@linlee what exact model is your mower and your engine? That way we can do more research into this issue. There will be no compression with the plug removed :-) You would need a compression tester to check that out. This could be something as simple as a misadjusted or otherwise malfunctioning valve etc. So we need a bit more information.
Let’s start off with a few things for you to keep right now. Here is the parts manual for your 21B807-1169-B1 engine 21B8071169B1.pdf and here is the operators manual 275935. First thing you want to do is to connect the spark plug to the spark plug socket. Then get it to a point where the bottom part of the plug touches the metal. When you try and start the engine, do you get spark on the plug? If so we know that your ignition works. Tell us more about the noise you heard. Possible that you ran over something? Smoke coming from the air intake could be caused by premature timing.
some general information that we may need later on. I add it here so we do not have to go back to find the information
Armature Air Gap-0.010-0.014in
Spark Plug Gap-0.030in
Valve clearance
Intake 0.003-0.005in
Exhaust 0.005-0.007in
Model Series 210000 Bore-3.44in Stroke 2.27in 21 cu.in. (344cc) displacement
Okay, this should work better for you. It’s the system check part of the manual for your engine. When they are talking about spark plug tester, that is a tool that you use instead of grounding the plug to see if it works :-)
OHV-BRIGGS-&-STRATTON.pdf