Oh yes, the Vacum advance units can go bad.
I put Diane in storage (my 64 1/2 with 289) for several months, when I brought her out, she had a REAL BAD BOG, and kinda run rough at certian throttles.
Checked the timing, it was good, checked the lines, they were okay, took my Vacumm advance unit off, and dust and chunks of crap came falling out. I got a hand pump and tested it. Needless to say, it wouldnt do crap.
I have a Dual Point Mallory Distributor, that I just recently found out they had stopped making like 15 years ago. Had a real FUN time trying to find a replacement Vacum Advance unit. Hell, half the people I talked to didnt even know what Vacumm Advance was.
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64 1/2 "D" code Red Mustang Coupe. 289, C4, Mallory duel point. Ported & Polished 65 heads shaved .01 with 351 valves, 11:1 comp, 1.7:1 rockers, blue wolverine lumpy cam, modified 4100 Hipo 4 barrel. GT Apperance pkg. And to many others to list
2000 Perf Red Mustang GT. 5spd. BBK Underdrive pulleys, Flotech off-road H pipe. Hurst T-Handle, 40 series Flowmasters, Steeda Tri-Ax
64 1/2 red 6cyl coupe. Future resto.