When ever a timing belt slips it means that the crankshaft and camshaft are no longer in proper sequence to allow the travel of the piston and opening of the valves to happen as required for engine function. That is why it would not run correctly.
When you say the plug was damaged, was any of the porcelan insulator at the bottom of the plug missing. That porcelan may be still in the cylinder. It is possible to pull the plug again and see if you are getting any denting of the piston or see any place where something may have knocked carbon off the piston and left a cleanspot here and there.
The bad side of your senario is the fact that the valves were opening when they should not. When this happens ( especially at highway speeds ) the valves may come in contact with the pistons. This can either slightly bend a valve ( which will then make a tapping sound as it is not closing perfectly in the seat, but can still run fairly smoothly ) or could totally wreck the valve train and destroy pistons.
The best thing that could happen is that you have only bent a rocker arm, push rod or collapsed lifter. This depends on which engine you have. This will also cause a tapping noise due to extra clearance in the valve operation. These are probably the cheapiest fix. You can remove the valve cover only to fix some of this.
You need to do a compression test to determine if you have a bent valve. If you have a 10-15 pound diference between any of the cylinders then it is possible a valve is bent. A bent valve will destroy itself and/or the cylinder head in a short while of driving.
Best I can do with information provided.
Tuesday, March 18th, 2008 AT 9:16 PM