I have B&S 15.5 engine on a Poulon 4000. Engine was running very rough and had no power. I changed out the carg and now only cranks and won' t start.

1. Make sure no water in gas tank.
2. Replace spark plug.
3. Be sure you are getting gas from the fuel line at the carb.
4. Check to be sure you are getting a good, blue spark. If not, then start looking at the multitude of safety electrical cut-off switches.
5. Check the irritating (my opinion) gas cut off at the bottom of the carb bowl to make sure it is not stuck and not letting gas up to the jet.

These are all the relatively simple things I can come up. Beyond that, I'd run a compression test.