Emissions Test

That is no where near enough information. It’s a ‘73 Plymouth, and it’s putting too many unburned hydrocarbons into the exhaust pipe at idle, that’s all you’re giving us. What engine is in it? Stock, modified, what modifications? Could be darn near anything, running rich at idle, carrying a dead cylinder, timing is off, plug is fouled… I mean nearly anything. 20* advanced at idle is too much per the factory, but that only applies if it’s bone stock.

What emission standard is even being applied to a ‘73 Plymouth to begin with? You may have to familiarize us with the relevant standards there, even in California there’s no emission testing for anything pre-‘75.