Priming Pulsewidth - Upon power up, Speeduino will fire all injectors for this period of time. This pulse can be used to clear out air that may have entered the fuel lines or help the engine start easier by providing engine with fuel before it's cranked over.
Usually priming pulsewidth is kept short but especially with low vaporization fuels (e85 etc.) longer priming pulsewidths are required for easy starting of the engine. Regardless of what fuel is used, keep this value as low as possible to avoid flooding the engine. Start tuning from low priming pulsewidths and try longer pulsewidths untill the engine starts easiest. Usually lower engine temps require longer priming pulsewidths.