Hi, I have a Briggs 13.5 OHV engine that I am having trouble with. Can the flywheel be off by 180 degreees?

It can be off by 180* but it won't fire correctly or run at all. (lots of back fire if running) Your best bet is to pop the flywheel and see if the key is split.

I already replaced the key. The old one was in perfect condition

Ah I see your thinking the flywheel could be off by manufacture defect?

No, actually I was wondering if it was possible that I put it back togather wrong. 180 degrees off

The key is what keeps the flywheel lined up correctly. So as long as you put it back on with the key lining up with the shaft it should be on correctly. Im wondering if your valves are closing correctly. If its not the valves or the flywheel position it must be the carb.