You Can clean the Piston tops with Gasket Remover and and Non Marring Gasket Scraper set from like Harbor Freight Come in a set of 5 i think and they are bright Orange.
This is what I did
1.Find TDC for the pistons Cylinder 1 and 4 will be both at there highest point. Same for 2 and 3
2.Put some Grease around the piston being sure to cover the space between piston and piston wall so that any gunk will get in the grease and not into piston grooves which you can later remove after the piston goes back down.
3. Get some Gasket Remover ( CRC Brand works awesome) Its in a can so i sprayed it in a container then scooped it up an put it on the carbon deposits on the piston being careful not put to much and let it run off. Let it sit for a while, i waited about 10 min maybe more or less depending if the gasket remover is drying.
4. Scrape the gunk off the pistons with the non marring scraper wipe what you can and vacuum anything you might need to and keep repeating the same steps until you're happy with the results.
5.Bring the pistons down slowly cleaning off the grease with a clean rag ( big box of rags) I then used a rag soaked in Brake cleaner to make sure it was really clean.
6. soak a rag in some Carb cleaner and clean up Burned stuff around the top of the cylinder walls
7. Spray some wd-40 on another clean rag and wipe the cylinder walls and piston tops so that when they travel up they will be lubricated and you can do the other pistons. I guess very light motor oil on a rag would be bette.
That's just what i did and i was very careful not to get grease on the outside of the pistons were the coolant flows and the same for the oil passages. You can try it or pass its up to you
Regards
J
Ps check out my threat with pics of before and after piston cleaning
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/514873-clicking-tapping-sound-engine-stumbles-then-poof.html