Dear Ole Mechanic;Lawn Mower

I've got a problem with the gas engine on my lawn mower.  It starts just fine.  It doesn’t smoke or use oil, and it runs just fine--for about 15 to 20 minutes.  Then it just shuts off like it has run out of gas.  By the time I get the starter cord wound back on, it starts right back up, but it has no power and usually shuts off again within a minute.  I figured that it was a plugged-up gas filter or a little piece of crap in the carburetor, but NO--the tank was clean, the lines and filter were clea,r and the carburetor was clean as a whistle inside.  I put the engine back together and it ran 16 minutes.  Thinking that I had missed something, I had a mechanic friend check my work.  He said the fuel system was fine, but that was all he had time to check.

Why won’t my mower keep running?    Thanks

Walkin’ in Tall Grass

 

Dear Walkin’;

Since you seem to have plenty of gas, I suspect that you are running out of spark.  It doesn’t sound like you have a vacuum leak between the carburetor and the engine.  A vacuum leak like that could cause the engine to run lean, which gets things real hot when the engine is working hard.  When the engine gets real hot, the piston and cylinder seize and get real tight until they cool down a lot.  If that has happened, then the engine will get real hard to start when it stops, and it takes quite a while to cool down.  Your engine cranks right back up, so nope, this doesn’t sound like your problem.

Seriously, it sounds like as soon as the engine gets up to operating temperature, the spark fails.  The first thing to try is a new spark plug,and please make sure that it is NOT a resistor plug, as a resistor plug is a good way to get a magneto coil to fail. 

The next thing to check is the points--if the engine has points.  I have run into some mower engines that do not have points.  These pointless engines usually have the coil on the outside of the flywheel and the flywheel has three magnets built into it instead of the usual two.  Also, there are only two wires coming out of the coil--one to the spark plug and one to the kill switch.  If there are three wires--one to the plug, one to the kill switch, and one disappearing under the flywheel--then the one going under the flywheel goes to the points.  If you cannot see a coil and there are two wires coming out from under the flywheel, then there are points (and a coil) under the flywheel.  If it has points, pull the flywheel off of the crankshaft, or get access to the points.  Make sure that they are clean and are opening and closing.  Check the points gap when the points are open.  There should be about .020 of an inch gap (.020 or 20 thousands of an inch is about the same as six thicknesses of a newspaper page.

If the points are OK, then you are probably fighting a coil or condenser that is starting to go bad.  Either can check OK when cold, but then short out internally when they heat up.  While there are sophisticated machines that can check the coil or condenser, they are few and far betwee

How do I check them?  I put in a known good one, or if I don’t have a good used one, then a new one.  First comes the condenser, as it is the cheapest of the two.  If the engine still quits after it gets warmed up, then it is a bad coil--as long as you haven’t run the engine out of gas and the fuel system is as clean as you say it is.

Herr Professor Nuzanbolts