318 Engine stumble

Choppiness can be due to many reasons. It could be the cam, or the cam and log-manifold combo. Or it could be AFR and or timing. Or it could be a mechanical deal. Or you could have the firing order wrong or a crossfire, or cracked plug insulator. Following is a test for you but it assumes that
the point-gap is stable, with the proper dwell occurring and not wandering. If the dwell is jumping around due to a worn-out D, or worn out point return spring, or a defective condenser, then the timing is similarly jumping around. So the first thing you gotta do is prove your timing is stable below 2400rpm. Disconnect the Vcan, and check it out. If stable, reconnect the Vcan and check it out. If that also passes, then start at Part I
Looking at your curve, I could see the choppiness being too much advance if the Vcan is hooked to manifold vacuum, or if it is bringing too much too early.

Here is a test for you; it assumes the ignition system is 100% working
Part I
Do not disconnect the Vcan.
Put the idle speed to 2400 and check the timing. This is the smooth out rpm right. Write it down.
Next put the rpm down to 2200, you said now it is choppy right. Put the timing light on it, and start taking out timing, by cranking the D, until it smooths out. If it never smooths out, add some back in until it does, and write it down. If it does not smooth out no matter what you do to the timing, then you have a different issue, so put the timing back, and go down to part II.
But to continue; assuming that you found a timing that was smooth, write it down. Continue this way in 200 rpm drops until the Vcan drops out, maybe around 1600rpm.
Plot it out on a graph. This is your maximum timing at those rpms, under no-load conditions. Now you have to modify your system to meet those needs.
Part II
if the problem was not in the timing, then perhaps it is in the fueling.
So disconnect the Vcan, and crank up the idle speed to perhaps 1600rpm. Then slowly take away air by covering the airhorn with a shoprag. If it smooths out, you have found your problem. But if it slows down a bunch;stop the test. Next, allow the rpm to stabilize, then pinch the fuel supply hose shut... and wait. Soon one of three things is gonna happen;
1) the engine will run choppy for a bit,possibly smooth out, then slow down and stall. If it smooths out, then it's rich. If it doesn't smooth out, then its probably normal, and the problem lies elsewhere.
2) the idle speed will rise, the engine will smooth out. Then the Rs will begin to drop,and as the engine struggles for fuel, things will be as at #1 . This is too rich
3) the engine will immediately lose Rs run worse and then die. This is probably lean.
So now, if #2 or #3, then you will have to prove the low speed circuits are clean, and if the are; then you will have to change the float level and/ or check the fuel pressure.
But if you got a #1, then we need to do some more digging, return the engine to idle. Go to Part III
Part III
Back up the bus. I think you missed something in the preliminary assessment. Go back looking for crossfires or cracked plug insulators, and a wrong firing order...
Run the engine up to 1600rpm until it gets choppy. Then one by one, remove the plug wires, until you find one that makes no difference if it is on or off, or it runs worse with it on. Write it down, and go look for another one. If you have two of them check the firing order, or if they follow eachother in the firing order, then separate the wires a couple of inches and if the problem persists then check the cap.But if the issue goes away with the wires separated, buy some decent wires. But if only one, then either replace the plug or sub in one from another cylinder. If the issue follows the plug then replace it. If the issue stays where you found it, replace the wire.
But if everything checks out OK here; then Part IV
Part IV
Time for a compression test
If the numbers come in even, then off with the valve covers. You will be looking for bent pushrods, and/or valve springs with no tension on them. The valves to be checked have to be on the base circle. That is to say with the piston for that cylinder close to TDC compression. If you can spin any by hand, you gotta fix that first. Sometimes I have done this with the engine idling. If you see any springs rotating or spinning, you got a problem.
But if the numbers are not even, within about 10 to 15 psi, you got a mechanical problem. To see where it lies, next comes a LeakDown test, ti find if it is the rings or the valves.
Part V
loose ends
To prove the low speed is working, with the engine idling, stick a couple of fingers down the primary side and block the Idle Air Bleeds. If the engine runs worse, at least one of them is working so test them individually. Of course the Transfer port exposure has to be close, heehee. If you have too much idle timing the throttles will be too far closed and the mixture screws will be cranked way out. The only cure for this is to back off the idle timing and open up the throttles a tad with the speed screw, then restore the mixture screw settings to the center of their range. Now repeat the IAB test. If you find one that doesn't make a difference, clean it out.
Part VI
Check the cam and cam-timing, maybe you got a wiped lobe or two, or a 292/292/108 in there lol.
Part VI
That's all I got, except to say you can jump to any part at any time if you suspect a particular problem. I for instance, just go straight to the compression test. While I'm taking the plugs out, I can check for cracked insulators, and I can check the wire routing and firing order. So maybe I catch something obvious and stay the compression test. And maybe not.
Part VII
Odds and Ends
Cracked insulators are or can be, very hard to see with the naked eye. Many times the diagnoses is by subbing in a new or different plug.
I remember one time in a 4 cylinder, swapping plugs with no different results. Turns out the plug I took from one hole and moved to the suspect hole, was also cracked. Four new plugs cured the problem. Then I had to figure out why it had two cracked insulators. I could not see the cracks until I put the magnifier on them. My boss was kindof ticked. He couldn't see the cracks either. The customer was kindof ticked too, cuz, he said, he had recently changed them. He couldn't see the cracks either until looking thru the magnifier. AJ was in the doghouse. Even moreso after I presented the bill for a replacement head installed, and a new knocksensor.. Turns out, the head was leaking water into the chambers, and the knocksenser wasn't sensing.
I don't think you have a water problem.
But you saidso yourself it was detonating at idle in N/P with no load on it. The detonation may have killed a plug or two.