Disconnect one plug wire at a time to see if it goes away. This can isolate the cylinder if it is something mechanical with the assembly. Careful with this though since it worked on my piston slap since I only had it in one cylinder.
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'68 C20 Longhorn 50th Anniversary 400/TH400
'68 C20 Longhorn 50th Anniversary 468/TH400w/buckets
'72 C20 Halfhorn (Longhorn w/o cab and front clip)
'69 Flxible Cruiser (look up ugly in the dictionary)
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