Just saw this post.
Had a similar experience back in the mid 1970’s with a 170 having a rough idle. It turned out to be a stretched timing chain.
Here is a simple procedure to check for timing chain stretch:
Step 1
Remove all spark plugs.
Step 2
Pull the distributor cap off the engine by unbolting it off. Observe the current rotor position.
Step 3
Take a breaker bar with a socket that will fit on the crankshaft pulley. Put it on the crankshaft damper pulley, making sure it fits.
Step 4
Turn the crankshaft pulley clockwise, slowly. Watch the rotor in the distributor. When the rotor starts moving, stop turning.
Step 5
Mark the damper pulley position with chalk or a marker so you can remember the exact position. The mark should be placed on the crankshaft damper.
Step 6
Turn the crankshaft in the opposite direction carefully and slowly. Pay attention to the rotor in the distributor. Once it begins to move, stop turning immediately.
Step 7
Mark the second crankshaft position again.
Step 8
Measure the number of degrees of rotation of the crankshaft. Wrap a measuring tape around the crankshaft damper where the marks are located to measure the circumference of the damper. Then measure the distance between the two marks made.
Step 9
Divide the distance between the two marks by the total circumference of the damper. Multiply the result by 360, which is the total number of degrees in a circle. The result will the give you the amount of degrees between the two marks.
A chain that isn't loose will have about three to five degrees of reverse motion before the distributor begins to turn. A timing chain that is very loose and needs to be replaced will have 10 or more degrees of reverse motion.