If you monitor the 3d printer forums you will see many common issues that most new Makerbot Replicator 1 and 2 owners have.
Most can be traced to two related issues, inconsistent diameter of both PLA and ABS filament and the Delrin plunger in both the MK7 and MK8 Stepstruder. The symptom that most users see of both issues is air printing. The printer keeps printing with no plastic coming out of the extruder.
The Delrin plunger is a black bullet shaped plunger (screw) inside of the MK7 and MK8 StepStruder. It tries to apply pressure on the plastic filament (ABS and PLA) to allow for proper drive wheel traction on the stepper motor.
The inconsistent diameter of PLA and ABS is caused by manufacturing issues and that filament does not age well. The biggest problem is absorption of moisture from the air.
So these two problems causes filament feed rate issues. You get too much or too little pressure on the filament and then you get too much or too little filament coming out of the extruder.
Lucky we have two upgrades or fixes for these issues on Thingiverse.
I would highly recommend that these two upgrades would be the first objects you print out when you get your Replicator 1 or 2.
Hopefully we will see this issue fixed in the MK9 StepStruder?