Welcome to BHQ !! Perform checks as mentioned. were pipes already on, or did you add them? Either way you need to check your jetting. Post up what you have in it, someone will point you in right direction. When you say that once it does start it screams, are you describing a great running motor or uncontrolable runaway engine? Comepletly disassemble carbs and re-clean, this means both jets. Make sure the pilot is clean, look at it very close, it's a very small hole and a tiny piece of crud will effect it . Make sure the bowl with the "bb" goes on left carb, slide ctuaway faces air box, choke crossover tube is installed between carbs. Best way to clean carbs is a bucket of Berrmans, let them soak a good while to break down any deposites that may be in small passages. Blow out all passages. Just spraying with a little carb cleaner will NOT get the job done. Post back with your results.