I find lots of old bikes and it is quite often necessary to free up the freewheel. Though I do not use WD40 as a lubricant, it is a good cleaner and freewheel freeeupper.

Remove the rear wheel and rotate the freewheel. Do you feel drag as you spin it? If so, lay the wheel on its side with the freewheel facing up and add a bit of WD40 to the joint, allow the liquid to seep in while spinning the freewheel. You might need to do this a couple of times but do not go crazy with the WD40. When the liquid does its job you will immediately feel the freewheel free up.

Next, add a really light oil to the same joint on the freewheel, again spinning the freewheel slowly to allow the light oil to seep in. I use very light transmission oil for this task. Hope this helps.