Two Questions, Rotor replacement and backfiring Options

[turnaround89]

I am putting all new rotors and pad on all four corners of my car and had a question about the front rotor inner grease seal. How is that put onto the rotor, i bought new ones(don't have them yet, get them tomorrow), but should the old ones just pull off the old rotor or are they glued on there?

My second problem is, after driving my car for awhile today is started backfiring like no other. The car drove fine this morning on my way to school but when i got out of class it would not accelerate smoothly without popping out the exhaust. Since this was going on i didn't take the car to work and let it sit till i got home. Took it out for a drive and now it accelerates smoothly until you reach 3800rpm, then it starts to pop, pulled the car into the garage and it was popping at idle. Whats going on? I did a complete tune up little over a month ago, new point, rotor, cap, wires, plugs, etc...timed the motor, but now its running like crap and ran fine this morning. Should i check my points again and set the gap? Is my timing off by a little?

Thanks everybody!!!!!