Small Engines (Lawn Mowers, etc.): Engine wont start, craftsman mowers, amazon

Question
Hi Fred -

Although not a brand you specifically list, everyone else is on vacation and it's more general than a brand-specific question:

What do I check after I replace a spark plug on a lawnmower and it still won't start?  

Both are Craftsman mowers - one is an oldie, but worked up until yesterday - it's model # 917.374670 - and the other is probably about 5-6 years old - model # 917.377110.  The older one was working fine, ran out of gas and when I added new gas (it was not stale) it just wouldn't start.  

The newer mower has the original spark plug in it as I have had difficult finding the RJ19HX that it requires.  I actually just located it on Amazon tonight, but if I replace the plug and it doesn't start, what else might I check?  

I'd like to get both of them working if possible.  

Thanks in advance for your time.

Fran

Answer
If you have spark on the mower it is fuel, usually.  Sometimes when the engine runs out of gas all the accumulated debris in the tank and carb bowl end up in the main nozzle.  Remove the carb and disassemble.  Purchase a carb kit for both mowers and install all the parts.  I always suggest using new mounting gaskets.  Next be sure there is good fuel flow and a clean fuel cap vent.  If there is a throttle and choke set them.  Check the little wire that goes from the ignition to the kill switch. If you are not shorting the spark plug to kill the engine there has to be a ground wire from the ignition to ground.  

Just some thoughts, hoped it helped.