Outdoor Power Equipment and Small Engines - 18.5 Briggs opposing twin backfires through carb

I have a 18.5 HP Briggs & Stratton opposing twin model 42A707type 2238 and a code of 9802185A that backfires through the carb. I replaced the flywheel(original one had a broken ring gear) and the magneto. At first I didn't have spark, so I cleaned up the new flywheel, cleaned up the contacts on the magneto and put everything back together. I had spark then!!!! So, I tried to start it.... it flooded, bad. I tore the carb apart, cleaned it out letting it soak, replaced all gaskets, springs and diaphrams (for the carb mounted fuel pump). Then I put it all back together. I tried to start and it fired!!!! How exciting.. till fire started blowing through the carb. Now, I go to start it, it backfires through the carb (with little fire!!!!!) and the flywheel key sheared off. Anyone have any idea what this could be???? Advice would be greatly appreciated!! Thank You.

sounds like a valve issue

Is the flywheel key sheared again? If so, you aren't tightening the flywheel bolt tight enough. Make sure there is no grease or oil where the flywheel fits onto the crankshaft.