Take out the decompressor and check it it is very easy to do. suck on the end of the decomp. (the small black end of course and see if the valve shuts. It sould require next to no effort be will take a few seconds to pull in. If you are still worried replace the decomp for a spark plug and place the end of the vaccumm hose on the end of the plug to block of the hose so an air leak is not created.Test run the engine and see if it solves the problem. Ensure the decomp is replace after the quick test as I dont recomend this as a solution to you problem.
