[dv657172]

65 GTO drag car
Holley 4150 DP 750 (4779-6)
1968 400 (461) with 16 heads

Ended last season with an idle AFR of 14.0 and an avg run AFR of 13.0

First time out this season my idle AFR was 21 and avg run AFR was 14.0
Increased jet size by 3 to get to 13.4 on a run, idle AFR unchanged of course.

Heard a small pop (lean backfire?) when I lifted at the finish line both times and my data logger shows my AFR shot up to 21-22 when I lifted after the finish line
Density Altitude was 300' higher to start this year vs. last years runs
Fuel levels look correct in both float bowls

The only thing changed vs. last year was adding jet extensions on the secondary side along with the notched float and going to a looser stall converter.

The car idles ok at 925 rpm, 6.5 lbs steady fuel pressure, 13 degrees advance, 13.5 inches steady of vacuum in park. Idle mixture screws are all 1/2 turn out.

Plugs look lean to me (see pic) although I am no expert but along with the backfire I feel pretty confident the AFR numbers I am getting are correct.

Any ideas what to check next or what the issue may be?

Turned idle mixture screws out a quarter turn and idle AFR only came down to about 18. Started going another quarter and vacuum started dropping and car ran rough.

I did a simple rebuild on the carb in Oct 2022, about 50 1/8th miles passes since then. Checked fuel filters pre & post carb before the season started and they looked fine.