I have a 18 h.p. twin cylinder Briggs and Stratton engine on my rider. When I try to start it it pops very loudly. I have checked the timing pin and it is o.k.. I pulled the heads and checked the valves and they appear to be o.k. Does anyone have any idea what could be causing this? It will not start at all, but it pops real loud sometimes. I appreciate any advise that I can get.