I'm New here 1st post and had same Problem. I replaced spark plug,fuel line, filter, syphoned gas and took off petcock and cleaned it with gunk carb cleaner. then tried new gas and got nothing. Then I pulled & cleaned carburetor unassembled it, set screws /jets,float and weight cleaned throttle cable slide needle. I also carefully looked at the reeds and reinstalled with a little "gas tolerant gasket seal" very little you don't want to clog the reeds with gasket seal. I didn't have new gaskets for that but wanted to make sure I didn't lose Vacuum from there.
That was last night and again No start. Went to bed woke up and I wasn't ready to admit defeat. So I retraced my steps two things 1# I replaced the vacuum line with larger diameter hose so I put old one back on. 2# I pulled off throttle slide and properly installed slide with indented side toward the front set screws and the slit side toward back of carb. I had installed it backward.I also adjusted throttle cable. It started right up and Hums! I was just about to give up Sheesh?