I just got the new engine in this 400 and it still has a small pop or sputter at idle.Sounds like an air leak but I cant find it.It did this with the old engine too.It has an HMF exhaust and every now and then it will back fire out the pipe it did this on the old engine too.Plug looked a little darker than I like but not black.Any recomendations or opinions needed.

 

Backfire and popping are symptoms of lean. Have you got a stock pilot jet, of did you step up to 40? Have you tried going richer on the mixture screw?
Check the rubber intake , between the engine and carb, it could be cracked, or loose.

 

When I put the pipe on the jets were what was called for but I think I will find him a Dynojet or K&N jet kit and go from there.I did richen the air screw up and got it better.