I just remembered, you said lower cylinder is the one with white smoke running cool. The lower cylinder is on right side so spraying carb cleaner on left side might not help if left seal is ok and issue is with center and / or right seal. But can't hurt to try. Pull plug on right side and have a look at it after running. Try a compression check which you can do without too much tearing down. Then consider leak down. Also check if water level in radiator has gone way down too.