SX/SXi/SXi Pro 93 Kawasaki 750sx - Exhaust Problems x10 (Back Firing)

I have a 93 750sx and I am having huge problems. Anyone help me out? This is the second time this has happened to me. I have a new engine that has no hours. I got the new engine and started it on the the stand hooked up to water. Everything ran great, no leaks or anything. Brought it to the water, as soon as the exhaust is submerged and started it back fires and blows the fiberglass box that the exhaust goes into. I replaced the box and the same thing happened when I put it in the water.

Any ideas as to why it keeps back firing and blowing the box apart? Anyone had the same problem, what caused it and how can i fix it? Winter is coming!!
 
Somehow, fuel is getting into the waterbox. Flooding the engine with too much choke, or a leaky n/s in the carb are the first things I can think of. If you need a new waterbox, PM me. I have a used 750sx waterbox in good shape I need to ditch.
 
Thanks I will keep that in mind. I need to find out why this is happening, don't want to go through a third water box..Thanks for your help I will look into it. I wil get back to a.s.a.p. How much are you looking to get for it?

Thanks
Kyle Ryan
 
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