Valve lapping 101
First I should start by telling you that unlike the old days, lapping valves in is not as important as it once was. The technique of grinding and surfacing valves and seats has been refined over the years to the point where many engine builders do not even do a traditional lapping job on valves. I personally do like to slightly lap my valves in, just to make sure everything is true. The key word here is SLIGHTLY. If you lap the valves too much, you will end up with a concave surface on the valve which is not the optimum for seating or cooling of the valve. Since most of the cooling of the valve takes place between the valves edge and the seat, you want the biggest surface area you can there.
With all that said, here is how I lap my valves and check for leaks.
You should only lap your valves after a valve job has been performed or when replacing valves in the head. If you are replacing valves without getting a three angle valve job done you should make sure the valve angle matches that of the head. Once you have your valves and everything is ready to assemble its time to lap. There are ways to accomplish this. The first is by turning the head upside down and using a lapping tool on the valves face. Do this by placing a SMALL amount of fine valve grinding compound on the valves angled surface and placing the valve in the head. Put your valve grinding tool on the valve and with very light pressure rotate the valve back and forth working the compound between the valve and seat. The second way is just like the first however it is performed with the head facing up. Once the valve is in place you pull the valve through and slide a piece of rubber hose down over the valve stem. With a slight upward pull, roll the rubber hose between your hands. Either way, this should only be done for about 10 seconds per valve. DO NOT OVER LAP YOUR VALVES; you will ruin the valves surface. Remove the valves and clean all grinding compound off before final installation.
To check for leaks simply install spark plugs, flip the head upside down and fill the chambers with parts cleaner. Let it sit for a few hours and check for any fluid that may have wept into the intake or exhaust ports. If there is fluid you can trace it back to which valve, and possibly where it?s leaking. Pull the valve and check for any problems.