Pull the plug wire out of the DISTRIBUTOR on the suspect cylinders to see if it idles differently. NO!!! you will not get shocked if you pull the wire from the distributor cap. Seriously trust me on this. Do it from the spark plug and you'll get it every time!!!
This is NOT good practice. Whether you disconnect at the distributor end, or the spark plug end, you are creating an OPEN circuit which drives the secondary voltage to MAXIMUM, leading to ignition coil, rotor, or distributor cap insulation failures.
Far better to GROUND the spark, which lowers the spark voltage to minimums, which makes life much easier on the insulation.
Use a typical "12-volt" test light. Ground the wire, put a dab of silicone grease on the pointy end, SLIDE the point between the plug boot and the plug wire--don't pierce the insulation. When the point gets close to the metal end of the plug wire, the spark grounds through the test light (it won't flash).
Or
Get eight thin nails. Grease the points, slide each nail between the boot and the wire as before. DON'T pierce the insulation. Use the grounded test light to touch each nail in turn, shorting the spark to the various spark plugs.
Either of these methods--with a vacuum gauge connected to manifold vacuum--works wonders when inspecting for weak or dead cylinders. When each cylinder is shorted in turn, the vacuum should drop an equal amount for each. If you ground a cylinder and the vacuum DOESN'T drop--or drops less than the other cylinders, you've found a dead or weak cylinder.
You must be registered for see images attach
Truck drives and accelerates fine. No miss or issues at highway speeds. If it is a burnt valve, is there any harm in running it and maybe putting some additives in the oil and gas?
If this is a burnt valve,--and I'm not convinced it is--no additives are going to help. It will get worse and worse, and it will be hard on the catalyst. A cranking compression test, or better still, a cylinder leakage test will identify a leaking valve.
Connect a scan tool, look at the data stream for anomalies with the sensors and outputs. Perhaps there are codes or pending codes.
Consider complete tune-up services--verify cap, rotor, test the ignition coil with a spark-tester calibrated for HEI. Inspect the plugs, air filter. Verify fuel pressure, consider a fresh fuel filter. Assure no vacuum leaks. Make sure the engine is running as it's supposed to, with no misfire.
