The valves cannot be ground.  The valves are made very light and thin, and I believe they have a very thin hardening layer applied.  If you grind them the edges get too thin, and they exhaust valves will burn away.

The valve seal pulls off....don't get them crooked and pry, get them off as straight as possible or you can break the valve guide.

You can lap the seals with "Valve Grinding Compound" paste to confirm they are bedded into the seats - you should not put a valve in a valve grinding machine and take any metal off the valve with a grinding wheel or cutter.

That fellow is overdoing it.....he said he had been grinding for about 5 minutes!

You just want to scuff the valves in to confirm the valves are seating.  Use a black marker to die the surface of the valve (or Prussian blue if you have it).  Then use the grinding past just until the color is scrubbed off where the valve is touching the seat.  If you have any areas that don't clean up with 15-20 second of grinding - the valve most likely will need to be replaced.  When you begin to grind you will hear the rough sound, then after a few seconds the sound will smooth out - lif the valve up and it will pull the compound back into the surface between the valve and seat, then grind again until the gritty sound goes away.....repeat this just 4 or 5 times and then pull the valve out and wipe it off and look at the valve....it should have a dull silver strip all the way around.

(Mower valves are different - they are thick and heavy and can be ground easily, as there is plenty of material that can be removed).

Such technique and advice deserves a place in the tech section.

A thing confusing me here is the word "lapped".

To my previous knowledge and according to translators the "correct" word should be grinding/ground.

It seems that in the context of valves, grinding involves a machine (milling, drilling, lathe) of some kind. Where using a grinding compound to grind valves lightly is called "lapping" the valves and valve seat.

Reason  for writing the above is to better understand this,  and to help others that may find this grinding/lapping confusing.

Please let me know if the above is incorrect.

English is my second/third language,  so...  

Thank you for the explanation.

As mentioned by justin_o_guy2, the subject regarding grinding/lapping valves really should be condensed into a how-to with tips and tricks in the tech section.

I'm willing to try and make a post with a write-up, but guess I will need help from someone to actually place it in the right place and so on.

Should I give it a go, and then send it to.... who?

Uneven carbon deposits around the valves and valve stems. I do not intend to just grind/lap away for the "fun"  of it.  

I intend to check with color on the surface of the valve seat first, then insert valve, rotate it back and forth a few degrees to get a feel of how oval and/or uneven it is. If all looks fine I will not do much if anything.
If however it looks like it's oval and/or uneven I will lap it a bit, then re-check... Rinse/repeat...