Year 1973).
This car is equipped with Zenit 40 Tin carburetors.
I already fixed the air with the syncrometer but now I have to work on the mixture.

Can anyone tell witch product is the best for me ?
I think I need a 14 mm spark plug type, but I'm not sure if I also need some extra adaptor.

Please help.
Thanks in advance.

[4flyboy]

Welcome Fizeus!

Where are you, post some pictures? I'm afraid that I cannot help with your questions but one of the wise sages will pop on and know what ails your 2.4 T..... here's a bump.

Scott

[spuggy]

Originally Posted by fizeus

Can anyone tell witch product is the best for me ?

Yes. A wide-band O2 sensor, with data-logging. Failing that, book dyno time and use their equipment to measure the mixture under load. Worst case, google "plug chop" and try to ensure the mixture isn't catastrophically weak anywhere in the rev band.

You can just ensure you have the "correct" size jet, and tune it until it doesn't run obviously badly. But for good/best results, you should probably do one of the above, if you dont have access to an old-time mechanic who knows how to do this.

A Gunson Colortune is only rated for setting mixture at idle, if I recall correctly. The one I bought way back when was utterly useless for that. Even if it wasn't, you'd need a dyno to read the mixture from it, becuse it'd be really, really hard to look in the engine bay while going down the road..

[1QuickS]

Get the standard, 14mm Colortune.

Colortunes are primarily for carburetted engines and those with MFI; fuel delivery systems like CIS would be better served with an O2 sensor.

I use Gunson Colortunes (one in each cylinder of my test engine) when I set idle mixtures on every Weber, Zenith and Solex carburetor project and have been using them since 1979 on my 67S and all carburetted engines since then. I use them primarily for setting idle mixtures but they can be a valuable asset when troubleshooting a running issue such as erratic combustion, lack of spark, etc.

They do also provide indication of progression circuit operation (richness or leanness) as the throttles are opened up.

The advantage of the Colortune over O2 sensors is the ability to adjust individual cylinders where the O2 sensors are "averaging" and cannot isolate individual cylinders. When setting idle mixtures I routinely see mixture variations that are adjusted by 1/8 turn of the mixture screws. Of course it is easier if you have six to use at once which is an expensive diagnostic tooling expense for a six cylinder engine.

Colortune is primarily a tool for adjusting idle mixture strength. Once the throttles are cracked open from idle, the progression circuit is adding more fuel with each increment of throttle opening. The fuel contribution from the idle mixture screw adds to the total fuel delivery during progression, transition and into the beginning of the main circuit. Any minor imperfection of fuel mixture at idle is pretty insignificant to fuel delivery during subsequent engine speed ranges.

When I use my Colortunes in my engine I easily see the combustion color by using a mechanic's inspection mirror (mirror on a wand handle). Do not use the tube with integral mirror supplied with the tool.

Here are some more tips for using Colortune:

• I used a countersink to bore an internal taper into the tip that screws onto the threaded end of the Colortune; it really helps to guide it on
• If the tool falls out of your socket either use a magnetic pick-up tool of a set of long, curved forceps to retrieve it
• My spark plug socket has been embellished with a bit of tubing that stays inside the socket and has an ID that captures the insulator on the end of the tool so that I seldom drop the tool down that long, dark hole
• If you have an OEM Porsche spark plug tool then the electrical lead may be passed through the tool and used to keep the Colortune in place during installation/removal
• Close idle mixture screw, adjust it open until a definite rich condition is indicated and then close until just begins to lose yellow (purple combustion color; wait for a count of ten after each mixture screw adjustment to allow the fuel delivery to stabilize...it is not instantaneous with screw adjustment)
• Repeat for all cylinders

Once idle speed and air balance has been reset after the Lean Best adjustments with Colortune you will find that idle mixtures will have been altered so repeating the Colortune procedure would be good. If you use the STE air flow meter (my highest recommendation to use them) then you will see an airflow reading of around 4 1/2 to 5 once Lean Best idle mixture is achieved. Higher readings indicate a rich mixture since it takes more air to burn the excessive fuel delivery.

However good the Colortune is for idle mixture setting I do recommend an O2 sensor for checking fuel mixtures during driving conditions.

[fizeus]

Hi Paul,
I'm very pleased to read your detailed reply. Thank you, and also thanks to spuggy.

I already own a STE SK syncrometer. And after reading your reply i'll surely buy a color tune.
My Porsche has sitted for long and my mechanic has tried to fix the carburation by ear.
His ear did the job but Zeniths are still popping thru Venturis.
I know this is generally a Lean condition (the mechanic told me that Lean is better then rich) and i'm just looking for an easy way to tune the carburetors.


Ok. I know i HAVE to clean the Zeniths but I'm a bit reluctant until I'll find old-stile mechanics in my country.

Thanks again to all !