Sounds like a fuel pump failure, diaphragm ripped or spring or pin on pivot let go. I’ve had the best luck with spectra premium pumps, Canadian made versus China. I gave up on nos GM pumps because they seem to have high failure rate, maybe just old diaphragms or not ethanol friendly.
The other thing that can mimic a pump failure is the cam eccentric coming lose and shearing its pin. More common on recently rebuilt engines rather than untouched engines.
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My Break Away Squad
1969 Fbird (base OHC, 350, 400HO & Sprint Cvt’s - 400HO & TA ht’s)
1969 Custom S 2dr ht
1969 LeMans (2dr & 4dr ht & Cvt)
1969 LeMans Safari wagon
1969 GTO (2 Cvt, 2 ht & Judge ht)
1969 Cat (3 Cvt’s & a 2dr ht)
1969 Ventura wagon 2 seat
1969 Exec 4dr post
1969 2 Bonnie Cvt's & 2 wagons
1969 Bonnie Brougham (4dr ht & Cvt)
1969 Grand Prix 2 SJ’s
1969 2+2 2dr ht (Canadian)
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