briggs & st 12hp?

If timing mark alignments on internal engine parts (ie crank & cam) have not been adheared to then it it very possible for valve timing to be out. If fitted it is also crucial to get possitioning of internal balance weights correct. You should be able to purchase a workshop manual from a Briggs dealer. You will need to quote the engine model numbers when ordering.
If the timing marks are correctly aligned check that the flywheel key has not partialy sheared as this will alter the timing by a degree or so.

Robin S
August 2008