(1970 fury3 w/ 318)
Dont have much experience with this but I'm wondering if an exhaust leak can cause a pulsating vibration that changes with rpms as well as hard starting and worsen fuel economy when cold.

All these symptoms came on all at once and they happened after sitting in a parking lot someone thought they would be funny got under my hood and unplugged the ignition coil, after cranking the motor for a bit(hard starting and long cranking has been an off an on problem for a while so i thought nothing of it at first and pumped quite a bit of fuel into the motor im sure) so i got under the hood found the problem and hooked it back up. after this i got the vibration problem i was speaking of immediately on my drive home, and a couple days later after having to crank up the idle 3 full turns to keep the motor running when it was first started cold i noticed there is a crack on a weld on my header right on a pipe about 2inchs from the motor in a Y shape a lil over an inch long that broke the entire length of the weld and with a 1/4 inch hole right in the middle. I have no idea if these events are related or have any bearing on the problem so I'm hoping i can get some input.

 

I hope you changed your oil after you got home. I hope your not hearing a rod knocking which could cause the motor to vibrate enough to crack something. Smell your oil and see if it smells like gas.

 

Before you get too excited, I'd just assume that your little prank caused you to fowl a plug.

Run a "backyard" cylinder balance test.

One way is to pull up the distributor boots a little, and if you have a long thin probe (line a 12V test lamp) you can slide the probe, grounded down along one plug wire at a time into the cap, and ground out one cylinder. As you go around, you'll hear the one that is not fireing

Another way, if it's easier, and you may want to use electricians fuse pullers (insulated), do the same as above, loosen all wires at the cap, then pull one out at a time, with one hand and slide your grounded probe in the tower with the other hand. This way, you don't have high voltage spark crossfiring all over, and that CAN DAMAGE electronic ignition systems.

Lacking the above, just pull all the plugs and inspect.

Your exhaust leak has nothing to do with vibration, unless the exhaust is touching the body somewhere.

 

I agree the exhaust should not be "the" problem, but should be remidied. I was wondering if your little gremlin didn't maybe change around a plug wire or two. This would cause it to run bad enough that you might want to try to raise the idle 3 turns. You are gonna need to start at the begining and do a diagnostic search to find your problem. Start by changing the oil, & filter, as mentioned, then check your firing order and do a compression test. Turn those screws back in where they were. This is where you want the motor to run.

 

I should have mentioned this, I guess I assumed he already checked. Good point

 

I had the same thought and changed out all the spark plugs before i posted. No misses and problem continued.

 

New plugs are fine but did you check the firing order of the plug wires... Look for a vacuum leak..