your timing has to be out . have you got the vacum hose onto the dizzy. have you timed it up with a light ? plugs ok ? no air leaks ? cheque with some carb cleaner when running. the engine will pick up quickly if there is a air gap and it will get sucked in and you will hear a different engine pitch sound for a secound or to.
bet its timed up wrong , all plug leads corrcetly fitted and to correct numbered plugs ?
i would start again with the timing light and double check everything before you start it again