bkarp@roadrunner.com 6 #1 Posted May 20 My m12s kohler engine will only run 45min. to 1 hour and then quits.If it sits for a few hours it will start again.I replaced the mag to no avail, any ideas on what could be wrong. thanks Share this post Link to post Share on other sites
Achto 27,565 #2 Posted May 20 (edited) 1 hour ago, bkarp@roadrunner.com said: My m12s kohler engine will only run 45min. to 1 hour and then quits.If it sits for a few hours it will start again. Could be a vacuum lock in the gas tank. Try opening the gas cap when it quits & see if it will run again. Could also be over heating, remove the engine tins and clean any debris out of the cylinder cooling fins. Edited May 20 by Achto 7 Share this post Link to post Share on other sites
WHX?? 48,814 #3 Posted May 20 13 minutes ago, Achto said: Could also be over heating, Ignition component over heating then open? After cool down starts again. 4 Share this post Link to post Share on other sites
gwest_ca-(File Mod) 11,040 #4 Posted May 20 See if you can drain the complete fuel tank by gravity. A screen in the tank outlet can plug up. When the fuel flow stops the debris will float away from the screen allowing fuel to flow again until the next opportunity. 1 Share this post Link to post Share on other sites
ranger 1,750 #5 Posted May 20 Early Magnum engines suffered from ‘Tight’ exhaust valve guide clearance. After running for about an hour the engine would cut out. The exhaust valve would stick in the guide and stay open. When it cooled down, it would close. When it cuts out, does it ‘coast’ to a stop, as if it has no compression, or does the engine stop as normal when the key is switched off? I had the same issue, mine was due to a ‘too restrictive’ muffler. You may have a carbon build up on the valve stem. The next time it happens, operate the starter and see if it spins faster than usual, and ‘coasts’ to a stop? Pull the spark plug, and tap the valve head with a punch or similar. The exhaust valve is directly under the spark plug. If the valve is stuck open, it should then close with a sharp click. Providing, of course, it’s not on the exhaust stroke! 3 2 Share this post Link to post Share on other sites
bkarp@roadrunner.com 6 #6 Posted May 21 thanks for all the input guys i changed fuel lines and gas tank filter screen,new mag,always run with gas cap a little loose. If it happens next week i will try the exhaust valve trick Share this post Link to post Share on other sites
Pullstart 62,834 #7 Posted May 21 20 hours ago, ranger said: Early Magnum engines suffered from ‘Tight’ exhaust valve guide clearance. After running for about an hour the engine would cut out. The exhaust valve would stick in the guide and stay open. When it cooled down, it would close. When it cuts out, does it ‘coast’ to a stop, as if it has no compression, or does the engine stop as normal when the key is switched off? I had the same issue, mine was due to a ‘too restrictive’ muffler. You may have a carbon build up on the valve stem. The next time it happens, operate the starter and see if it spins faster than usual, and ‘coasts’ to a stop? Pull the spark plug, and tap the valve head with a punch or similar. The exhaust valve is directly under the spark plug. If the valve is stuck open, it should then close with a sharp click. Providing, of course, it’s not on the exhaust stroke! that is great info, I’ve never heard that! Better yet, you could place the punch gently onto the valve, and spin the engine checking for open and close operations… 2 Share this post Link to post Share on other sites
ranger 1,750 #8 Posted May 21 58 minutes ago, Pullstart said: that is great info, I’ve never heard that! Better yet, you could place the punch gently onto the valve, and spin the engine checking for open and close operations… I read about this issue some years ago. Apparently there was a service bulletin issued instructing dealers to ream the guide to a slightly greater clearance. This only applied to the Magnum engines, I wonder if the problem may have been a consequence of the horizontally mounted muffler impeding the hot air exiting the cooling fins and causing the exhaust valve to overheat? 🤔 I know that my stainless muffler caused the issue with my 312-8, and after changing it, the problem disappeared. I sat in the middle of one of the horse paddocks after it cut out whilst mowing, waiting for the engine to cool down, then I heard a very distinctive “Click”, the engine fired up straight away. The next time it happened, I pulled the muffler and ran with an open exhaust, it ran for ages mowing without any more problems. No problems since with a Raider10 muffler stuck onto a 90deg bend! ( only temporary)😂 1 Share this post Link to post Share on other sites
ranger 1,750 #9 Posted May 21 (edited) 1 hour ago, Pullstart said: that is great info, I’ve never heard that! Better yet, you could place the punch gently onto the valve, and spin the engine checking for open and close operations… I assume you mean “Spinning the engine over”, by hand, 🖐️ and not on the starter🙀🙀 Edited May 21 by ranger 1 Share this post Link to post Share on other sites
Pullstart 62,834 #10 Posted May 21 2 hours ago, ranger said: I assume you mean “Spinning the engine over”, by hand, 🖐️ and not on the starter🙀🙀 Yes, absolutely. 1 Share this post Link to post Share on other sites
