It's probably running a little lean, if it's what I'm used to hearing. Does it stop when it's under a load? It should.

No it doesn't............ I haven't put a real heavy load on it but have taken it for a spin around the yard a few times with the blades engaged. It's not a real loud noise but definately noticable.
I just noticed that the muffler is loose to the touch. It's under a guard and shroud so not going there tonight. Do you think that could be causing it??

Thanks for your help.

I'll check on adjusting the carb to make sure it's not running lean. Also thinking I'll need to check the valves............... Hope it's something simple.

I would tighten the muffler first. If your carb is adjustable, most aren't, turn the main jet in until it lightly seats and back out 1 1/2 turns. Then start the motor and let it warm up a bit. Turn the screw slightly in or out until it evens out the roughness while not under a load. Turn on the blades and see if any final adjustment is necessary. If it has an adjustable idle air screw, most don't, adjust that to one turn out. After the engine is warm, put the throttle to idle and quickly move it to high speed. It should accelerate smoothly to high speed. If not, slightly adjust the main jet and/or the idle air screw until it does. At high speed, adjust the main jet screw, under a load, outward until the engine begins to misfire and then back in until it smooths out.

Your engine probably has a fixed jet. You will have to remove them and clean them with spray carb cleaner. Some are a long tube that goes up through the center of the carb and some are just the bolt that holds on the fuel bowl. There are many types.

Regardless if the jet is adjustable or not, they all have some sort of small holes drilled in them that get varnished shut and must be cleaned. Never let gas set in the carb for a long period of time.........3 to 6 months or more.

Below are pictures of lawnmower jets, not necessarily yours, but they will give you an idea of what to look for.

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Thank you very much ! Great info, thanks for taking the time to share your knowledge.......................

Great Post Bayme . Picture #4 with the red arrow. was my intro into Tecumseh carb maintenance. Generally speaking, most old lawnmower carbs respond to to a gentle tear down helped along with spritzes of carb cleaner through a straw..

Not kidding here but unless this backfire is something that recently developed it may just be the nature of the beast (B&S). Two of my neighbors recently purchased mowers that came with B&S motors as well as what is call an auto throttle (no throttle lever on the mower at all). When they/I fired them up it was hard to not notice the lack of smooth ideling. They both seemed to misfire/backfire a bit.