I've got an '83 3/4t Silverado. On a 300 mile round trip last weekend, it started to miss and backfire through the exhaust. I replaced the plugs on the road and this cured it. So I get to my destination and pick up my truck camper (reason for trip).
Cruising toward home and the ignition cuts out again; longer this time as when it caught, it blew both mufflers apart! Hit and miss until I stopped and replaced the cap and rotor (I think the rotor was grounding out as it was REAL ugly).
It ran ok for the last 100 miles home; hard to hear though due to said exhaust eruption.
Fast forward to today: Exhaust is fixed so I can hear. It sounds like a lean backfire coming through the carb. It is definately popping through the carb when you give it throttle. Starts and idles fine.
I've replaced cap/rotor/plugs/coil/wires/module/fuel pump; pick-up coil tests fine. Ran a compression check: great in 7 cylinders but #7 is at 90. (others are 160-165) That said, all plugs look the same. Pulled plug wires one by one while running, trying to isolate the culprit cylinder; no difference in killing any one cylinder.
Has a full 12V to the HEI. Also has a fairly new Quadrajet.
Tried to fire up my buddies "circa 1985" Allen scope yesterday, but 20 years of dust was causing us to truobleshoot the troubleshooter
Any advice before I give up my shadetree antics and drop her at the shop???
Thanks in advance - Nick