Small engine puzzler

[Roger Thornhill]

I had my tiller [powered by a 5hp Briggs 4-stroke] quit on me after about 10 minutes of use. It simply died; no sputtering or rough running. After determining that it had fuel and wasn't overheated, I removed the spark plug, pulled the starter cord several times, and determined that it had no spark.

I then took off the shroud, got out the ohmmeter, and made the following observations:

- continuity good through the mag coil winding

- continuity good through high-tension lead

- grounding lead not shorted

- spark plug good

I'd messed around long enough and needed to get the tilling finished, so I went down and bought an entire new ignition module, and a new spark plug. The magnets are still strong on the flywheel, and the flywheel is properly timed on the crankshaft (keyway is undamaged). I replaced the coil module, set the clearance (.010"), put in the new plug, and buttoned it all up. No spark.

Shit.

So I triple checked everything I knew to check, and couldn't find the problem. The basic laws of physics say that when you pass a spinning magnet over an induction coil, it WILL generate an electrical pulse. Not in this case.

Any small engine mechanics here know what might be wrong?

 

[don24mac]

Look for a safety switch somewhere. On all newer equipment, there's safety this and safety that. Like on a lawn tractor there's a switch under the seat, on the shifter, on the blade control and on the clutch/brake pedal that any one of them will short out the ignition if they aren't in the proper configuration. Even my push lawn mower has two.

Check the controls on your tiller, especially the one that controlls the tines, for a switch somewhere. Look for wires on the handles or even a simple switch on the lever where it starts the tines.

Could likely be something like that.

 

[Hamilton Felix]

You don't have a low oil cutout, do you?

 

[Roger Thornhill]

Mystery solved

First off, thanks for your responses and suggestions. In the end, the problem turned out to be a classic case of assuming that a new part is good.

I had checked for anything shorting the mag coil to ground, including completely disconnecting the coil shorting wire (don't know the exact terminology - on aircraft mags we called them 'P'-leads). Everything checked O.K. with a simple continuity/resistance check.I finally took the old mag coil unit and the new one in to work and checked them on a Wien bridge inductance tester. The old unit was open through the high-tension winding, so I didn't scrutinize it any further.

The NEW coil did indeed have continuity through the winding - a little TOO much; the leads going into and out of the coil were directly shorted, and the coil had no impedance at all. Went to the dealer and swapped it for a THIRD coil, installed it, and the bugger fired right up.

I should have remembered the classic admonition - never assume anything. Including, in this case, assuming that just because a part is brand-new, it can't be defective.

Thanks again for your help.