Small Engines (Lawn Mowers, etc.): Rear tine tiller prob, rear tine tiller, dremel rotary tool

Question
Good morning,

Hoping you can help.  I have a Snapper rear-tine tiller with a Tecumseh engine on it (approx 7-8 years old).  The tiller was in storage for a few years and I recently brought it back out.  It's had a tune up including new plug, oil, & air filter.  Starts up like a dream.  But.....

I was using it a few weeks ago in my yard and it ran great for about 15 minutes or so.  Then all of a sudden it shut down and was difficult to restart.  When it DID start up, it only lasted about a minute and then shut down again.

So far here are the diagnostic checks I've done: breather, plug (even though it's brand new), and fuel cap.  Advice I've gotten from a local shop: they said if it ran for 15 minutes, then it can not be a carb problem.  They claimed the magneto may have been rusted over and (as a result) when it gets hot, it's no longer functions properly (until it cools down again).  So I located the magneto and they were correct with the rust part.  I cleaned the rust off the magneto with a spinning wire brush (looks brand new ), and did the best I could with the magents on the flywheel with a dremel rotary tool (they don't look AS clean, but still better than before).

Upon putting everything back together (fyi - I have no engine manual - just going by eye), I start it up (again - starts great), but NOW it shuts down after a minute!  It ran better BEFORE I cleaned the rust off.  Now it runs worse.

Do you have any suggestions on what might be wrong?  Did I possibly place the magneto back on wrong or is there a certain gap requirement between the magneto and flywheel?  Did I possibly ruin the magnet's polarity by using the dremel rotary tool?  If this isn't the problem, are there any other diagnostic checks I can do?  It's such a small engine and I can't imagine it being THAT difficult.

Thanks for any tips or advice you can give me.

Joe

Answer
The air gap from armature to flywheel should be 0.010 - 0.012".  Here is something you should try before you go any further.  Loosen the fuel cap off a little and see if it will continue to run.  If it does, you need to clean or replace the fuel cap because the vent is plugged.  If this doesn't help, you may have an ignition problem related to heat.  You wouldn't be able to affect a permanent magnet's polarity unless you broke it.  Quickly check for spark (jump a 1/8" gap to ground) after it dies.  If it has no spark, you have an ignition problem.  If this is an electronic ignition, there is no valid test for the armature and it will need to be replaced.  If it has spark, you will need to consider either a fuel or compression (problem with exhaust valve) issue.  Let me know what you find and ask a new question for more direction.  Hope this helps.