I have a three year old sears craftsman tractor with a 19.5 hp briggs and stratton engine. It was running rough so I replaced the spark plug. It turned over great but no spark.
I replaced the magnetto and used the enclosed card to set the gap. I drained the fuel tank and replaced it with fresh gas. I cleaned out the ignition harness and any other wire to wire connections I can find. It turns over great and compression sounds good but still no spark. I pulled the spark plug and it had fuel on it. When I pulled the plastic engine cover and cranked it, puffs of gas came back out of the carb.
Help.

Was the replacment magneto a Magnetron? If so, did you install it properly, it should be marked as to which side goes toward the cylinder head. In other words, you can put it on up side down, still get a spark, but the timing will be off.

Ask me how I know this.

I did. It says this side up and it is identical to the one that came with the engine, with the ground wire plug on the underside.

If you are getting puffs of smoke you must be getting a spark. Timing of spark must be off. The replacement I received came with instructions to connect ground spade to engine head. That would be correct if the connection at the head is isolated from head but only if it is isolated. Was not the case for me.

Suggest you check, I thought I had installed properly but had not. If all else fails, turn it over to see what happens.

P.S.
You said puff of gas, I am assuming you mean puffs of smoke.

What's part number of mag?

Hi,
Suggest checking the flywheel key. Also, while in there, unplug the ground wire to the armature and try to see if it is grounded elsewhere.
Peace,
Clarke