Small Engines (Lawn Mowers, etc.): 12.5 briggs & Stratton ride on lawn mower, briggs stratton, grass clippings

Question
Hi There,

I have an oldish 12.5 briggs %26 Stratton ride on lawn mower. After about 15mins of mowing it conks out like it run out of gas and then goes puff and stops. When you try to restart it will not spark, I then replace the spark plug and first pop it goes for another 15 mins then the same thing. Apparently this is a carbon clogging it up? But is there anything I can do to prevent this or stop this from happening so that I dont have to conitue to buy spark plugs?

Cheers

Answer
Kieran

There can be numerous reasons for the engine to quit running when it gets up to operating temperature.. expansion of metal parts where bolts/screws aren't tight enough, breakdown of the condenser for the ignition points (if equipped), or a possibility of a crack somewhere. An overall inspection of the tightness of fasteners would be a good place to start.

   * If the miss is a miss, the problem probably has to do with ignition or the fuel mixture. Check the tightness of the carburetor mounting bolts and where the intake manifold/air-fuel mix tube fastens to the engine block.

   * If instead, the miss is popping through the exhaust, this might be caused by a bad exhaust valve or valve seat.

   * If there is puffing or popping near the engine head, the head bolts might not be tight.

   * If the engine spins over too freely when it's hot.. indicating much lower compression, check the torque of the head bolts.. if they were loose, the engine should now restart.

   * If it won't restart until it's cooled down, the exhaust valve (and/or valve seat) may be the problem. If you can remove and disassemble the engine yourself, this isn't expensive to have corrected. If not, the shop labor for tear-down and reassembly will be fairly costly. A common reason for exhaust valve failure is that grass clippings become embedded in the cooling fins around the exhaust port area.

   * If the engine is just failing to fire the spark plug when it's hot, there's a good chance that it's time to change the (points and) condenser. This can usually be confirmed by quickly removing the spark plug after it quits running, and grounding the sparkplug base securely to the engine (away from fuel vapors).. then spin the engine to see if there is a hot blue spark with an audible snap, snap noise.

As soon as the engine shuts down, turn the flywheel by hand or DISCONNECT the plug wire and turn the blade by hand to see if there is any compression. I have had several B&S engines that did not have enough valve clearance and when hot, the valve stem expands in length enough that the valve does not properly close. After cooling, engine will be OK. Also could be a bad condenser or the other faults listed above.

Problems with the ignition coil can make an engine impossible to start when warm (or even stop running when it warms up), apparently this happens on motorbike engines quite often and is caused by a break in the HT coil.

A carbon track forms inside the coil where the wire is broken. When the engine is cold the carbon track has a low resistance and it conducts well enough so the spark is strong enough to start the engine, when the engine is warmer the track resistance is greater and the spark less strong. This problem gets worse over time as the carbon track gets bigger and bigger, the only remedy is to change the coil or electronic ignition module.