FUMES 73 Plymouth Valiant

Then there is tuneup / engine condition/ carburetor.

If you don't have one, wander over to MyMopar.com and download the 73 factory service manual. That one, and several others, GOT THERE from the efforts of guys on HERE

Has anyone evaluated the engine, AKA leak down test, compression? Where is the timing set, and has anyone verified that the vacuum advance and centrifugal advance is working properly?

I don't remember when Ma got away from adjustable valves on slant sixes, you need to investigate if the valves are adjustable, and SET them if they are so

Has the carburetor been "gone through" and cleaned, etc? ---and this gets back to watching the spark plugs color. If they want to run blackish, the carb is too rich

Also does the engine run up to proper temperature? An engine with no stat/ stat failed open may run COLD and not run properly