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 takeing the timing back and enriching the carbs but nothing seems to effect the sound/feel that I percieve as detonation. I have triedit withand without the vacum advance connected. Currently driving without any vacum advance but the vac advance when conected adds 24 degrees. Suggestions?

[whamo]

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.

[Curtis240Z]

I set total advance at 36 degrees.

[mayitin]

Well is hard to tell without hearing it, I was going to tell you to change the plugs. If you have a leak you will find it by looking for the black sud. you may have broken stud. Is it stock headers?

[whamo]

The car has a stock exhaust manifold and the MSA exhaust system. I'm going to check the torque on the exhaust manifold today.....