I have an older mtd with a 12.5hp briggs, I mowed with it about a week ago and everything was fine. This time when I went to mow the tractor won't fire. I have spark at the plug, I can see fuel in the carb throat and can smell it. It cranks fine but won't fire, every once in a while it almost catches and blows some smoke out the exhaust. Any ideas would be appreciated.

Thanks,
Jeff

Hi,
First thing to try is a new plug, and to be sure that you gap it to 0.030" as some now come new with about one and one half that amount of gap.
Peace,
Clarke

Hi! Clarke.

If the fuse went bad it wouldn't even try to "catch an blow smoke" would it?

K

Not a fuse if you are getting spark. You get nothing with a blown fuse.

Its been a long day.

Clarke,
I already replaced the spark plug as it was the original one from1994. I also replaced the ignition switch because I wasn't getting consistent contact through the pins when I turned the key. The spark is very strong which made me think fuel problem but it seems to be getting that also.

Jeff

Clarke will be back I am sure and his answers have been great. Have you dumped the carb bowl and tank, refilled with freshly bought fuel? Just a very small amount of water can cause the engine to not fire but you still smell gas. Drain your tank into a glass jar, if you have any water it will be very evident. Gas with 10% alcohol absorbs water and can be a pain in the neck.

I will try that. The gas I use from the local station does contain 10% ethanol. Thank you everyone for the help.

Jeff

Hi,
Well, you say it is getting fuel. What is the chance that something other than gas was introduced into the tank. I only ask because you say it blows smoke. Engines will crank badly if at all cold, and smoke like the ens, if the fuel is contaminated with kerosene. Try taking off the air cleaner and spraying a highly flammable liquid such as carb cleaner or a flammable brake cleaner (not all are.) If it cranks and smokes, it is probably the fuel. A trained nose will know what it has in it.
Engines will run off kerosene, but with a 25% power penalty. They are cranked off gas and warmed before changing the fuels over. They use a double tank, one section with gas and one with kerosene. They have dry carbs when not running. To crank the tank with gas has its shufoff opened and the carb fills, the engine cranked and warmed. The shutoff is closed and the one to the kerosene tank is opened. To shut the engine down, the kerosene tank valve is closed and the carb allowed to run dry.
Peace,
Clarke

I don't believe I have any other type of fuel mixed in as I always use the same gas can and I have been using the gas in this can for several refills nows with no problems in any of my other equipment. I did spray my carb cleaner in previously. I thought the carb may have been dirty. It didn't fire up with carb cleaner either.

Jeff

Clarke, dare we say starter fluid?

Hi,
Well, it is rare but it may be time to check that flywheel key. Ignitions have been such that with the spark advance systems that have had to be built in because of EPA regulations; they can crank badly and run with reduced power.
Of course the subject of compression has yet to rear its ugly head. It is checked by taking off that same blower housing that gets you to the flywheel and key. You turn the engine pasr the power stroke about 90 degrees or so, grab the fins of the flywheel and spin backwards sharply. It should "bounce" back 90 degrees with healthy compression. You do it backwards because the ACR works in the other direction.
Peace,
Clarke
PS A shot of starter fluid would not hurt, just do not use it habitually. I have found a "neat" trick is to remove the air filter, soak a rag with starter fluid and place the rag over the intake to the carb loosely. I use that with Yanmar diesels as well, they are not supposed to use it either.

If you see fuel in the carb throat it could be flooded. That will make it not start. What color is the smoke?

The smoke is white.

Hey Everybody thanks for all the ideas. Today I ripped the head off after trying all kinds of other things. As I was spinnning the flywheel to see if the valves were moving properly I noticed when the magnet on the flywheel came around to where it would fire the plug, my intake valve was still open. It turns out that my key on the flywheel sheared and the flywheel slipped on the shaft throwing everything out of time. I have no idea what caused the key to shear, but Monday I'll get a new one and it should be good to go. Thanks again.
Jeff