Mower Engine making exploding sounds

I just got a MTD garden tractor/mower. It hasn't been used for several years, so it needed a new gas line, and some tires replaced. But the 12.5 HP Briggs engine fired right up.
This engine is missing a muffler. It just has a piece of pipe sticking out about 4". Although the engine runs well, there are blue flames coming out of that pipe, which shoot out as far almost a foot. I'd guess that explains why the muffler is gone.
But what puzzles me, is that several times per minute it sounds like a gun is fired, (explosive sound) and an extra big yellowish flame shoots out. This does not affect the engine speed, and it keeps running well. But it is unusual. What could cause this? It almost seems like a valve is leaking, but I'd think that would also cause the engine to run poorly.
One thing I have noted is that after I have run this mower for about an hour, these explosions have become much few per minute, but they still occur.
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Jerry,
You need to replace the missing muffler and do a tune up. Clean replace the spark plug and gap it. Go on-line and be sure the right plug is being used. While on-line get the valve clearance specs. Adjust the valves. Replace the oil. Put in fresh gas. Once you've done all of that you can diagnose any problems that occur.
Dave M.
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David L. Martel wrote:

Back firing? Result of too rich mixture. Carb. and/or timing issue?
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I can try to lean the mixture via the carb screws. As far as timing, I dont recall ever seeing any sort of timing adjustment on small 4 cycle engines like this. This is a 12.5 HP so it may have more to it than the common 3 or 3.5 HP mower engines. I've worked on plenty of those 3 to 5 HP engines, and the only "timing" adjustment was to make sure the coil was the thickness of a sheet of newspaper from the flywheel, and of course point gap. Those must be correct, but that's not a "timing adjustment".
And from tearing apart many of those 3 to 5 HP engines, I dont recall any way to adjust the valves either (aside from replacement of the valves or the cam).
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On Friday, June 26, 2015 at 12:44:55 PM UTC-5, snipped-for-privacy@spamblocked.com wrote:

Timing would refer to a partially sheared fly key...and if you re-seat the valves (lapping) you have to re-gap...
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wrote:

I have heard that cool air is sucked in by not having the muffler on and can warp the exhaust valve or cause other problems by having cool air sucked in.
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On Fri, 26 Jun 2015 03:16:35 -0500, snipped-for-privacy@spamblocked.com wrote:

Sounds like an ignition failure
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Unburned fuel is exploding in the hot pipe...should you be messing with this considering your views on chainsaws?
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