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

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 rust doesn't harm the ignition system...it's a myth.

It could still be a carburetor problem...or a bad spark plug; I've had new plugs out of the wrapper cause the same problem.

Since it is a Tecumseh, it is prone to other odd problems.
I had one Tecumseh where the head bolts, on the exhaust side, loosen and cause the very same problem.  Fortunately, or unfortunately, depending on on how you look at it, it was my own mower.  I spent months trying to figure out the problem.  Similar to you, I checked the ignition system, kept thinking it must be a carb problem, until I was ready to throw the mower away.

A friend just happened to hold the trouble-light in the right spot and we saw tiny bubbles coming from the head gasket...torqued the head bolts to 200 inch pounds and it ran great.

So, replace the spark plug, set the coil gap to .010, make sure the intake gaskets are good and torque the head bolts to 200 inch pounds.

Let me know what happens.
Eric