Hello all, I'm new to the forum and in my first message, I've got to ask for help

My father asked me to try and find some help with a problem he's having with a truck. Here's the specifics:
It's a 1987 GMC Pickup 1/2 Ton with 5.0 liter engine. It has a Throttle Body fuel injection system.
Problem: He can't get it to pull over 10 inches of vacuum psi. He needs 17.
Here's what he's done thus far:
1.) Cleaned the throttle body.
2.) Set the timing to specifications.
Any suggestions? The new guy would really appreciate it and I could score some points with my father as well!
Rustyinmonroe