Are you setting your timing at 38 degrees btdc as total timing or is that initial timing? If initial that is way too much, you shouldn't go over 40 total as a max, usually a total of 36 is where you want to go. Initial timing can vary a bunch depending on how much total advance your distributor has, you need to know that to really set your timing properly. You can also get one of those dial in timing lights to get your timing set or put some timing tape on your harmonic balancer.