I've got a Poulan pro rider with a Briggs 18hp ohv Intek plus engine,model# 445777,type0129E1,code 001128YG. I bought the mower used and have had it for almost 6 years with no real engine problems until now. Mower always ran strong but this mowing season it would take 3-4 seconds of turning over before it would fire up and then sometimes would idle roughly. Every now and then I might get a slight puttering or small backfire when at full throttle but would usually smooth out after a few min. The other day mower was running fine but then while mowing it suddenly seemed to loose power and wanted to die unless I pulled the choke then it would pick back up for a few seconds and want to die again unless I pushed choke back in.engine did die but was able to be restarted but then sounded like it was running on one cylinder.I checked all fuel lines,filter,plugs,all the obvious easy stuff then got mower to start but there was some slight backfiring and puttering at full throttle.turned engine off and now can't get engine to start,it will turn over easily and fire up for a second then die. I've never checked the valve timing and think that may be the problem but would definitely like to have the opinion of a more seasoned small-engine mechanic.PLEASE HELP!!!