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    Question 1981 2.3 Fairmont 5200 lean on Primary

    I have been struggling a bit with CA smog, but recently got it to pass just barely. The problem was high NOX and O2, but I think there is a lingering problem with it running lean.

    With the EGR vacuum capped, I get low voltage on the O2 sensor ( 0.0 - 0.2 v ) from about just off-idle to around 50% throttle. It drops closer to zero the higher in throttle I get. When I get really close to secondary, the power drops a lot, like I took my foot off the gas. When I hit secondary, it shoots up to 0.5 and settles at 0.45 with a increase of power. Additionally, any time I hit the accelerator to some spot within the primary, the voltage goes up to 0.45 and then, as I hold it in the new position, the voltage drops to 0.2 or less and the power backs off.

    During this time when I am within the primary zone and the voltage drops down, the vacuum regulator (feedback system) starts increasing vacuum to the booster valve. It eventually reaches a max of 5 inches and brings the voltage of the O2 up slightly, but it doesn't seem to compensate for much. I have triple checked the flow and cleanliness of the booster and think it should be working fine. 5 inches of vacuum applied moved the diaphragm plunger as expected when I had it apart.

    Yes I had a vacuum tube and long wire leads running out of my hood onto my dash while driving around to get an idea of what was happening.

    I think what is happening is it is feeding off the accelerator pump and then leaning out for some reason, but it only drops in mixture while in primary. As soon as it transitions into secondary, the power jumps up and volts hold 0.45.

    With the EGR plugged in, the voltage reads higher overall. This is likely due to the dilution, and I don't really know how to interpret it with said dilution, but the behavior feels the same. Power drops after the accelerator pump (actually the power overall is a bit worse in general).

    I did a full kit rebuild on the carburetor, then proceeded to take it back apart again another 3 times just to really go through and clean everything. I checked through and noted all the numbers stamped on the bits, hoping I could reference them online somewhere to see if they were all in the correct place, but I could not find any references. I tried switching the primary with the secondary jets just wondering if someone put them in backwards, but it didn't seem to make a difference.

    This is what I found
    Secondary: jet 191, well tube 93, hs bleed 150
    Primary: jet 175, well tube 94, hs bleed 185

    I'm wondering if my primary isn't big enough. My father said when he was young, he put a larger primary in his pinto and it helped him a lot, but that reasoning isn't logical enough for me to try it. Wondering if there is anything else that could cause lean on primary, or if this arrangement is wrong?
    Last edited by FusRohPotato; 04-02-2024 at 08:55 PM.

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