You swapped your old flywheel onto the replacement engine? Very possible the key slipped out of the keyway and the ignition timing is off.
The method for placing the key into the keyway is often done improperly. The correct method is to set the flywheel in place on the crankshaft taper and THEN...you gently work the key into the keyway, lightly tapping it down to seat it. Some folks try to put the key into the keyway first and then slip the flywheel down over it. Not good because if the key slips down, it may fall out or worse, the flywheel can be tightened down off center. Go back and check your flywheel key. Even if the flywheel was already on the engine, the popping you hear is a good indication of "out of time spark".

I'll check that and get back.

thanks for the response!

took it apart and the key was sheared and the flywheel had kicked 25% around from where it should be. Picking up a new key tomorrow and we'll see what happens.

thanks,

That would certainly incite an engine to make a big bang out the exhaust. Just follow my instructions on installing the key properly. Briggs wants 100 ft lb of torque when tightening the flywheel nut. Under torquing can cause key failure later.

update: replaced the key tapped it in tight. started right up. ran great. let it go for 20 mins. stopped an started it a few times. a few hours later i came back, and it didn't start. I checked and the key sheared again! when it stopped before it kicked a little and I guess that was enough to shear the key. When I put the flywheel on the shaft, it doesn't seem to "lock in" like it should.....I'm going to try another key, and torque it correctly. is there anything else to consider? I'm thinking about putting foil around the key to tighten it up but I don't think that's the issue....can't believe how sensitive this is. but I will say when the engine is running it sounds better than it ever did!

thanks,

Make certain the taper surfaces of the crankshaft and the flywheel are clean and free from any "burrs" or score marks. DO NOT put any kind of lubricant or anti-seize on the taper surfaces. Clean the taper surfaces with a solvent type cleaner that will evaporate completely (spray carb cleaner or lacquer thinner on a rag). Use a genuine Briggs key.

put in the new key - it seemed to fit better. wiped down the shaft and flywheel with carb cleaner. The whole thing seemed to go together better. most importantly i torqued it to 100 ft lbs. too tired to start it tonite but I'm confident that this time it will work. thanks again. if it's more problems, i'll let you know.

ok, just did the whole yard. I'm good to go.

thanks for your advice - it was very helpful and encouraging when I was getting pretty frustrated!