I have a John Deere 217 riding mower that the battery is dead. Is it safe to jumpstart it with an car battery? Thanks, Randy

Does the mower have a 12V battery? If so, then no problem - 12 volts is 12 volts, regardless of the size of the battery. I've used my car to jump start my riding mower many times. Be sure to take the usual precautions (get the polarity right, connect the negative of the car battery to the chassis of the mower, not the battery itself).

Regarding jump starting a riding lawn mower... I want to use a portable jump starter, and as you say 12v is 12v, but what about the amperage, does it matter.

No - it does not matter, as long as the portable starter can put out suficient cranking amps to turn the starter motor. If the jump starter is designed to crank an automobile it will certainly work on your riding mower. And you don't need to worry about the jump starter being "too strong" - the actual current that will be pulled from it is limited by the design of the starter motor on the mower. Same principal as when you hook a small 5 watt christmas tree light to the 200 amp electrical service feeding your house.