Lawnmower wont start, help?

When it yanks the cord back it could be an idication of the ignition timing being off. The only way for this to happen is to shear the flywheel key. To check it you will have to remove the engine cowling and the nut off the flywheel. At that point you should be able to see if the key ways are alligned. If they are not alligned you will have to pull the flywheel and replace the key. Shearing the key usually happens when you hit something with the blade. Use a puller to remove the flywheel and torque the nut back on to about 45 ft lbs.

I had this problem with my lawn mower last week. Mainly because I left it on the back part of my property outside all winter in the rain & discovered it under a tree. I had to disassemble the tinwork down to the electronic coil, remove it and sand the rust off of it and the flywheel. Only then did I get spark. Also had to drain some water out of the carburator. Normal electrical problem if no spark is (1) The above (2) The wire that kills the engine to the ground (may be in conjunction with the engine brake as well) may be grounding out or not adjusted properly. (3) Spark Plugs (4) the high tension lead break or worn out spot (5) Corrosion in the part of the high tension lead from the coil that clips on the spark plug. (5) If you messed with your coil assembly, the air gap might not be correct. (Usually too large) The coil assy part with the plug wire isn't expensive so when I suspect that might have any problem like in the plug wire, I just replace it also.

If it is backfiring I would check the valves. When you pulled the head were the valves OK?

try spraying a little starter fluid in intake if it runs than shuts off chances are its the carb

clean sparkplug... may need to clean magneedle.... possible water in fuel, (all it takes is a drop) .... carb float might be sticking (if it has a float)... carb itself may need to be cleaned...the lines that has a filter for the fuel, the filter might be dirty...... hope it helps