Lawnmower cuts out?

Question

Basically I have an old Super 11 SL tracotr lownmower, with a 4 stroke briggs 11hp 399cc single cylinder engine.just changed spark plug and head gasket, cylinder bore is perfect, not scored etc..

after 15 mins use it just starts backfiring and eventually cuts out... then its completely impossible to start...no matter if fuel on / off, revs low / high / choke on / off..... etc....

no, its not flooded i know quite a bit about engines....

However, it makes some quite loud backfires (poss thru carb?!) when trying to get it running again... actully have to let it rest for 15 mins, at which point it will start, but not without a bit of a fight! It has done this for ages, but now I need it to work properly and its getting on my ****!

Fresh petrol etc.. I'm assuming it's overheating, but ive made sure the mixture isnt too weak (which could provoke overheating), the filter is clean, etc etc.. the oil is a tad high though, but that would cool it done more, surely?

Anyone have any ideas?

Thanks!!

Answer 1

Sometimes the problem you describe can be caused by timing, which is controlled by the wood-ruff key between the fly wheel and the shaft. Just a slight "crimp" will alter timing enough to cause all the problems you have. Good luck!!!

Answer 2

Check your float bowl.  A swollen needle valve seat will slowly starve the motor as the feed can't keep up.

Backfiring could be a timing issue. Didja check the flywheel shear key and keyway?  If you ever hit an object and the motor was stopped by it the flywheel could have advanced itself.

Just some ideas.. Good luck!

Answer 3

Are the new spark plugs the proper ones?

Answer 4

install the magnitron ignition

Answer 5

It sounds like a shir pin problem. witch would through it out of timing,Pull the fly wheel check the Shir pin.

Answer 6

yes, buy a new one. :)

Answer 7

A "Magnitron" ignition is a breakerless (solid state) ignition system.

Have you taken the cover off the engine to see if the cooling fins are free of debris? Clogged fins will cause a overheating problem.

When the engine backfires and goes dead, does it still have spark? If not it could be the ignition module is going bad, the flywheel key is sheared, or the stop wire is pinched somewhere and grounding out the spark.

Please check the items out and let us know what you find!

Answer 8

does it have a magnetic fly wheel,if so pull it see if the pin has sheared ,they make them weak so it won't damage the shaft if it hits something that stalls it out,if the flywheel turns on the shaft it puts everything out of time .sounds like what you have but a big job...............tom

Answer 9

You might be cutting to much grass at once or the blade is real dull

Answer 10

Sounds like a distributor problem. Whenever an engine backfires, it's either due to too much fuel in the cylinder or mistiming or not enough spark to ignite the fuel immediately. I would inspect and clean your distributor and check the generator to make sure it is not weak and not producing enough electricity.

Good luck.