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I've been saying all along that a dyno run without an air/fuel plot was useless. I screwed up. I was wrong and I apologize for having provided incorrect information.
I had a lengthy conversation with Marc Salvisberg of Factory Pro about the validity of exhaust gas analysis with gasohol fuel.
Out of that conversation came an "Ohhhh...DUH!" moment. While I was concerned with CO levels versus O2, Mark informed me that the gas curves were largely meaningless on a sweep test and the only thing that mattered was the maximum horsepower reading!
The rpm is read directly from the engine and is an instantaneous value, but the results from the exhaust gas sensors are several seconds old by the time they're plotted onto the graph at the current engine rpm.
It takes time (seconds) for the exhaust gas to reach the sensors and for the sensors to make their readings. That means that the A/F ratio plotted at 8,000 rpm might be the result of combustion at 3,000 or 4,000 rpm and useless for tuning!
Marc says the correct technique is to set the main jet for max power and then try and figure out the A/F curves later -- or ignore them altogether! That means multiple test runs with increasing jet sizes until the peak power drops off and then stepping back to the set that produced the best power. That's the only real option since the A/F curves are useless for high rpm testing.
For me that means more carb teardowns, more dyno runs and another handful of main jets. I had previously tested with #150 mains but the A/F curve showed extremely rich conditions at high rpm. After changing to #145 main jets and adjusting the needles the A/F curve looked pretty good -- but 4 horsepower down from the peak reading with the #150 jets. I chalked the difference up to air density variation since the A/F ratio looked pretty good.
I think now that I was seeing the A/F ratio from the incorrect needle adjustment when I thought I was looking at the main jet mixture. That means the #150 mains go back in for a verification of the previous run and then trying #155 and maybe #160 jets to see what happens on the power curves.
Agan, my apologies for providing incorrect information.
Jim
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