not enough total ignition advance?
 

I have a SBC (5.0GL Volvo Penta) that is down on power, runs great otherwise just reduced power (I have another post with more details of what I have checked/tried ). Anyhow I am thinking it might be because not be getting enough total timing advance. My factory manual has the ignition advance at various RPMs and the initial timing is right per (3deg BTDC) at 600 RPMs and tracks up the speeds to about 2200 RPM (20deg), then it stops advancing. Manual says it should go to 26 deg total at 3600RPMs. Weights are free and springs look to be fine. Is it possible the curve was not right ever ? It is mechanical thing (tolerance in the slots etc. (then it is not my problem). My question is would ~5or 6 deg total advance make a huge difference in power in a SBC?

I would check to see if everything is moving freely. No binding or anything stopping the weights from moving. If all is good, run the engine up to the max timing rpm and set timing at that speed. You'll need a timing light with an adjustable advance system.
Example; say max timing occurs at 3000 rpm and tops at 32 degrees. Set timing light to 32 degrees (timing should read 0 degree on the tab at rpm), run engine up to 3000 and set timing. Drop it to idle, set timing light advance to zero, and see where the initial timing falls.