How do I replace a drive belt on a 1999 harley

On a Big Twin, you must pull the entire primary drive off the engine.

Disconnect the battery. Take the outer primary cover off the engine. Remove the compensator (engine sprocket) nut. This nut is very tight and has Loctite thread locker on it. 150-165 foot pounds of torque. You'll need to something to lock the chain up so the engine does not turn while loosening or tightening this nut. We use what is called a "locking bar". Remove the small plate with the screw and locknut in the center of the clutch assembly. Remove the mainshaft nut up inside the clutch assembly. It torques to 60-80 foot pounds LEFT HANDED TREADS. Remove the nut from the primary chain tensioner. Grab the compensator sprocket and the clutch assembly and lift the primary drive out of the cover.

Take the small screw out of the center of the jackshaft and remove the starter jackshaft.

From the right side of the bike, remove whatever you have to remove to get to the starter, exhaust pipes or whatever. Disconnect the battery cable and the small wire from the starter. Remove the two bolts holding the starter on inner primary cover.

Go back to the left side of the bike. Remove the four bolts that holds the inner primary to the engine, and the two that hold the primary to the transmission. There's one bolt coming in from the backside that has a braided wire on it on the early models. There may be four bolts holding the inner primary onto the transmission. When you have all the bolts out, the primary will come off easily. If it does not seem to want to come off, you may still have a bolt somewhere. Do not beat on the primary cover with even a rubber hammer. It should come right off.

Now, you'll have to remove the rear wheel, rear shocks, belt guards, and remove the swing arm.

As you can see, it's a pretty involved job on a Big Twin.

Good luck
Steve