Classic/Antique Car Repair: rough idle, vacuum leaks, stock cam

Question
QUESTION: I own a 67 impala with a 396 engine, the short block is from a 1969 top of motor is original 67.It idles rough I have tried three different carburetors with no change in idle.I have an excellent ignition system and NO vacuum leaks currently running 18 inches of vacuum.I can hear a slight miss on the right side of the exhaust.The car runs very well off idle but at an idle in drive it shakes the whole car, very annoying  thanks  Jon

ANSWER: I have some questions for you. What do you know about the history of the block? Is it stock? Does it have the stock cam? has it been bored out? Have you taken a compression test on the engine? Once you get back to me we will go on to the next step.
Brad

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QUESTION: Brad I dont know the history on the motor but I believe the motor and cam are stock or very close it doesnt have any lope.I know the block is from a 69 chevelle and I can still read the numbers on the front pad so I dont think the block was ever  decked, but as far bored out not sure?  I have not yet done  a compression test because the car has good power even for a big car and all my plugs are burning fine. I bought this car last August it had a Demon carb with performer RPM  intake, all the original parts came with the car so i returned it to the stock look with factory intake and Qjet carb. The roughness in idle was there before with the demon and still here with the Qjet. I`m thinking maybe somethings just not balanced internally?

Answer
That could very well be and this will turn out to be a 007 Sherlock Holmes check everything mystery, but I would start with a compression test as I have found big engine will hide a low compression cylinder once you get it wound up and it will show up at idle.
Brad