Hello all,
Need some expert advice. The 350 chevy engine with a quadrajet carb in my boat is experiencing a strange problem. It only happens above 1900 RPMS.

It backfires through the carb. It will idle all day long up to about 1800-1900 RPMs but then when you put a load on it the problem starts. We took it out last weekend with no issues, but then it did it again yesterday. All standard parts are new (plugs, wires, cap, rotor) rebuilt carb. It never fully shuts the engine off, it just starts cutting out until I bring it down to idle and then it is fine there

One side note. the crossover passages on the intake manifold that feeds warm air to the carb base have turned white on both sides of the carb.

Any advice would be much appreciated! thank you, Don