Mower won’t start after complete engine rebuild

Ok, this is going to be lengthy. I have completely rebuilt my Briggs and Stratton model 31C707-0026-G. It all started with a broken piston Rod arm. I figured while I was rebuilding and the engine was old I replaced multiple parts. (Cam shaft, cam shaft timing gear, tappet valves, all gaskets) I just put everything back together, started it up with out the spark plug, turned the ignition key and the flywheel spins freely. Put the spark plug in, tried to turn it over and the fly wheel doesn’t want to turn. There is a lot of resistance. The flywheel just doesn’t want to turn. I did a little research and found that it could be a valve issue. I have a Briggs and Stratton service manual and adjusted the rocker valves to the required specs. Put the valve cover back on, cranked it over and had the same results. I now have back firing out of the carb and exhaust. It’s also very hard to turn over to the point where my starter was smoking, no issues with the battery and the starter is fine. After adjusting the valves, the flywheel will spin much better when I have the throttle cable to the lowest setting. When it’s at max, flywheel barely turns. I am suspecting that something is messed up with the timing in the cam shaft even though I lined the marks up. Does anyone have any suggestions?

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Are you aware 4-stroke engine crankshaft rotation requires 720 degrees of crankshaft rotation for spark to occur? The diy method of mechanical timing is to insert a straw or length of dowel rod into cylinder #1 and mark the straw/dowel when the piston is closest to the spark plug hole. Mark it again with piston bottoming. Knowing when piston is all the way up, rotate the crankshaft until top dead center occurs and examine examine timing marks. Either they're not aligned and requires another 180 degrees of crankshaft rotation for timing marks to align or you misaligned timing. At TDC, intake and exhaust valves are closed; and turning the engine should show the exhaust valve begin to open. If you aren't familiar with 4-stroke cycles, search for sequencing of 4-stroke engines.