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The answer is simple, you have a bad cylinder. Your mechanic told you. One man's "rock or shake" is another man's vibration.
If you remember which hole was bad, pull the spark plug wire on it and see if it makes any difference in the engine's behavior. I bet it gets slightly worse, but doesn't change a lot.
Whatever you do, don't go making a lot of changes all at once. This is a simple problem with an expensive solution. You probably need a different engine or at least a valve job on your existing heads.
If it is a valve, you can sometimes tell by pulling the coil wire and turning the engine. Listen for a hiss every other revolution. If it comes through the carb it's an intake valve. But dollars to doughnuts it's an exhaust valve and the hiss will be heard at the exhaust.
If this is the case, I'd suggest looking for a mid-80s Fifth Avenue for heads or complete 318.
Also, unless your 273 came with a factory four-barrel carb there's no reason to put in 91 octane fuel.
Keep us posted. There's more collective brainpower on this board than in most garages.
R.
As far as I can tell it's a stock 273 with a 2-barrel (most likely original). I did check the casting on the block shortly after I got it and confirmed it was a 273. How much can a valve-job run up to? If it's more expensive it may not be worth it since 318's and 360's are available. I've got an 8 3/4 in my garage I could swap instead of the 7 1/4 just in case I want to go to more cubes.
It depends, usually around 250-300 in my area, plus new valves if you want them. If it were me, and this was just a 273 that I was going to cruise around, I'd take the heads off and inspect it. If it was a burned valve but was not really sunken and the seat looked okay I would probably buy a new valve and hand lap it to the seat and let it fly, this is not the best way, but will work fine.
Usually a burned valve will give you no compression in a cylinder, but you could be right at the tipping point. If its not a burned valve you've probably got a badly recessed seat, and may need to go larger with the valves or have new seats installed. Unfortunately this is a case of you don't know until you open it up.
Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street
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