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1986 351w Econoline 350 22' RV, 66,000 miles. Dual tanks, smog deleted, new OEM 4V carb from Rock Auto, new mechanical fuel pump, new fuel filters (<1000 miles)
I have fixed this problem but I don't know why what I did fixed it. After running the RV for more than 30 minutes, starting with a cold engine, the RV will start to sputter and stall when going up hills at 45 MPH or when climbing on the interstate at 65 MPH. Typically it started when my foot was to the floor on the accelerator pedal, but this does not happen when it is cold. It will not recover from this condition, even on flat roads, until I shut it down and let it rest for a few hours. I was on a 1000 mile trip and the first 400 miles were fine, but I was leaving NY in January heading South and the air was very cold until VA when the problem started. Also replaced the coil in an attempt to fix. At one shop, while having the problem the mechanic told me there was no fuel coming to the carb when he looked into the carb and manually moved the throttle. His theory was clogged fuel filters, but he didn't have time to fix it for me that day.

We knew there was a problem with the Fuel Tank Selector valve, so we by-passed the front tank directly to the engine. This did not resolve the issue. We ran 93 octane fuel, but while we had more power when it was cold, it didn't fix the issue. We parked the RV in South Carolina and went back last week with the tools to by-pass the whole fuel system with a 12 gallon marine fuel tank, electric fuel pump, wix fuel filters, fuel line, clamps, etc.

We first started South driving the RV without any fixes and after 40 miles it started to act up then we pulled over to fix. We added an Edelbrock 7psi electric fuel pump in line off of the front tank before the mechanical fuel pump. The idea was to connect the marine fuel tank to the inlet of this to test if the tank pick-up was clogged, but on a whim, I put the electric inline with the front tank and gave it a test. The first 40 miles went fine. The next 40 miles went fine. We got onto the interstate and were able to go 70 MPH! - It never was able to do that before unless going down-hill with full pedal. We went another 400 miles at 75 MPH, up hills and with pedal to spare. We drove it straight to Florida without issue. When I shut off the electric fuel pump, the RV does just fine around town at slow speeds.

Why does running both an electric fuel pump back at the tanks and the mechanical pump fix this? This model does not have fuel pumps in the tanks. I can't find any fuses or references to fuel pumps in the tanks. Maybe it does and they failed? Would there be an E-350 stripped chassis for RV's that used both Electric pumps in the tanks AND a mechanical on the engine? This unit has fuel vapor return lines to the tanks and an orifice on the output of the mechanical fuel pump (1 intake, 2 outputs). Added pressure to the intake of the mechanical pump causes fuel circulation and cooling to avoid vapor lock? Maybe the mechanical pump wasn't designed for a 4V carburetor?
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