I just re-installed my Crane HI-6 ignition. After doing that, my timing started to behave really wierd. Initial timing and mechanical advance work correctly. However, when I hook up my vacume advance, the timing increases a little. Then, as more throttle is added, the timing comes back close to zero and begins to jump around. Is it possible that my vacume advance is bad, or is it something else. The distributor is only about 3 months old. It's non-computer controlled HEI w/ vacume advance.

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82 Camaro:
350, Erson cam, ported heads, Crane Hi-6 ignition, Accel supercoil, Comp Cams 1.6:1 rocker arms, TH350 tranny with 2500 stall.
SLP 1 3/4" headers