Occassionally you get lucky when rebuilding an engine, and find that the valves and seats are in good shape and don't need to be completely reground. Even so, there will be carbon buildup and minor imperfections in the mating surfaces between the two from use, that should be corrected. Lapping the valves is a simple process that will accomplish this. "Lapping" is the use of an abrasive compound to mate the margin to the seat perfectly, grinding away carbon and minor imperfections. A valve in block engine such as in the Cub does not readily lend itself to being taken to a nearby shop as does a head, so a do it yourself approach might be best.
Two different tools are most commonly used to lap valves. The first is a simple dowel rod that has different sized suction cups on each end to adhere to the face of a smooth valve. ($4 to $7) Valves with cast letters, pitting, or indentations such as holes or slots are difficult to keep attached to this type of tool as an air seal cannot be maintained. Cub valves are especially small and the same sort of tool as used on small air cooled engines would have the correct size of suction cup, but unfortunately, Cub valves have indentations, making them ineffective.
The second type of tool is a mechanical type which comes with both suction cups and small plates, designed to engage the small holes in the top of valves as found on the Cub. ($13 to $19) These used to be found in nearly every NAPA, farm supply store or other automotive parts place, but have become more difficult to find. J C Whitney still carries them for $13.95 and shipping)
Having valves to lap, and not wanting to wait for a mail order, I decided to make my own tool, a hybrid cross of the mechanical and the suction cup dowel type. I accomplished this by brazing two small pins (nails) on either side of a 3/8" steel rod about 10" long. I ground the tips of the pins to fit the indentations on the valves and had the perfect tool for my task. Using this tool along with some Felpro grinding compound, I was able to lap all of my current projects valves easily.
To correctly lap a valve, you must first thoroughly clean the carbon from the valve and guide so the valve will turn freely. Even after cleaned up, the valve margin and seat will most likely appear dark.
Apply a small amount of the grinding compound to the valve margin, insert it back in the guide, and apply the tool of choice.
Grinding is accomplished by turning the valve back and forth in the seat, NOT by turning it in one direction, as with a drill. Turn the valve 1/4 to 1/2 turn between "sessions" to uniformly grind the surfaces. If you are using a dowel type tool, you must do this by hand. Start at the top of the dowel, and spin it back and forth between your palms as they descend on the rod. You should feel a gritty resistance at first, that becomes smoother as the compound is forced out from between the surfaces. Lift the valve from the seat between grindings, allowing compound to pull back under the surface, and repeat. You may have to do this several times, depending on the amount of imperfection in the seat.
Remove the valve periodically and wipe it and the seat off. When you have a clean, gray, uniform surface on the margin and valve seat, you're done.
Make sure and thoroughly clean the grinding compound from the engine and reassemble with a quality prelube. Its that easy.