1968 GMC 250 Inline 6 4.1L Chugging

[RichardJ]

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Confirmed vacuum advance arm is working -- vac adv should be connected to Ported Vacuum port with no advance @ idle with 0 in.hg.
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Checked vacuum at carb -- Vacuum = ? in.hg. @ rpm, checked WHERE ? Port on manifold or does you carb have manifold vacuum port?
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Confirmed points/dwell set properly -- With vacuum hose plugged, does dwell stay @31 when engine is revved, With engine @idle, use hand vacuum pump to operate vacuum advance through its full range. Watch timing on balancer with timing light. Again check that dwell stays @ the 31
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Confirmed RPMs -- What is the Idle rpm with choke off and engine warm
New exhaust -- Did you check the Intake/Exhaust gasket against the head? Look for black soot on the block below the manifolds.

NEW Exhaust? Did you get the LMC exhaust?