Small Engines (Lawn Mowers, etc.): Snowblower, crank shaft, bolens snowblower

Question
I have not removed the points, the coil, or the flywheel key.  I took it to the shop last year, I was told it worked fine for last winter.  I haven't touched it since.  All I did was replace the breather assembly, and the seal around the crank shaft.  

It is a Bolens 524 with a 5 HP Techumseh HS-50 #67073C.
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Followup To
Question -
I checked the flywheel key, looks fine.  Checked the Valve Overlap, looks good.  However, the gap between the magneto(?) and the inside of the flywheel is approx 1/2".  That seems to much to me, I wouldn't think it would fire though and I have a good spark.  Also, the gap between the points is .017, seems to operate good.
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Followup To
Question -
I have an old Bolens Snowblower with a 5HP Techumseh Engine.  The seals were bad in it and leaking oil, so I replaced the seals.  When I took it apart it was running fine.  When I was putting it back together the cam gear fell out, I allined the dot and hash on each gear and put it all back together.  It runs but back fires out the carb while running and runs really rough.  From what I know and what I've read it seems like the timing is not allined correctly.  Do you have any advice?
Answer -
Have you checked the flywheel key?

You can check the timing by removing the cylinder head.  Of course you will have to replace the head gasket and re-torque the head.  This may be quicker than removing the engine and checking the timeing marks.   With the head off, rotate the flywheel until the piston is at top dead center (TDC).  You should be able to see both valves just barely open at TDC.  This is know as valve overlap and all engines have this behavior.  You may have to rotate the engine a few times to see the valve overlap or you may use a valve lapper or you finger to see if the valves rotate at TDC (this means they are slightly open).

Check the flywheel key first, then the mechanical timing.
Let me know what you find.
Eric

Answer -
Did you ever take the magneto/coil off?  Replace the flywheel key?  Have the points out?

Some of those older engines have to be timed with a dial indicator.

What's the engine model number and serial number?
Let me know.
Eric

Answer
Try removing the points, re-installing and setting the gap to .019, a snug .019".  Sometimes a thousandths or two makes a difference.

Also, have you inspected the carburetor bowl for sediment deposits.  Does the carb have any adjustments?  Is the carburetor a diaphragm or float bowl type?  

Let me know about the carb and points.  The backfiring and rough running could be caused by a lean carb.

Keep trying. I had a 7 hp Tecumseh that I almost threw away because of rough running.  After about a month of trial and errors I found some head bolts that were barely loose and would loosen even more when the engine got hot causing it to run worse.  Finally got it fix and it runs great.

Let me know about the carb and points.
Eric