Fuel air mixture

[John Vardanian]

How do you ensure that the fuel/air mixture passing through all six throats-- in case of a 3-carb engine, for instance-- is even?

Thanks in advance.

john

[whturner]

Hi John:

For mixture information at operating speeds - high enough the idle circuit is not influincing the results - there is the old tried and true spark plug inspection/analysis. It is not super accurate and can be affected by other things. I won't fill up space since it has been described in a thread earlier.
At idle speed,the idle screws set the mixture, which you tweak until you get the best idle. That assumes all other factors which affect idle are OK.

In both cases, in a triple dual-throat-carb arrangement, each carb setting affects 2 cylinders of a 12 cylinder engine.

Thats about all a shade tree mechanic can do. the racers of course with unlimited bucks and equipment can do much more.

Cheers
Warren

[Rudy van Daalen Wetters]

John,

I'd first start with an air flow meter on each carb and see that they all draw the same amount of air at idle. Then, using a prop rod on the accelerator pedal, open the throttle to say 3000 rpm and see if they all still draw the same amount of air.

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

[John Vardanian]

Thanks guys.

I have two spark plugs on the same venturi that suggest too lean of a mixture. Would enriching the mixture with the idle screw remedy this?

john

[Michael Bayer]

John That depends on how much idle vs road time you had on those plugs, if very little I would suspect the high speed jets on that carb for some kind of tiny blockage. You might warm up the engine, clean the plugs, promptly drive the car a good bit at highway speeds then shut it off and look for changes. M

[whturner]

Thanks guys.

I have two spark plugs on the same venturi that suggest too lean of a mixture. Would enriching the mixture with the idle screw remedy this?

john

Hi John:

Your suggestion is a good place to start.

On my car the idle circuit affects the mixture up to (about) 4000 - 4500 rpm. So a good part of normal operation will be at speeds where the mixture is transitioning from the idle circuit to the high speed circuit.
In fact, on mine, I can't get best road operation with the same idle mixture setting that gives best idle. I am still looking into that. But in the meantime, guess which setting I use!
In my case, driving along a fenced road lets me hear whether the engine is running evenly on all 12 cylinders.

To be more quantitative Rudy and Micheal have the right advice. If I can find it (haven't used it since my triple carb Ford flathead V-8 days), I am going to get out my Unisyn and do the same thing.

Cheers
Warren

[John Vardanian]

Thanks again guys. I will start with the idle mixture screw since the plugs are all cleaned ant set. I will also remove the jets and inspect.

I had a thought and wonder what you technical guys think about it... what if you were able to measure the temperature of each exhaust port at various RPM's and then compare. Would a leaner mixture indicate a noticeably hotter reading?

I bought a probe type thermometer from McMaster-Carr with high hopes of exploring this, but the second I got it within two feet of the running engine it went nuts. There was too much interference from the ignition, I suppose.

john

[Rudy van Daalen Wetters]

John,

Maybe or maybe not on the exhaust temperature readings is my thought. Temperatures may not always correlate at different RPMs. The best indicator would be the plugs after a good trip with no idling. Hooking up an oscilliscope and a gas analyzer would be a cool way to explore mixture settings at various speeds.

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

[whturner]

Exhaust temperatures can tell a lot - but getting them is a trick.

When my engine was on the dyno after rebuild the exhaust temperatures were part of the measurements recorded. A thermocouple was inserted (had to drill a small hole) in each header to get the measurement. This is approximate, but gives a relative temperature for each cylinder. For an more accurate measurement of gas temperatures, a suction pyrometer would be needed.

(Where the spark plugs can be seen there used to be a device which let you see the color of the flame in the cylinder.) I have no idea how that could be used with most of todays engines).

If an exhaust gas analyser is available, a tailpipe measurement can be used if the exhaust system is tight enough. On a Ferrari the 2 banks of cylinders can be compared.

Cheers
Warren

[John Vardanian]

Thanks guys.

Warren, just curious, the small holes in the headers you mentioned, were these holes in your car's headers or the testbed headers?

john

[whturner]

Hi John:

That is a good question! I assumed my headers were used - will have to find out now, as you say, just for curiosity.

Cheers
Warren

[330GT]

...(Where the spark plugs can be seen there used to be a device which let you see the color of the flame in the cylinder.) I have no idea how that could be used with most of todays engines).
...
Cheers
Warren

Warren is talking about Colortune spark plugs. They have a clear ceramic shell, so one can see the color of the burning gases. See http://www.gunson.co.uk/item.aspx?item=1822&cat=486 for more info. Still applicable to carb cars and motorcycles as a way to adjust the idle mixture. However, for a 3 carb V-12, six would be needed to do a good job. That ends up being expensive. There's a FerrariChat thread where someone uses them on a 308. A picture shows a 4 plug kit, but I don't see that option as being currently available.

[whturner]

Wow Kerry:

I had no idea they were still available. Of course when spark plugs were easily visible, and the older cars plugs had no boots, they were easier to use. But they were also not usually needed, when you only had one or two idle screws to fuss with. And I couldn't afford one anyway in those days.

Cheers
Warren

[sjvalin]

I have a single ColorTune somewhere. They are expecially useful for tuning SU carburettors as the mixture on these typically is constant from idle through higher RPM's. JohnV, you are welcome to borrow it, assuming I can find it in my garage!

-steve

[Rudy van Daalen Wetters]

The colortune sparkplug seems to work really well on SU carburetors. My understanding is, however, that it doesn't work as well on Weber carburetors. This has been my experience also. I have heard that on Webers, the car must be under load to give an accurate reading for setting the mixture using the colortune plug.

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705