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Chris Miller
Prolific User
Username: cjm51213

Post Number: 178
Registered: 5-2013
Posted on Sunday, 10 August, 2014 - 01:09 am:   Edit PostDelete PostView Post/Check IP

Hi Folks,

How do you measure exhaust emissions and engine performance for optimum carburetor tuning?

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Randy Roberson
Prolific User
Username: wascator

Post Number: 281
Registered: 5-2009
Posted on Sunday, 10 August, 2014 - 02:14 am:   Edit PostDelete PostView Post/Check IP

Absent another reply, I will offer this:
there are portable analyzers which will measure the components in the exhaust and you can adjust while reading the output of the box: we do it on occasion with large engines. In general: richening the mixture beyond the ideal (stoichiometric) will result in increasing carbon monoxide (CO) and decreasing NOx (oxides of nitrogen). Conversely, leaning will result in lower CO and higher NOx. "Rich-burn" (i.e. our old car) engines run best just a little rich of stoichiometric. Too lean and there will often be a stumble on acceleration, and way too lean will result in lean misfire as the mixture becomes hard to ignite reliably. Too rich and there will be black sooty smoke and slow combustion.
On an industrial engine with a governor, as a generator set, for example, one can adjust the mixture while observing the manifold depression. As the mixture becomes more suitable the engine's manifold vacuum will increase which reflects the governor closing the throttle to keep the engine at the same speed (it will produce a little more power at a given throttle setting,as the engine gets "happier" with the mixture so the speed will try to increase). We would adjust the mix to the maximum manifold depression (the most manifold vacuum or the throttle the least open) under load, then richen it a little: reduce the manifold vacuum a little. The generator would take the load best a little rich.
In a Car with the mixture adjustments available to the driver, you can play with the adjustment under a steady load, and actually feel the difference in engine output. Many drivers never mastered this when the Cars were new, I am sure, and just left the adjustment where it would run OK. Just like many people could not tune pianos because they could not hear the beats the tuner uses to compare the tones.
Many people have adjusted the carburetor on a lawnmower; same principle. You turn it rich until it runs poorly, then lean until it runs poorly, then sort of set it in between, hopefully a little rich. In the case of the lawnmower, you get more obvious feedback, because you can hear what's going on.
I know you are confused fault for not being clearer.
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Jan Forrest
Grand Master
Username: got_one

Post Number: 605
Registered: 1-2008
Posted on Sunday, 10 August, 2014 - 09:05 pm:   Edit PostDelete PostView Post/Check IP

The simplest and cheapest option (other than getting some other beggar to do it) is to get hold of a "Gunson's Colour Tune" device. It's a simple spark plug replacement with a clear window in the top through which you can view the actual colour of the combustion process.

They have 2 disadvantages:

1) They have a limited range of rpm useage due to the relative fragility of the bond between the glass and metal bits. It's not unknown for them to part company with bits of shrapnel heading in your direction at a significant rate of knotts (OUCH!). Fortunately this is very rare and normally only happens if you go potty with the loud pedal.

2) The field of view is quite limited which means you can only see the top of the plug using a mirror on RR/B cars. With some of the more modern designs it's impossible to see the plug at all due to it being hidden by the plug lead and/or a coil pack.

As Randy says; there's little around to beat a pukka exhaust gas analyzer. For the hobbyist Gunson's used to make a pretty good one. I bought a used one several years ago for just the price of a middle-of-the-road bottle of single malt. Other advantages are the inclusion of other testing equipment in a single package such as:

Switchable between 4, 6 or 8 cylinders
Dwell meter
Precise rpm readout
Battery voltage display
Points bounce indication

I wouldn't be without it these days so if it ever turns it's toes up I'll be looking for a replacement immediately if not sooner!
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David Lacey
Experienced User
Username: dlacey

Post Number: 22
Registered: 11-2010
Posted on Monday, 11 August, 2014 - 10:37 am:   Edit PostDelete PostView Post/Check IP

Hi Chris,

On a Jaguar v12 Quad carburettor car I have installed Bosch 'Lambda' oxygen sensors in the exhaust. This is done by drilling & welding a threaded fixture onto the exhaust pipe as close as possible to the manifold, to which the sensor is mounted.
These sensors are connected to LED 'bargraph' displays temporarily mounted at the dashboard.
With this I can 'see' the mixture of each carburettor individually when driving, under all speed/load conditions.
It works very well, only getting confused by misfires, as these appear to be 'lean' when infact they could be rich misfire or ignition misfire (residual oxygen in the exhaust is interpreted as lean mixture).
On an RR I imagine it's not so easy to see each carb mixture individually as both carbs feed both banks, but still you get a good average reading for the engine.
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Jan Forrest
Grand Master
Username: got_one

Post Number: 611
Registered: 1-2008
Posted on Monday, 11 August, 2014 - 10:30 pm:   Edit PostDelete PostView Post/Check IP

Depending on the model one lambda sensor may not be enough. My Shadow has a single exhaust system where each manifold joins at the front of the 'B' bank. A single sensor located at this point would give an average reading for both carbs, so one could be rich, the other weak, but the combined reading could be 'just right'.

Fortunately this doesn't matter much as I'll only be adding a single lambda when I (eventually) fit the 'closed loop' LPG upgrade. As the Rolls runs a single, large evaporator which simultaneously feeds both banks the same quantity of gas, both banks should be running the at the same air/fuel ratio.

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