Well, it's an extreme case, but it's a very large motor. A 535 has a lot of swept area, so it's possible that a strong downward stroke with the exhaust valve open could create a decent amount of exhaust vacuum.
During cranking, if cylinder 1 fires while cylinder 5 exhaust is open, you could get a strong toke on the #5 primary tube.
If a set of headers made me tear a motor down twice, that would be one former set of headers!
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I could explain all this to the girl at the parts store, but she'd probably call the asylum.
White '67 LeMans 407/TH350/Ford 3.89... RIP
Red '67 LeMans. 407/TH400/Ford 3.25
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