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Hello, my name is***** will assist you.
you are correct on the schrader valves. One is fuel and the other is air. Speaking of them, have you actually tested the pressures on both sides to see what it is getting up to?
Have you tested both electric pumps on both engines. Since you have twins, you will have a total of 4 electric pumps.
You should be able to rent fuel pressure testing gauge from an auto parts store. Make sure that it can read at least 100 PSI. The fuel pressure should read at 90 and the air pressure should be 80 when engine is running. Fuel pressure will go to 40 at only key up and then it will fall off when the pumps turn off. It should hold around 20 PSI or so.