Small Engines (Lawn Mowers, etc.): briggs 18 hp, crank shaft, hp briggs

Question
i rebuilt a 18 hp briggs 422707 1516 01 every thing is torqued and speced as i have rebuilt engins small engines before but the motor wont start spark plugs ar firing and it is back firing through the carb and exhaust. i originally aligned the crank and cam with timing marks together then took the bottom end off my bs 16 hp and the timing marks were set in sequence both facing the same side of the engine re assembled my 18 hp like that and the same problem i cant find any thing to help with the timing marks  i now am not sure if that is the whole problem ?????

Answer
The timing marks on the crank shaft gear and camshaft your must align.  If they are I would look at valve seating and adjustments.  Look for a bent pushrod.  As far as the other engine being out of time depends upon where in the cylinder the piston is when the valves are open or close and the plugs fire.  Backfiring through the carburetor typical of the engine being out of time or a valve keeper came off.  The other thing could be  or electronic ignition but I doubt it.

You have not mentioned fuel but again this would not be a symptom of a fuel system problem.  It most certainly has to be ignition/valve timing.  I hope this information has helped if not please get back to us with a little more information like application.