Here is what I would suggest:
The question I hear often is how do I set the timing on my LaSalle or Cadillac for the altitude in Colorado?
One answer is to advance the timing of the distributor so that it just begins to ping under hard acceleration, then back it off until the ping stops. Works, to a degree, but takes a few tries.
My answer for adjusting timing for our high altitude in Colorado, for the gas we use here, and specifically for your engine (whether it is newly rebuilt or has lots of miles on it), is use a vacuum gauge. You canít go wrong with this method.
How does one go about adjusting the timing using a vacuum gauge? First you need a few tools 1) a good vacuum gauge 2) a good wrench to loosen the distributor nut and 3) something to plug the vacuum advance line which comes from the carburetor to the vacuum advance on the distributor.
Note: It is fairly easy to loosen the nut that holds the distributor in place on flat head engines using regular tools. But when Cadillac went to the overhead engine the distributor was still at the rear of the engines (1949 to 1962). This configuration makes it difficult to get a wrench on the distributor hold down nut. An aftermarket tool is available that makes loosening this ĹĒ nut much easier. Iíve attached a picture of two of these tools. The vacuum advance was first introduced on LaSalle and Cadillac in 1940, so prior to 1940 you donít need to worry about it. If the advance line is rubber plugging it is easy. A golf tee or a loose screw should plug the line. If the vacuum line is metal then use tape or a rubber cap to plug the line.
Now the first thing to do is find a good source of vacuum on the intake manifold where the vacuum gauge can be connected. Make sure that if you pull off a line to accomplish this task, you plug it, just to ensure that there are no vacuum leaks. Most flat head engines have a good source of vacuum in the intake manifold that connects to the double action fuel pump. To adjust the timing correctly you must disconnect the vacuum line to the double action fuel pump anyway, or you will get a false reading. Just remember to plug the line to the fuel pump.
The second task is to loosen the nut on the distributor before the engine gets hot. Trust me it is easier loosening this nut on a cold engine. Just loosen it enough so that the distributor will turn, but still with resistance. You donít want it to move easily or it will cause you problems.
The third task is to start the engine and let it run until it reaches operating temperature.
The fourth task is to adjust the idle so it runs smoothly and you get a constant vacuum reading. Then disconnect the vacuum line to the vacuum advance on the distributor. Remember to plug the vacuum line.
Hold on, the exciting part is coming.
The fifth task is to advance the timing by turning the distributor. If the idle increases (i.e. the engine speeds up) you are going in the right direction. Now watch the vacuum gauge and adjust the distributor to obtain the highest vacuum reading on the gauge. A Cadillac or LaSalle engine in good shape will draw 18 to 22 inches of vacuum at sea level. Vacuum decreases by approximately one inch per 1,000 feet of altitude (see attached chart). So at 6,500 feet, where I live you would expect the vacuum to be in range between 13 to 17 inches. The other variation is in atmospheric conditions; any change will cause a change in the readings on the chart, just as a barometer will rise and fall with weather conditions. This is why we are looking for the highest reading of the gauge, not a listed set limit. So what we want to do is find the distributor setting that will draw the highest vacuum, within limits. (Note: if your engine idles below the vacuum limits listed on the chart it could indicate late ignition timing, if it is higher it could indicate early ignition timing). Now once you have determined the highest setting of vacuum, drop it back into the range you think it should run. Living in Elizabeth, I would try to get it close to 17 inches of vacuum. Setting the vacuum correctly should be enough to prevent pinging, but if your car pings on the first try, not to worry (oh at this point a test drive will be in order, so just disconnect the gauge and plug the line). The gas we use makes this setting a variable. So if you do get pinging at hard acceleration, drop the vacuum back about a half inch. If that fails to get rid of the pinging try one half inch increments till you stop the pinging (make sure the gas is fairly new or your car will run worse once you put new gas in the car). Once you get it correctly timed, verify the timing using a timing light. Write it down for future reference, with the octane of the gas, and the altitude. You now have a reference point for your car to verify that the timing has not changed.
The sixth task is to tighten the distributor down without moving it. You might want to turn the engine off to do this task. After it is tightened down start the engine and check the vacuum reading to make sure it did not change.
The seventh task is optional but strongly recommended. Adjust the idle adjusting screws (there are two of these for both two and 4 barrel carburetors). What you want to do is adjust one screw at a time. The goal is to improve the vacuum reading, but maintain a steady reading. If the vacuum gauge needle begins to bounce you went too far. To adjust turn the screw clockwise till the vacuum starts to drop, then turn the screw counterclockwise until you get maximum vacuum or as an alternative if you have a RPM gauge adjust until you reach the maximum RPM reading or highest vacuum, but no further. You may have to play with this to get it correct. Do both screws, then do it a second time to see if you might improve the vacuum reading just a little more.
If the vacuum gauge needle will not stay steady and you have verified your work then you may have other engine problems. The older Motors Auto Repair Manuals have an excellent section on determining problems via the vacuum gauge, as do others.
Finally, check the idle. Verify that it is set to specifications or as close as you can get it. Connect the vacuum line back into the vacuum advance. Disconnect the vacuum gauge and connect up any hoses or lines that go to the vacuum fitting you used.