The most common cause is "Hydraulic lock" meaning too much fuel is getting in the engine when it is not running. (Remember liquids do not compress) To cure this ALWAYS empty the fuel thank when working on or storing the car. And always keep the carb higher than the fuel tank when it is not running. You can use a forceps (surgical clamp) to pinch off the line between runs but just using caution is all I do! You might also go to my web site [to third page] in the drop down menu there are lots of tips you may be able too use! Check it out and enjoy