Cracked pre chamber maybe? Those engines are very similer to my 603, so the pre chamber is screwed into the head with a sealing ring. The ring may be leaking or the chamber itself may have a crack in it.

Not a big deal they run about $60ish last I checked but you need a special socket to yank them.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200