Re: johnson 200 no spark
If actually backfiring (out the exhaust... loudly!), I'd suspect that you've had the flywheel off and did not torque the flywheel nut to specifications (145 foot pounds) which would result in a sheared flywheel key, effectively throwing the engine out of time.
If on the other hand, it's a mild spitting back type of thing, that's usually due to a fouled carburetor, a restricted slow speed fuel passageway, or a wrong size (too large) idle air bleed jet, or leakage at a fuel related gasket somewhere.