Like Frank said, dot to dot and valve timing has to be correct unless you have a sheared crank gear key. For each revolution of the crank the pistons end up in the same place. They may be on a different stroke but the cam and valves determine that. If you installed an arc flywheel then make sure you have it set correctly on the hub and the bolts aren't loose. I have had flywheel keys shear after installing flywheels that looked like new and they sheared as soon as I fired it up. Pulled my hair out cause I knew it looked like new but when I pulled the nut off it was sheared, new key and ran fine. Frank asked a good question about what cam as well, the composite cams are bad about stripping but the solid can't.