I've changed the gasket between manifold and head to fix a vakum leak. #4 was running lean, spark plug was all white. The old gasket was damaged in several places. Engine runs better now, but still not dead smooth, maybe it's not suppose to? While i was checking the timing it was 10 deg at idle, i started to hear detonation, or what i belived was detonation I have never heard detonation in a volvo before but i now what it sounds like on aircooled vw, but i have never heard it a idle. The sound disappeared when i revved the engine. I drove the car a short trip on the freeway and the rattling is now gone. Could it be something else i heard? Anyway the engine runs a bit smoother but still not smooth enough, it's a lot better than before though and all the spark plugs read light brownish colour. The engine doesn't have a lot of power, but i guess that's just what a B21A feels like.