Not only do you need to make sure your chokes are sufficiently closed when they snap shut as you prime the carbs with the gas pedal (if there is any gas in your float bowls), you also have to make sure your choke pull-off is adjusted properly so that upon start up you don't just flood the engine with too rich a mixture once the engine does catch. Vacuum created once the engine catches has to be enough to pull open the chokes a bit to let enough air in so you still have a needed rich mixture, but not closed off from all air trying to get down the throat of the carbs. Manual has the specs on how far the choke should open when pull off is adjusted properly.