Engine is a Briggs 27 hp twin V installed on a Scag mower. Engine sputters, pops, but does not run. Here's what I think happened: removed engine and cowling then placed engine upside down to remove oil pan. Drained oil before doing this but did not remove oil filter. Replaced governor assembly and re-installed pan. Installed engine back on Scag and set new governor rod. Trying to start engine just drained battery while engine just sputtered and popped. I figure that sitting the engine upside down drained sufficient oil into the combustion chambers to interfere with operation. The starter does appear to be working a little harder to rotate the engine. Any thoughts and directions toward fixing this will be appreciated