Mr. Giant writes, "still idles rough."
I like this advice carbking up there gives, put it back together the way its supposed to be.
Readers really need to hear the idle to make decent guesses.
There are two types of rough idling. One is repetitive, the other is random. Both point to certain types of problems.
A repetitive rough idle keeps time with your rpm. This occurs at very regular intervals, precisely the same amount time between rough spots like a clock ticking.
A repetitive rough idle points to a problem with a specific cylinder. Low compression, bad plug, faulty wire, arcing in the distributor cap or, most likely, a leaky valve. Your big bolt GMC odometer has probably rolled over once or twice. High milage on a GMC usually means leaky valves. Reason for this is modern gasoline, since around the Seventies, is blended to destroy your engine. I think this is a conspiracy between Chevron and NAPA.
Have your engine idle, pull off one spark plug at a time beginning with number one cylinder. If the rough idle worsens dramatically, not the bad cylinder, slip your spark plug wire back on. Should you come across a cylinder when pulling the plug wire only slightly worsens the idle, this is your bad cylinder. Might have more than one bad cylinder. This is a cylinder which is most likely suffering a leaky valve. Might be a plug, a rocker arm adjusted way too tight, a bad compression ring, but this problem is specific to this cylinder.
A random rough idle cannot be timed, simply comes and goes with no discernible pattern. This is like a mouse running loose inside your intake manifold. Mouse stops over here, runs over there, stops somewhere else, completely random.
This points to an electrical problem, a carburetor problem, a leaky intake manifold gasket, plug going bad, dirty points or a hot failing spark coil. More often this is a points problem or a spark plug problem. However, an overworked coil will do this, a coil missing a step down resistor or simply plumb worn out. Might be a bad spark plug connector at the distributor cap or at the spark plug. Carbon fiber spark plug wires, if bent, if damaged, will mimic a random problem.
If a random rough idle, pulling off spark plug wires, one at a time, will make no difference, not a specific bad cylinder.
Overall, repetitive problem or random problem, this comes down to three major components: fuel, compression and ignition.
A fuel problem usually kills your engine or robs your engine of power. Typically this is "smooth" problem, not a rough idle.
A compression problem, such as a leaky valve, will always be repetitive, you can set your watch to this. This is a very reliable repeating problem.
An ignition problem will drive you nuts. Only cure is a step-by-step correction. Start by inspecting, cleaning and setting your points. Check the rotor for looseness and arc damage which erodes away the copper tip. Look at the cap for obvious problems like too short of a center electrode, broken or chipped electrodes, loose fitting. Check the overall condition of your distributor, no looseness anywhere. Check for a possible vacuum leak which is random, maybe a hose flopping around.
Replace your condenser, always. Those do not simply quit working, those gradually go bad often causing a random "miss" in your engine, like your engine stumbles for a moment.
Be sure to carefully set your timing. Usually this acts like a fuel problem; robs power or causes backfiring, not random.
Spark plug wires and plugs, no way to test, simply replace those. Same with the coil.
Two types of coils. One requires an external resistor, the other has an internal resistor. One or the other, has to be right. When an external resistor is missing, a coil will heat up and slowly begin to fail. Works great while cool or just warm. When hot, problems show up and as heat increases, problems become more severe. This is usually the lacquer coating on the wire windings inside frying causing minor short circuits; does not quit but the spark periodically becomes very weak.
One other often overlooked problem, a corroded wire somewhere, like your key to coil wire. Corrosion changes properties slightly with temperature. Shaking of a corroded connector will cause a random like problem; voltages drops, wavers, spikes.
A reliable check for a repetitive problem is running a compression test with a good quality compression tester. This is actually the best way. Pay attention to how quick or how slow compression bleeds off for each cylinder.
A reliable check for a random problem is simply flip a coin and go for it, could be just about anything.
Oh yes, brand new parts is not a guarantee of having good parts. Never assume a brand new part will work correctly, most often a new part is broken right out of the box; Murphy's Law.
Probably Chevron and NAPA are behind this broken new parts thing.
Billyray
