detonation?

[whamo]

I have had an issue with my 240z for as long as I can remember. It sounds like it is experiencing detonation at part throttle under a load. The engine is stock with the exception of a 280zx EI distributor. The initial timing is set at 7 degrees at 750 rpm. The total mechanical advance is all in at 2700 rpm. The total timing (initial 7) plus (mechanical 18) totals 25 degrees. This seems pretty tame to me. A typical performance v8 usually produces best power at 30-35 degees of total timing. I'm not certain how much a datsun inline 6 needs. I have tried taking the timing back eve further and enriching the carbs but nothing seems to effect the sound/feel that I percieve as detonation. I have tried it with and without the vacum advance connected. Currently Im driving without any vacum advance but the vac advance when conected adds 24 degrees. I have a wideband sensor and when the sound occurs the AF ratio is in the mid 12:1 range..... Suggestions? so far I have been ignoring it but its beginning to drive me crazy. I suppose the real question I have is how much timing is safe with premium pump gas in a stock l24.

[aarang]

Does it still detonate w/ the vac advance disconnected? The ZX distributor vac advance needs to be hooked to a ported vac source as apposed to direct manifold vac if I remember correctly.

[whamo]

Yes the noise is still present with the vac advance disconnected. Im pretty certain its not detonation now. I retarded the total timing to 18 degrees, zero initial and the noise is still there but the power is way down. Still looking for input on best total advance for this engine....... I think the noise may be from an exhaust leak..... although none is evident at idle.

[kiwi303]

I believe a blown exhaust gasket between the head and exhaust header can make sounds like that.