We built a shooter and it was able to get into the high goal with 90% accuracy but when we added an encoder to it it stopped working. It ran super fast then slowed to a stop. Any ideas on how to fix?

What kind of shooter are we talking about here? Is it a flywheel, linear puncher, catapult, etc.? Sounds like the encoder adds a lot of friction to your system.

FlyWheel. We took it off and now it works better then before.

Seems to be a friction issue. Make sure that the encoder isn’t binding up or adding any more friction to the system.

Probably friction

Another thing to note is do not put the encoder on the same axle as your flywheel. Put it on the gear ratio before.