Yes. They will jump fire under load. You could remove the cap and take a picture and post it up. Somebody could see the the fire jumping from post to post. I had the same problem with my Nissan pickup. Drove me crazy misfiring under acceleration and all I needed was a new cap. $7.

It looks like a spider web. FYI

 I've seen carbon trails inside the cap from one contact to the contact next to it that caused a misfire. The carbon trail (track) looks like a line drawn with a pencil. The cause was a poor spark plug wire connector. The path from one contact to another inside the cap was an easier path to complete the circuit than jumping the bad connector in the spark plug wire. @jalexquijano Have you ever compared the resistance in all the spark plug wires with an OHM meter? They should read about the same as each other. No need to pull them from the engine. Just disconnect both ends and stick a probe in each end.

 

This cap is brand new. Ill take a picture inside and measure resistance from both ends of each of the 6 wires. 

1 hour ago, Mark Maras said:

 I've seen carbon trails inside the cap from one contact to the contact next to it that caused a misfire. The carbon trail (track) looks like a line drawn with a pencil. The cause was a poor spark plug wire connector. The path from one contact to another inside the cap was an easier path to complete the circuit than jumping the bad connector in the spark plug wire. @jalexquijano Have you ever compared the resistance in all the spark plug wires with an OHM meter? They should read about the same as each other. No need to pull them from the engine. Just disconnect both ends and stick a probe in each end.

 

Inside cap spark plug 4 terminal circled red

  Looking carefully at the pics, I think I may see the carbon trail. The horiz. flat #4 boss that holds the contact has a very faint vertical line on it, then looking at the next contact down in the pic I see what appears to be another or the (kinda wavy) same line under the top of the lid that runs down from left to right. The carbon trail that I originally discovered on my 521 was very faint too. Do you have a volt-ohm meter, I don't remember.

Yes i have a multimeter. What do you want me to measure? Slide s spark plug in one end and measure resistance? I alrrady swapped cables 3 and 4 and still no change in idle on cyl 4. Could the distributor be bad?

I don't see how the distributor can be bad. The coil wire leads to the center of the rotor, which spins and distributes the energy to the contacts in the cap, then to the wire, then to the plugs. If the problem is electrical it has to be somewhere between the #4 internal cap contact and the connection at the plug. (you did check to see if the screw on connector on the plug tip was tight, right?) I think the best way to test the resistance is to remove the cap with the wires intact. Check the resistance in all six as an assembly by touching one probe to a contact inside the cap and the other probe on the sparkplug contact inside the spark-plug boot. Check the resistance in all six. Actual numbers aren't as critical as major differences, especially in #4.

20 hours ago, Mark Maras said:

I don't see how the distributor can be bad. The coil wire leads to the center of the rotor, which spins and distributes the energy to the contacts in the cap, then to the wire, then to the plugs. If the problem is electrical it has to be somewhere between the #4 internal cap contact and the connection at the plug. (you did check to see if the screw on connector on the plug tip was tight, right?) I think the best way to test the resistance is to remove the cap with the wires intact. Check the resistance in all six as an assembly by touching one probe to a contact inside the cap and the other probe on the sparkplug contact inside the spark-plug boot. Check the resistance in all six. Actual numbers aren't as critical as major differences, especially in #4.

Resistance measurent plugs 4 and 3 from inside cap terminal to spark plug boot

I removed spark plug number 4 and it came out oily and with the gap closed. Weird.

 Have any of the #4 plugs you've pulled in the past had no gap? Is it possible that you dropped the spark plug or bumped it against something that closed the gap previous to installation? If it were mine I would re-gap that plug, install it, disconnect the coil wire and turn the engine over with the starter for a couple of revolutions. Then pull the plug and see if the gap is closed again. If the gap is OK, replace that plug with a new one and take it out for a test run. If the gap closes again take a pic and report back. The resistance readings on the two wires look OK to this non-electrician.

Ok. Im doing a compression test right now. This car has not the stock camshaft. It has a schneider 274f cam and oversized pistons. Should i consider sourcing a stock camshaft?