I use Autoglym Engine and Machine Cleaner first to dislodge grease and grime. Then shampoo and rinse. Then Gtecniq tar remover. Then Peek metal polish which seems to outperform Autosol mentioned above. Nylon pan scouring the carbon build up on the tips with the Peek if bad but don't do the cylinders - just the carbonated edge of the tips. Then polish off with a clean (but not new) microfiber and seal with Poorboys wheel sealant. Bit of bother and I don't do this every time (obviously) but if you want to restore yours this regime works well.