Help. mower will not start. have new fuel, new plug. fire going all the way thru plug. can spray starting fluid in cylinder with no help. have fire, have fuel, in time (checked shear pin), have compression. but no start. belonged to my late wife, so has great sentimental value, loaned to my son for two seasons, when i got it back, it had this problem. battery will not hold a charge, but can put it on trickle charger overnite and it spins over great. could that be the problem? not enough amps in battery? don't want to spend money on battery if i have another problem. any help is greatly appreciated!