Fun with air/fuel ratio meters

[Coloradocobra]

I finally installed my Autometer air/fuel ratio meter last weekend to attempt to fine tune my new carb. I installed the O2 sensor about 4 inches downstream from the collector and 3 inches before the muffler on the driver's side. It immediately confirmed my rich condition and showed my idle swinging wildly as I was attempting previously to overcome a rich condition by leaning the idle circuit. A drop from 70 to 68 jets in front improved the idle and now my cruise mixture shows that it is in the "rich" range right next to "ideal". So, I have gone from way rich to just rich.

My question is, if I can drop one more jet size and get my cruise and idle in the ideal range without creating a lean condition under full load, is this desireable or should I leave it slightly rich? In Denver, the only place to go is even higher in altitude and it would seem that making it run just right down here would help prevent loading it up when making a mountain run.

Anybody have experience with these meters on a carbed engine? There was a great article on the web at one point on how to use these to tune Holleys but the link seems to be broken. I just want to make sure I don't mess something up.

Gary

[ediamond]

Does the Autometer pickup from both headers or just one? I am going to install a fuel meter on my car and want O2 sensors on both pipes to hit the settings dead on. I have a Demon and Holley so this should be very interesting. Demon claims that they have less issues with loosing their settings than the Holley's....we will see.

[Coloradocobra]

The gauge I have only supports one O2 sensor. I suppose a switch could be used but that makes driving a little difficult. For ultimate tuning, two sensors should probably be used as it is possible to end up with slight differences from side to side, which is especially important if you are running towards the lean side. A fellow in Denver with a pretty radical GT-350 runs two gauges in his car. For now, I think I'll stay a little on the rich side while monitoring only the left bank to have a margin of safety on the right side. I may weld in a bung on the right side in the future just to compare.

I was surprised how constant the fuel ratio was across the rpm range running from 40 to about 80 in third gear. With more rapid acceleration you can actually see a temporary lean condition until the power valve opens and then the mixture returns close to the original value. Full measurement with the secondaries is a little tough because I build up speed a little too rapidly for legal limits.

I had this Holley custom built to match my engine and aside from some float/needle and seat issues with our goofy fuel at this altitude, it runs MUCH better than the 3310 I used to have on it.

The arguments between Demon and Holley will continue but I think ultimately none of them are "perfect" out of the box and they require fine tuning. As long as the gaskets are good and the bowls are tight I can't see how a Holley would drift any more than a Demon but then when you add in sun spots and phases of the moon, anything can happen.

[JAM1775]

Could you tune the secondaries in a similar way by staying in 2nd gear and running up the revs ? I have considered installing the sensors also but I would probably not permanently mount the gauge in the car. I would hook it up, test and tune then just leave the sensors in the pipes. Or do you think that is too much trouble ?

[Coloradocobra]

My installation is temporary as I really don't have room for the gauge and it does not match my SW gauges. Assuming that you don't expose the O2 sensor to an excessively rich condition or to leaded fuel there is no problem leaving it in. The sensors are not cheap, however, so I intend to move mine to my Mustang after I get the Cobra sorted out. They supply a threaded plug with the bung kit that seals the bung when the sensor is removed. The O2 sensor that I bought from Autometer (Summit) is a three wire design - it has a heater. The gauge will work with other sensors and I beleive it also supports a one wire sensor. Apparently they all have the same voltage calibration.

It is my understanding that you can also tune secondaries - mechanical or vacuum. Better try it with a buddy in the car as it is hard to watch the gauge and the road while accelerating. The article I read suggested fully tuning the primary, idle, and power valve first. Power valve is tuned by selecting the proper vacuum setting and then modifying the fuel passage diameters. This involves restricting or opening and there is a kit someone sells that actually uses small jets to adjust this important setting. For example, I hear that the passages on doublepumpers are often small and the carbs are set rich. The combination works. If you lean out the primaries, you will end up lean under acceleration. In Colorado, the vacuum secondary carbs are set up with too big of a passage and it has to be restricted to lean it out.

Once these are set, then it is on to the secondaries. The same techniques apply and you now only adjust the secondary jets to acheive the best mixture. I understand that a trick some people use is to set the primaries a little towards the lean side and then run the secondaries a little fat. The thinking is that the richer condition will protect under full load and that the primaries are usually used under light loads. The danger here is that you can go lean before secondary tip in - the point when fuel actually begins to flow that could be harmful. With the air/fuel ratio gauge you can detect and correct the situation before any harm is done.

Gary

[oleyellar]

Check out this procedure. It should answer all your questions about air/fuel ratio and how to get there from wherever you are.
http://www.bob2000.com/carb.htm

[Coloradocobra]

Thanks,

This is the web site I was looking for. He has changed his address, evidently. Good stuff in there!

Gary