My MTD walk behind lawn mower won't start. There's gas and spark (I think---when I looked at spark plug, it was pretty dirty and had a lot of dark greasy residue on it so I cleaned it off, but it still won't start)

You need several things for a running internal combustion engine:

• Air
• Spark (to start)
• Fuel
• Compression
• Lubrication
• Exhaust.

You've shown that you have a workable spark plug so far. Going from the above, consider as well

• Fuel. Bad / old / contaminated fuel. Clogged fuel filter. If it's a motor that needs gas/oil mix, does your gas have the right oil mix?
• Air: Dirty, clogged air filter or air inlet.
• Spark. Is any electricity making it to that proven-workable spark plug?
• Compression. For this one, the most likely problem (other than a toss-the-engine thing like a broken piston) would be if the spark plug is not tightened into the mount hole, allowing the air to leak out rather than compress.
• Exhaust. Most lawn mowers will have a spark arrestor on the exhaust to make sure you don't set a dry yard on fire with your exhaust. It's often overlooked in diagnosing two-stroke engines. Make sure it's not clogged or blocked.
• Lubrication. Is there oil of the right type, and up to a proper level on the dipstick. No oil equals way too much friction in the engine.

Good Luck!

DH

Your best bet is to take your old plug out and a parts store can duplicate it.

Usually when a mower dies, it is a fuel delivery problem. Have you changed the filter? Is the gas fresh. Also has the air filter and spark plug been changed recently? the Carburetor may need to cleaned. Take the air cleaner off and spray some carb cleaner in the throat of the carb. Also, take about 1/3 bottle of carb/fuel system cleaner and add it to the gas in the tank.. For the new mower engines, there is no fine-tune adjustments on the carburetor. Also clean the exterior of the carb with carb spray.

I have attached an article about how to change the spark plug on a walk behind mower.

The directions should be roughly the same despite the difference in brand and model.

I hope this helps!


How to Change the Spark Plug on Toro Recycler Self Propelled Mower

It seems to be an obstruction in your fuel delivery system. Get a can of ether and a spray a short burst of it into the breather area near the carburetor, if the mower starts up and then stalls out, then you may need to get your carburetor serviced.

unplug the wires going to the coil if still no spark replace the coil

A good place to start would be to remove air cleaner assembly and give it a good wash and service and take a close look at choke assembly/lever to be sure it isn't sticking in on position, while trying to run it with air filter off..

Have a look at these tips for troubleshooting starting problems with lawnmowers:

http://www.fixya.com/support/r9118692-lawnmower_not_starting

Hope this helps!

The three most common causes (IMHO) of your problem are: 1.) Carb (dirty or gunked up) 2.) Gas (old, maybe has water in it, or wrong octane/type. ie., no e-85 in mower) 3.) Spark plug (worn may need replacement, or gap checked) Lawn mowers are simple internal combustion engines. Like any other, all they need is spark, air/fuel, and compression to run. Think about those three things as you trouble-shoot. I hope this helps.

All lawn mower engines have a drain plug for draining the oil. Look for a square nut on the side of the engine