Dual 392 engines can not share a single fuel pickup tube from gas tank

May have realized what I did not want to know, a single 3/8 " fuel line can not supply enough gas to twin v8 392 engines, without the farthest one bogging down due to fuel starvation.
I had been running both large v8 engines off port fuel tank using the cross over valve. While this works for slow speed running, it does not when the farthest engine gets rpm pushed up closer to 2000.

Does that make sense to you, that each engine must have its own fuel pickup line and not share a single fuel line? I plan to fillup the starboard tank and close the cross over valve and see what happens next time out. In my strange thinking, I had been wondering if the other engine had something wrong with it mechanically, but maybe not.

So even though I when I first got this boat, ran 2 tanks for 2 engines, I switched over to one tank for 2 engines and yet somehow never made the connection that could be a problem.

 

What type of fuel pump?

 

Electric Facet pumps

 

Ok, a couple of things to consider. One, where are the water separators/filters?...Upstream or downstream the pumps? Secondly, how is the valving arranged?...How many valves and fittings between the pump and the carbs? Finally, how is the tank pickup arranged?... Radiused or squared off ?...Sock?
Poor pickup and/or valve geometry can easily double or triple the pressure/flow loss in a given system. See the Crane Fluid Flow Handbook Crane - Flow of Fluids through Valves, Fittings & Pipe (Imperial Edition) https://www.academia.edu/36962581/Crane_Flow_of_Fluids_through_Valves_Fittings_and_Pipe_Imperial_Edition_

 

I put 5 gallons in the other tank today, will get it up to 20 gallons and take it out and let you know how it runs. Each tank is 150 gallons.
The main filters filter the fuel going to each engine before the pump. Pump legally has to be mounted on the engine to limit the pressure line length. I have a small screw in filter in the pump inlet and a small can filter in the output line to the carbs. The large main filters are really big. They are basically fuel filters made for fuel pumps at gas stations. So like 7 inchs tall and 4 inch width. They could be plumber with 1 1/4 inch pipe, But I have reducers to 1/4" inch pipe on them.

All lines are 3/8 except the output line from each pump is 5/16 to the carbs.

Tank pickup is 3/8 copper tube with a flared end, and sits about 1/4 " off the tank bottom, there is no sock or strainer on it. Of course each tank outlet has a shutoff valve.

The selector valve manifold is sitting in center of the boat with equal length lines from each tank

 

I happen to have a pic of my valve selector manifold, 2 inputs on each end rubber hose from tanks, center cross over valve, 4 outlets possible , one to gen, two to engines, and a forth for hooking up an emergency portable tank or for testing.

 

Here is one of my main filters, I have 2 of these upstream of pump.
Right now I have Purolators on these bases, not Wix.

 

I dare say you have checked the two fuel pumps for their being up to spec ? I guess you could swap them over, and if the engine that is suffering fuel starvation swaps with them, that will tell you a pump is not working properly.

 

The engine that bogs and surges is the furthest away from that tank. I am really thinking it is due to running of one pickup. Years ago before E10 gas, I did use both tanks, one for each engine. Then I filled with E10 and got gobsmacked with massive crud coming loose in the tanks, (boat is 1970) plus then I had a phase separation, so I cleaned both tanks, and settled on using just one tank for years, but of course I also started noticing the boat did not perform like it used to do. I was paranoid about putting gas into tanks and having twice as much trouble, so simply was using just the one tank and living with it as is. So maybe it was a fuel flow issue all along for the last few years. So lately been thinking of figuring out some things.

 

Is it flat-out illegal to have an in-line pump placed before the "fork in the road" ? It would not need to be much capacity, and could be rigged to run whenever the ignition was turned on.

 

It's a simple thing, the engines consume so much fuel per hour at full throttle, that fuel must be delivered to the carburators at a designed pressure for them to operate properly. The fuel moves trough a line with a fixed diameter so in order to move more fuel you must either increase the velocity of the fuel, or increase the fuel line diameter. A 3/8" line can supply gasoline for 1000HP if the pump is up to it.
Google "fuel line size" or similar wording and you will find a lot of explaining, this is a common topic for hot-rodders, racers, etc. right down to the hydraulic theory and calculations.

 

I dont have such a thing. My pump is mounted on the engine, the pump output goes to small can filter and right to carb, about 3 feet away...and it is rigged to run when oil pressure is there, if no oil pressure, then no pumping. I do have a prime push button switch to manually prime the carbs. And when the engine cranks the pump runs. But it wont just run if the ignition is on, that would be bad.

 

I may also change the needle valve seat to a larger diameter hole to allow more fuel to flow in easier with less pressure. Since these are quadrajets, they are easy to get in various sizes. Plus windowed ones are best. Certainly what is in there is old and stock from decades past. example use a large 0.135 seat
JET Needle and Seat Assemblies 201135 https://www.summitracing.com/parts/jet-201135

 

If there is a gauze filter on the fuel line at the engine, make sure it isn't clogged.

 

are they international 392s