Rough idle in D not P

[LT5PLA]

Hi all, 1970 250 with the carbies, seems to have developed a rough idle.. but only in D and not when in P.

Spark plug issue perhaps..nothing has really changed on the car.. so its odd.

things I should check? Replace all spark plugs perhaps, I did remove them about 2 weeks ago to inspect and clean them.

[Todd Miller]

Cleaning spark plugs destroys them by causing the spark to scatter.

[LT5PLA]

so you reckon just replace with new plugs?

[LT5PLA]

ordered fresh spark plugs, lets see if they do the trick

[Todd Miller]

Something to consider when working on any car, especially one with a driveability issue; is the importance of using OE/OEM parts, like the car was originally built with, AND returning the car (or maintaining the car) in "as-it-left-the-factory" condition. That means installing the same spark plugs that it originally came with, the same spark plug wires, the same coil wire, same distributor cap & rotor, etc.

Having a work ethic like this is much more efficient, and much less expensive than parts that are replicas and "are supposed to fit fine." The use of fancy spark plugs, and pretty, replica spark plug wires, and replica cap and rotor sets will always lead to 2 things:
1) The car develops some sort of driveability issue
2) The person who installed the parts believes it cannot be the parts, because they're brand new, and were just installed.

I've repaired many vehicles by simply taking off all the "new" replica parts that someone else has installed, and returning the vehicle to "as-built."

There are many causes of rough idle at loaded rpm, but in diagnostics, the path is always from the most simple cause, toward the most complex. Your statement about having cleaned the spark plugs becomes the initial starting point, even if it's not the full cause of the issue, because it's the most initially evident cause, and it falls into the category of: Put the car back to how it came from the factory, because it ran perfect at that time... The car did not come with used spark plugs that had been cleaned.

[LT5PLA]

thanks mate, appreciate the advice.. what were the original spark plugs for this model?