Classic/Antique Car Repair: bumpy engine, smog shop, buick riviera

Question
Dear Mr. Benjamin;
  I have a '78 Buick Riviera with a 403 engine.  It has a slight "bumpiness" particualrly noticeable at idle and start-up speeds.  I have new sparkplugs and cables. It has a rebuilt carburator.  Oils consumption is less than one might expect with an engine with 250,000 miles on it.
    The engine usually runs very smoothly and I'm real fussy about "my baby".  What would you suggest that I could do smooth its present unevenness?  Thanks    Davis Goss

Answer
I'm really shooting in the dark here without being able to hear it, but here is a list of things to check:

1.  Carburetor:  I have had almost zero success with "rebuilt" carburetors. Unless I've rebuilt it myself, I just don't trust them.  You can try adjusting the idle mixture yourself, or take it to a smog shop and ask them to do it while monitoring the fuel/air ratio. If they can set it to 13:1 and it still idles rough, that wasn't it.

2.  Exhaust valve leakage:  Have a mechanic check the compression of all 8 cylinders, and compare the results.  If any cylinder is more than 10% lower than the average, ask him to do a leakdown test on that cylinder to see where the leakage is.  Burnt exhaust valves will make the car idle rough (and you can hear the "chuffing" from the exhaust pipe), but will have very little effect on the running at road speeds.  If you check the compression yourself, be sure to block the throttle all the way open, and remove all 8 spark plugs before you begin.   Start with a warm engine, and write down the results.  Go all the way through all 8 cylinders, then do it again, and note any big differences as the battery gets tired.  Crank each cylinder until the pressure reading stops increasing - usually about 4 compresssion strokes, or 8 engine revolutions.

3.  If you know of a shop with an "ignition analyser", take it there and have them verify there isn't anything wrong with your points/condensor/coil/rotor/distributor cap/wires/spark plugs.  Your car probably has the HEI ignition system, in which case you don't have points etc, but the other stuff is still important.  I know you have already replaced some of that stuff, but not all of it, and there may still be a problem there.

250,000 is getting up there, but many of these engines go a lot further without problems.  I'd expect your timing hardware is getting very tired, though, so you may well need a new timing set for it.

Good Luck!

Dick