I got an older Simplicty 18 hp. Rider. Everything runs great and works fine, until I engages the blades. It just bogs right down. The only thing I can think may be the problem is the wrong gas cap. My grandson ran over the old / original one. I have a replacement one, but not original. Would this be the problem. What else could it be.

Hi,
I would say the symptom is more indicative a problem with the deck; seizing spindle(s), something wrapped around blade/spindle(s) or seized idler(s.)
Disconnect deck from engine and check parts for freedom of movement.
Peace,
Clarke

The deck was removed, all works freely.

Hi,
Okay, let's go in a different direction. What engine do you have, brand, model etc.
Peace,
Clarke