Here it is: my admission of total "brainlock" diagnosing single cylinder engine failure. This could happen on any brand so don't get all huffy when I reveal the engine is a 10.5 horse Briggs and Stratton! This sits on top of a trail-behind lawn mower deck. Its only job is to spin the two blades to cut grass. So, we were mowing one day and the engine falters like it was running out of fuel and finally stops. Has oil, has fuel, will not restart. Pulled the plug it check for spark which appears VERY weak. Ah-HA! Spent 40 bucks onna new ignitor (Hey! The Local Briggs dealer actually has PARTS in stock!). The new spark appears much stronger so everything is re-assembled and it won't start! It has a pull cord type starter so it is obvious. Normally, one pull does the trick so, yes it is obvious the problem is NOT the ignitor. Thinking the easiest place to start inspecting is under the overhead valve cover, I remove the cover to find the exhaust rocker clearance is HUGE and the exhaust valve is not opening. The valve train is pure Chevy: stamped steel rockers with a spherical nut for a pivot. This nut is on a 1/4-28 stud and the locking device is a setscrew pushing on the end of the stud! This all came loose and the little cup came off the end of the exhaust valve. This cup only increases the flat area for the rocker to push but the valve lash increases dramatically when it is not there! That was the cause of the failure.
Conclusion: I am used to seeing a really fat spark that shows well even in shaded sunlight. This type of ignition, while elegant and ingenious, produces a weak spark in comarison. Not wrong, just different. Must use more effective "shading" when trouble shooting this type of ignition!
The valve train including the setscrew reassembled with Loctite!
Karl
