Check the copper ground wire that is attached to the coil. I was having the same problem with my engine bogging at the start and it took forever to get up to speed. I rebuilt the carb twice, I changed the carb, I changed the air filter, I messed with the needle settings, I was about to start adjusting the clutch when I finally noticed that ground wire was very loose. So I removed the coil, tightened it down, and the next time out the engine ran awesome. Give it a shot and good luck.