Chevrolet Repair: 1988 chevy truck idles rough, spark plug wires, vacuum leaks

Question
i have a 1988 chevy truck that i replaced head gaskets ,now the engine misses there is like 3 or 4 spark plugs not working. the old plugs were black i replaced them with new ones and now there all black .the firing order is correct and the spark plug wires are good.checked compression on all cylinders there 110-120psi
checked vacuum leak around the intake and throttle body with wd40 and it checked out.valve lash was adjusted by chiltons specification.

Answer
Hello Miguel,
Cheking for vacuum leaks is always good, but a vacuum leak would usually cause a lean condition, and yours sounds like a rich condition, or just lack of spark.
Some things I would do to check for the rich condition would be to check the fuel pressure, which is a pain on a 88 or so model, because they didn't put a test port on it.
But the throttle body fuel systems need about 13 PSI while running. If for some reason you have significantly more than that, you could be forcing too much fuel in.
Another thing right along the same lines, and you check it while checking the fuel pressure, is the pressure regulator.
If it is bad, the vacuum hose to it can suck raw gas into the intake manifold, so pull the vacuum hose from it, and see if there is any sign of gasoline in the hose, or vacuum side of the regulator.
You can connect a vacuum hand pump to it and see if you get gas.
Leaks in the housing seals of the throttle body are another source of dumping fuel into the intake. The throttle body is easier to work on than a carburetor, so check it out.

Your other possibility is lack of spark.
Timing may be a possibility, but sounds like you probably petty much checked that stuff. There is a wire that needs to be unhooked while checking the timing. EST bypass it is called. A brown wire, with white trace, I believe. The connector should be somewhere near the master cylinder on the firewall harness.
Did you have good spark?
Make sure the ground wire from the rear of the right side of the intake manifold, is connected to the firewall.

Back on the fuel, the throttle position sensor, if not giving an accurate reading, could contribute to a rich condition, as well as the coolant temperature sensor.
Are there any fault codes stored?

Van