backfiring out of carbs

Hi ive got a small ford 4cyl motor in one of my commerical mowers. it sat for a good amoutn of time so whn i went to start it again the other day all it did was backfire out of the carbs. i cheacked the valves all looked to bemoving up and down. cleand the carb. cleaned the maniflds and exchaust. i dont no what it could be any more. its ran off a distributer so im not sure to cheak the timing or not. if nay one know what to do or try please help

thanks joe