Do I have lightweight heads?

[erichy]

Hi guys I'm trying to figure out if my motor has lightweight heads. The manual says if I have lightweight heads I should time it 13 degrees BTDC but if not it's 7.4 degrees BTDC. The book says to look for a " scalloped exhaust wall" on the head to see if they are lightweight. What the heck are they talking about? The motor runs great at 13 degrees but at 7 it barely runs. I think I have other timing issues but need to know what to start with. Thanks in advance Eric.