Its sounds like you might have the wrong spark plugs installed and they are to cold, can you double check this please?
The only other reason the engine would do this is a lean backfire. The rebuild carburetor will be lean because that's the way the government makes them do it.
Maybe you should get the old one rebuilt or take it part and do it yourself
Let me know what you find
Saturday, June 18th, 2016 AT 9:54 AM