Just got engine rebuilt and after 150 miles engine knocks under load? When the spark is retarded 3 or 4 clicks to abount 15 degrees advanced knock goes away? Could the 5.1 Snyders head have this effect or something more serious? I drove this car about 30 MPH for about the first 50 miles to break her in. Any Ideas as to what could be wrong, I would hate to pull this engine again. Thanks, Phil