I have a 2000 Imp Viper with twin L29 454's. I recently purchased the boat and took it out several times before I ran into any issues. My fuel gauge is apparently about a 1/4 off and I found this out the hard way. Got it towed to a gas dock and refueled and either motor would not start. I attempted to prime the motors with the key several times and could not get it re-started. I let the boat set while I ran home to grab my fuel pressure gauge. Hooked the gauge up and was only getting about 10 psi, eventually I was able to prime the system on both motors and get about 40-43psi to the test port on the fuel rail. Both motors started right up and would hold 35-38 psi. PHEW. All ok. Lesson learned... This weekend I went out Friday and the boat ran great. I took the boat out Saturday and got to a buddy's dock, hung-out for 3 hours and the boat would not start. Eventually I got my port motor to re-fire by working the throttle up down and priming the engine. After several attempts I got the starboard side to run. Once they quit stumbling the boat ran great at 55-60 all the way home without any issues. I figured maybe I had picked debris up from running the tank low and the fuel filters were my issue. I replaced the fuel/water filters before hitting the water on Sunday. Primed the motors and they started up and seemed to run good. Took the boat up about 10 miles, had lunch, went back to the boat and only the starboard motor would run. It did stumble at first but cleaned out. The port motor would start and stumble but not stay running... This time I brought my fuel pressure gauge along and the port motor was only at 10-15psi while priming it. Eventually I got rail pressure to 40-43 by priming with the key and working the throttles while trying to start. It finally started and ran for about 20 secs at 35psi. It started to stumble and the fuel pressure dropped from 35 all the way to 14 psi and it died. Limped it home on the port motor as it would not re-start. I would say I have bad fuel pumps but its hard to believe that both failed at the same time. Any advice would be appreciated. I have never replaced pumps on the L29 efi engines and I am guessing the bank 1 manifold has to be removed to do so? HELP!

Any help would be appreciated! I had a friend suggest the anti-siphon valves?

Sounds to me like vapor lock (VL). VL happens after the motor is all warmed up and folks stop for an hour or so, then it doesn't want to start (fuel is heat soaked). Moving the throttles doesn't do anything with a EFI motor, unless you go full throttle and that just shuts fuel off (used when motor is flooded).

Best way I know of clearing VL is attach pressure gauge and have a rag handy. If you pressure gauge has a bleed off port, press it when turning motor to run (don't start). Turn key back off, and back on a few times until you get a good stream of gas out. If you don't have a bleed port, press in on shrader valve to let air out