So I have reread through the entire 14 pages of this thread and am impressed at the effort made by Bartamos in particular and others in trying to solve Jimmys issue. One thing that I missed on the first read was the latest compression test results. Cylinder #6 had a reading of 70 and then 0 during a subsequent test. I don't recall any mention about oil pressure PSI or the removal of the valve covers to see if the top end (much less the rest of the engine)is being properly lubricated. A reading of 70 PSI and then 0 would indicate to me that a component is sticking for a low reading of 70, or a reading of 0 indicating (to me at least) either a stuck lifter in the block bore or inadequate lubrication of the rocker shaft (which would affect the travel of all the rocker arms), or both, resulting in at least some if not all,valves not closing or opening effectively causing low compression readings. I would also suspect possible cam lobe damage as well. To state the obvious, head gaskets and timing chains are not going to momentarily repair themselves and then fail. Also, metal components, as we all know, expand when they heat up and contract when they cool.
I am more convinced than ever, based on what I have read, that the issue is lubricant related. If it were my engine, I would first remove the valve covers and assess how much fresh oil is visible on the rocker arm shaft, rocker arms and head in general ( it will also provide a good general assessment of the lubrication of the engine parts not visible). If lubrication is fresh and obviously sufficient, I would then assess the condition of the pushrods (at piston TDC you should be able to spin them with your fingers); if they look to be unbent I would proceed to turn the engine over BY HAND (wrench on crankshaft) and assess the rocker arm and valve travel. If all rockers and valves seem to be moving freely, I would then adjust all the valves, one cylinder at a time, and then do another compression test. Compression test readings are always going to be low if the engine has not been warmed up; as in this situation that is not possible, look for consistently close readings between cylinders (10 to 20 PSI would be optimal). Using a oil can to spray oil into the cylinder top and wall gap with oil before testing will temporarily help seal the ring / bore gap and raise the PSI reading. Two adjacent cylinders having a low reading in contrast to normal readings in the other cylinders often indicates a 1 blown head gasket, 2 a cracked cylinder wall between the cylinders, and 3 a cracked head; all three are most often a result of overheating the engine.
Having done all that and everything looks good, I would imagine it would come back around to timing and ignition settings. Below I have provided some valve and ignition info that hopefully will help get it going, from the Service trade Edition of Motor Truck and Diesel Repair Manual 28th Edition:
1966 GM 305-E V-6 engine: Spark Plug 45ns Plug GAP .035 Distributor Point Gap .019 Timing Mark 5 degrees Firing Order 1-6-5-4-3-2 Valve Lash Intake .012 HOT Exhaust .018 HOT ( Cold, I would look for intake .015 to .017 and exhaust .021 to .024 and plan on readjusting them once the engine is running and hot ). Valve spring pressure (LBS @ inch length) 200 @1 1/2 Compression Pressure at cranking Speed 135 PSI
Valve Adjustment Procedure (verbatim from manual): With engine at operating temperature and rocker arm covers removed, turn adjusting nut on each rocker arm until proper clearance is obtained between valve stem and rocker arm. Best of luck
