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351 wont run properly

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The boat is a 1977 century raven 250 with a 351 Mercruiser. When we leave the dock and motor out to our spot the boat usually runs fine. During trolling speed the boat runs ok. When we go to throttle up at the end of fishing the boat doesn't want to run well. It misses really bad and will back fire horrible if you try to push it past about ten mph. We have installed new plugs, wires, cap, points and condenser. We also replace the fuel filter. A professional mechanic put a new coil in it and a different thermostat. He also tightened the timing chain which was slack. We took it back out and it did the same thing. Now I am starting to think its a fuel issue. I think most likely the line is pinching off because the tank isn't venting properly, or there is something in the tank clogging the fuel passage through the line. The reason I believe this is because every possible electronic device has been replaced on the engine and we still have the same symptonms. Also the reason I think its a fuel issue is because while the engine is running rough and you try to give it more throttle the engines carb  is making a loud sucking sound like its getting air but no fuel. Its an old boat that looks pretty good. My grandpa bought it brand new in 77 from the factory in Manistee and fished it for years out of the port  of Pentwater. We are not ready to throw in the towel on this boat, mostly because it has been part of our family since 77. Any suggestions are welcome. Just trying to keep the BER-BEN on the water. Thanks

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Check the screen at the fuel pick up in the tank.  My boat has a screen there, a fuel filter and another screen at the carb.  I had this problem (but no backfiring) and resolved it cleaning the 2 screens and a new fuel filter. 

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