Need some direction on a 1952 chevy my son just purchased. Car seems to have been sitting a while so we just gave the car a full tune up including new wires plugs (R45s) a 6volt coil distributor cap and rotor new condenser and points. We did not replace the vacuum advance but distributor does move when we accelerate the gas pedal. Problem were having is when we set the timing it seems to hold but after accelerating it seems to lose timing .

Here are some notes on the car. 6 volt 216 engine that does seem to have a knock sounds like its coming from under the valve cover and we did replace the carb but the carb sat for like 2 years on the shelf it was running before we removed it two years ago from another vehicle. And also we set the points but it was are first time attempting to set points. With the old points still in  we rotated fly wheel until points were fully opened removed the points installed new points and set them too 18. So with all that any suggestions on why timing could not be holding. Thanks for any advice.