the engine pulsates???

it starts fine, at idle or at full speed the engine pulsates in rpm. up then down. repeats over and over again. if i hold one of the levers on top of the carbureator in place it runs steady. when i let go it seems like a vacuum leak is pulling it back and forth, or the spring in the linkage for the kill switch assembly is not tight enough to prevent this from happening. any help would be much appreciated.