I have an 18.5 HP Briggs and Strattion on a Craftsman riding mower.

Lost oil and engine failed. I replaced several internal parts and everything looks great. No scoring in the cylinder, blah blah.

I put the engine all back together. I aligned the dots on the camshaft gear with the dot on the drive gear that is on the crank shaft. The fly wheel is aligned and has the key in it.

The engine turns over just fine but it won't start. Exhaust is coming out of the exhaust but instead of the engine sucking air/fuel in, it seems like I'm getting air blowing out of the engine through the carburetor. There is popping like it wants to run as well as the occasional backfire out of the carburetor. I have also tried spraying starter fluid into the air intake next to the carburetor so I don't think it's a fuel issue. I'm getting spark as evidenced by the popping and backfires. It seems like a timing issue but the fly wheel is in place and I aligned the dots on the camshaft gear when I assembled it. Any ideas?