Outdoor Power Equipment and Small Engines - briggs and stratton misfire

hi. last weekend i blew the con rod on my b&s 12.5hp lawnmower. i managed to track down another engine that had no carb etc and salvaged the crank and connecting rod from that. yesterday i put the new crank and con rod into my old block, using the original piston etc and re-fitted engine to mower. after replacing oil and topping fuel tank up i cranked the engine over and it tries to fire but will not continue running when i disengage starter. sometimes it will backfire out through the carb but most times it is like it is on the verge of firing up but its not quite there. as far as i remember i lined the cam wheel up with the crank (two little holes). can anyone offer an explanation as i am a little puzzled. cheers.

Check for spark, if you have good spark then perhaps the flywheel key has sheared. Properly torque the flywheel nut and make sure the flywheel and shaft are clean and dry, no lube.

it wasn't that the flywheel pin had sheared, i'd forgotten to put it in !! thanks for your help.

DOH! :D
Glad ya got it just the same.