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MG MGB Technical - Gunson CO meter in motion?

If I securely attach the probe from a gunson CO meter to my tail pipe, can I use it to check my CO level under load while driving? I thought it could serve as a poor man's "rolling road" to help tune my SU HIF4s.

I don't presently own a Gunson, but it seems the most cost effective way for me to accurately tune.

I've thoroughly explored an air/fuel meter and have decided to forgo that route for the time being.
Steve Lipofsky

Steve. Not really. For most accurate operation, the CO meter should be slightly lower, and upwind, of the exhaust pipe. This allows any moisture in the system to drain out properly. As to accuracy of the Gunson CO meter, I do not see any. It is adjusted to read 2.0% CO in ambient air. According to the factory, this is some form of "majic number" going back to ancient Anglo-Saxon and Scandinavian mythology which allows you to set their machine to read out your CO level without having any form of actual calibration standard. (Such as inserting the probe into a known mixture of air/CO and setting the machine to read that known mixture.)

Do not know if it is the heavily polluted air in the Phoenix area or what, but when the machine is set up and adjusted according to the factory specified procedure, it will not cross check with the local emissions testing station machines nor with the much more expensive four and five gas analyzers owned by local mechanics. We have experimented with this using three different Gunson's CO meters. The machines will cross check with each other, indicating some level of accuracy. But the actual CO readings of the Gunson meter do not seem to bear any relationship to what the readings on the professional grade machines are. (Latest example. I set the carb to show 2.5% CO on the Gunson machine. Emission station reading was .19% CO, a rather significant deviation.)

The best use of the Gunson CO meter is as a comparison tool. If the car fails emissions, I hook up the meter and calibrate it per factory instructions (realizing that it is not going to read what the emissions stations figures are). I then use the meter to monitor my adjustments in the direction they need to be going to pass the emissions requirements. Often times, with a few trips through the emissions test station, you can set the emissions yourself, and pass, using the Gunson meter. But, my experience is that the meter does not accurately reflect the CO emissions and does not accurately reflect the air/fuel ratio (which has to be made by inference from the CO reading).

Some of the other people here have, over the years, experimented with O2 sensors and dash mounted read outs. For tuning purposes, this sounds like the best way to go, but I have not had time to test it myself. Les
Les Bengtson

Thanks, Les!

I guess I'll need to revisit the Air/Fuel meter or see if there is a better CO meter on the market that's affordable.

I've got a household CO meter with a digital readout that I expect is reasonablt accurate. It shouldn't be that tough to make a good inexpensive automotive CO meter.
Steve Lipofsky

Steve,
http://www.innovatemotorsports.com/
Although this AF meter isn't cheap at $349 it has a wideband O2 sensor compared to the narrow band sensor used in the cheaper models. I tried one oof the narrow band units about two years ago and it made a nice light show but didn't provide any useful data. Clifton
Clifton Gordon

Go for a nice drive on the highway than stop the car and pull the plugs to see what they look like.

I have a Kal-Equip a/f meter that I use on the road to see how my needle selection is w/a custom engine I built. I can't verify its accuracy but it does work and gives me a nice overall picture of what is going on.

The problem is the twin SUs as one can be lean and the other rich.
Mike MaGee

The CO reading on a Gunsons reduces as the revs increase. Too high a revs can damage the sensor, and as Les implies 'calibrating' to 2% in free air resulting in an approximately accurate exhaust reading at idle is simply a quirk of physics. Although having said that mine is 2% pessimistic, but I compensate for that before the annual test.

The colour of the plugs is only an indication of what the average mixture was in the recent period before removing and examining them. If your needle profile is incorrect for your particular application you could be significantly weak or rich at other piston heights.

It might not be as easy as falling off a log to get twin SUs balanced for mixture as well as air-flow, but it isn't impossible.
Paul Hunt

I use the Kal equip to see how the neeedles are performing at different RPMs and different loads. Although it is not exact, I can see if the car is running lean or rich and it helps me make a decision about my needle selection methods and what I need to look for when selecting another set of needles to buy. Thankfully Joe Curto is very close by and puts up w/me, although he will have nothing to do with the Jap SUs I have in my modified Datsun Roadster.
Mike MaGee

Clifton: What is that system other then an O2 sensor and and a volt meter? Sounds like a lot of cash for what you really get. RIC
R E L Lloyd

RIC, I don't own one and I'm not endorsing it, but I believe the wide band O2 sonsors cost lots more than the narrow band sensor. I passed the information on from an ad I looked at in Grass Roots Motorsports.
Clifton
Clifton Gordon

This thread was discussed between 08/04/2004 and 10/04/2004

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