On my 12 HP Briggs i run 1" for about 8 inches, then it goes to 2 1/2"
the rest of the way(about 32"). However when at full throttle, if i quickley
let off down to an iddle, it makes a few loud pops.

Is this because of back pressure, if so can i cut the pipe about 15" back and weld
a 2.5" washer with a 1 1/4 " hole in it, the weld the other half back on?
Also, this it the first time i have ever had this problem, 1-because i never use pipe this large, i only use 1-1.5" on my other racers.
2- Because all my pipes come out in front of the rear wheel, this one comes out behind the front left wheel.
This exhaust was an experiment, and sounds good idelling.
Andrew