Briggs & stratton model 120000 won't start it trys to turn over but won't

i ran the number and its no good. 120000 L head is 140 ft/lbs
120000 ohv is 210 ft?lbs

The cooling fins are probably clogged with leaves, grass, mouse nest.

what kind of mower

I'd contact Briggs and Stratton directly. More than likely they use a lock-nut with a reverse thread to hold the blade on to the drive shaft.

governor out of adjustment

check for spark is there a low oil shut off and the oil level is a little low ?

magnito problem sound like

trry a shot of starting fliud see if it wont start then make sure your getting a spark to

valves unseated warranty defect take to dealer to have it fixed

The crankshaft should have a marked "tooth" either with a "dot" or a "-"mark, the camshaft will also have a corresponding dot or dash. By rotating the crankshaft, align the marks on the crankshaft & the camshaft so that they are directly opposite each other. Drop the camshaft into place and the engine is now "timed"--if the marks are not exactly opposite one another, lift the camshaft (just enough to disengage the gear teeth) and try again, being "one tooth" out can make a huge difference--so take your time and make sure the "marks" are directly opposite each other.