Preheat the grill to medium heat, about 350 degrees.
Remove the tough ends of the asparagus.
Combine the balsamic vinegar, garlic, thyme, and pepper in a large shallow dish. Add the asparagus and toss to coat. Marinate at room temperature for 15 minutes. Reserve the marinade.
Bundle about 8 or 9 spears together and wrap in bacon. The bacon will hold the spears together without any toothpicks or skewers. Continue with the remaining asparagus and bacon. You should have 8 bundles.
Grill the bundles for 8 minutes, turning 3 times, or until the bacon is done and the asparagus is tender. Transfer the bundles to a serving platter and drizzle with the reserved marinade.