Colortune vs Old School carb mixture adjustment

[CraigLandrum]

I recently bought a Gunson Colortune to try and be a bit more scientific about setting the mixture on the twin H6 SUs in our TR3A. The concept is quite attractive-by observing the color of combustion thru a special glass topped spark plug, you can exactly set your mixture to achieve the most efficient combustion, thereby maximizing both power and fuel economy. The target color is a neon blue-the more yellow you see, the richer you are running and the more fuel you are wasting. Installing the plug is easy and-in a shaded area-the combustion color is easy to see.

My main problem is the closer I got to blue, the weaker the engine ran and-using the old school method of raising the piston 1/8 inch and listening to engine revs(increase=too rich, decrease=too lean, no change=just right) the leaner it got. I finally ignored the color and went old school. The engine is running fine, although the yellow combustion color(and the black deposits on my plugs) tell me the mix is too rich.

I was never able to get to pure blue, even with lean needles installed and the mixture nut screwed all the way up. My son informed me that when he rebuilt the carbs he may have used HS6 rebuild kits instead of H6-don't know if that matters.

I'm worried since the plug deposits and the Colortune say I'm running too rich, but old school apparently tells me I'm dead on. FYI, timing is set at 6 deg btdc, floats verified to be set at 7/16, and there is no fuel overflow.

Can anyone here vouch for the accuracy of the Colortune/blue combustion method with respect to SU carbs? Any suggestions on why I seem to be running too rich but the old school tests would tell me I'm right on?

 

[Geo Hahn]

I have used the Colortune -- lots of fun if you like gadgets -- and was able to get the blue and what seems to be a good mixture. To be sure, I do not think it is any easier or more accurate than the traditional method.

BTW - with the lift-the-pin method you're listening for the engine to speed up slightly then return to where is was -- right?

I have found I can get some additional accuracy in the lift-the-pin thing if I use a clip-on tachometer to see the changes in RPM instead of relying on my hearing.

 

[martx-5]

I have never used the Colortune, but what I did do was install an heated oxygen sensor it the exhaust header of my TR3 to monitor the air fuel ratio. I've done some engine modifications and want to keep track of my air fuel ratio under different conditions. At idle, this thing runs pig rich...you can smell it, and it reacts as being rich when lifting the pistons. But, the car runs great! What I do observe, is that the air fuel ratio changes quite a bit while driving. Cruising I get an almost dead on stoich reading...14-15. Acceleration shows some lean tip in followed by a nice rich climbing of the AFR gauge, up to the 12-13 point. Trailing throttle, back to pig rich. These are good readings, and all of this tells me that I'll just leave things as they are for awhile and disregard what is happening at idle.

What you set while at idle is not what you will neccessarily see while driving. Idle adjustments are just a starting point. You really need to tune for actual driving. Sometimes the butt-o-meter is more useful then some of the fancy tools that are out there. The big advantage to the O2 sensor is that you can monitor the AFR on the fly under many driving conditions.

Edit: Here's a pic of the O2 sensor and the AFR gauge...in the glove box.

 

[NickMorgan]

I tend to think that lifting the piston 1/8 inch is a little too much. I think that 1/16th would be more appropriate and produce a more accurate result.

 

[CraigLandrum]

Could a worn throttle rod allow extra air to enter the mixture after the bridge, perhaps reducing the airflow over the bridge, thereby forcing me to adjust the mixture rich just to get enough fuel to the engine?

 

[NickMorgan]

Certainly a worn throttle spindle won't help! If it is bad it will make tuning the carbs almost impossible. I think that the spark plugs can tell you a lot and I would be tempted to go by their colour.

 

[martx-5]

Yes, but you can check that easily by squirting some starting fluid or something around the throttle shafts and see what the reaction of the engine is. If they aren't leaking, there should be no reaction.

I thought you guys re-did the carbs!

 

[CraigLandrum]

My throttle shaft question was purely theoretical-they do not appear to be bad, but I'll try the carb cleaner trick to see if air is getting in.

 

[charlie74]

not to hijack this thread, but i have been interested in a colour-tune for my ZS's. i typically use the plug colour as a guide for air/fuel mixture but had recently been wondering lately how much variation in colour can one expect to see as ambient temperatures change? i live in the northeast and we have had some very big changes in weather so far this spring.
thanks,
c74

 

[poolboy]

I don't know about temperature and color being related, but mixture and temperature/ altitude, yes.
Here's a pretty complete color guide; https://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

 

[TR3driver]

The concept is quite attractive-by observing the color of combustion thru a special glass topped spark plug, you can exactly set your mixture to achieve the most efficient combustion, thereby maximizing both power and fuel economy.

Unfortunately, that theory is fundamentally flawed! The problem is that the best mixture for fuel economy is DIFFERENT than the best mixture for power.

A 'perfect' (aka stoichiometric) mixture is when there is just enough oxygen to (theoretically) burn all the fuel, and no more. But, best fuel economy is achieved with a little extra oxygen; while best power needs a little extra fuel.

Plus of course, mixture at idle has little or nothing to do with mixture at cruise or WOT. Unless you know for certain what mixture curve your carbs produce, any attempt to set 'perfect' mixture at idle is likely to screw up one of the other conditions.

So I vote for the old school "lift the piston". After some experiments along the same line Art is following; I feel it gives the best overall performance. Plus it gives me a double-check on the remainder of engine tuning/condition ... If I don't get a clear, sharp peak in response to the LTP method (just one flat of the mixture nut should produce a detectable difference), then there is something else wrong (valve adjustment, ignition timing, etc.).

PS, even running slightly lean for best economy, there is still some portion of the fuel that is not burned completely.

 

[martx-5]

Altitude has little or no effect on variable venturi or constant depression carbs such as the SUs and Zeniths. They are self adjusting for altitude by the nature of their design.

 

[poolboy]

Altitude has little or no effect on variable venturi or constant depression carbs such as the SUs and Zeniths. They are self adjusting for altitude by the nature of their design.

Oh, I was thinking that the oxidizing agent (O2) might vary percentage-wise in the volumn of air with altitude and require a different adjustment to compensate...oh.

 

[TR3driver]

Altitude has little or no effect on variable venturi or constant depression carbs such as the SUs and Zeniths.

Certainly less sensitive than fixed venturi carbs; but they are still sensitive to altitude. VTR 2001 (I think it was) was held at some 9,000 feet elevation and you never saw so many Triumphs blowing black smoke.

 

[pjsmetana]

Thanks Martx-5! Now I have to order a wide band O2 sensor and gauge

 

[tdskip]

So in full disclosure I haven't hooked up an Q2 sensor to any of my cars yetr, but after fooling around tuning my friends car with one I can confirmt that they are;

1) A huge help

2) A huge amount of fun

It's on the list of to-do items for the TR8 for sure....

 

[martx-5]

Re: Colortune vs Old School carb mixture adjustmen

Thanks Martx-5! Now I have to order a wide band O2 sensor and gauge

A wide band is the way to go if you want to spend that kind of money, but I don't feel it's neccessary. I used a generic $30 narrow band with a Westach analog AFR gauge that cost about $50. The analog gauge is buffered so the needle doesn't act so skittish.

I'm just looking for lean vs. rich readings, and I think the narrow band provides enough info my my purposes.

 

[pjsmetana]

Re: Colortune vs Old School carb mixture adjustmen

Thanks Martx-5! Now I have to order a wide band O2 sensor and gauge

A wide band is the way to go if you want to spend that kind of money, but I don't feel it's neccessary. I used a generic $30 narrow band with a Westach analog AFR gauge that cost about $50. The analog gauge is buffered so the needle doesn't act so skittish.

I'm just looking for lean vs. rich readings, and I think the narrow band provides enough info my my purposes.

I guess that's a good point... as I don't see my little cream puff getting on the Dyno any time soon.