fuel returned to tank

[qz16]

I am in the process of restoring a 1973 240z.  It is powered by an L28 (twin 3 screw round top SU) with a 5 speed gear box.

The engine has been completely rebuilt and is now back in the car.  I was in the process of refinishing various parts including the fuel filler and cap so I did not want to use the fuel tank as a source.  Instead I supplied fuel from a 2 gallon jerry can, I do not know precisely how much fuel was in the can.  Thankfully the engine started almost immediately.  As you know there is some fiddling to be done when you don’t have an electrical harness and your remote starting while trying to adjust carbs and timing.  Well, it was really a very short time before I was out of gas.  Now if the car was running badly that would be one thing, but it wasn’t, it actually was running fairly smooth.  Never the less I was out of gas.  Okay, so I go down and get 2 more gallons.  I removed a plug and it was not sooty and had no smell so the mixture could not have been too rich. I ran the engine again but ran out of gas much sooner than I expected.  The fuel inlet line is a hardline that is U shaped with ports that connect each carb via a rubber hose and terminates with a rubber hose to the hardline that returns to the fuel tank.  This connection was in place and there was no sign of fuel leakage in the car, or under the car.  I decided to take the return line to a second jerry can.  Another two gallons. A huge amount of gas was being sucked out of the jerry can with the gas and a huge amount was being deposited into the return jerry can.  The amounts were not accurately measured but it seemed like at least 50% of the gas was being returned.

 

Sorry for taking the long way around, but here’s my question:  Is this normal behavior?  I am running a mechanical fuel pump.  It is new.  The SUs have been rebuilt and are working quite well.  The fuel lines are either new or clear.  The fuel filter is new.

Is the mechanical fuel pump supplying more gal/min than it should?  Because of the physics of the setup I don’t think that the mechanical fuel pump is supposed to respond to pressure caused by the float bowls becoming full and the needle valves shutting the inlet to the bowl.  The carbs are not leaking.

 

I would appreciate any insight that you might have.

Thank you.

[Captain Obvious]

Two gallons of gas should provide you quite a bit of fiddling time. Hours maybe? And to answer one of your questions... Yes, the fuel pump is always supposed to supply more fuel to the rail than the carbs need. There is supposed to be a small restriction in the return line of the rail that acts as a rudimentary fuel pressure regulator and allows a small amount of fuel pressure to build in the rail. If you do not have that orifice in place, you'll pump more fuel (in a circle) than necessary and maybe not have enough pressure in the rail to push fuel into the carbs.

So I'm not exactly sure what it is you're describing here... When you were running out of gas while fiddling, I know you were pulling fuel out of the jerry can and sending it to the carbs. But how did you have the return line connected? Were you returning to the same jerry can, or were you returning the excess to the main fuel tank?

Edited November 9, 2018 by Captain Obvious

[Zed Head]

Pretty sure that the mechanical pumps also self-regulate pressure.  So the restriction in the return line that CO describes really serves two functions - to allow pressure to build to the level of the fuel pump pressure relief, and to restrict flow so that the fuel isn't flowing in a useless circle, absorbing engine bay heat.      I think I just changed CO's "regulator" definition to a "pressure builder" definition.

Looks like you're missing the restriction, in the return line, the tiny orifice.  You have no flow control.

 

My description of pressure regulation in the pump could be way off.  I will open an FSM now to see if I can verify...

[Zed Head]

I think that this "static" test is with the outlet completely blocked.  "Clipped".  Not super clear.

[Zed Head]

Here's why you're having problems.  The pump will pump a lot if the hose end is open.  .42 gallons per minute at 1000 RPM.

Edited November 9, 2018 by Zed Head

[Zed Head]

Kind of got carried away there....

[qz16]

First of all thanks for the quick responses. 

Captain Obvious, I was pulling fuel from one jerry can and had the return line connected and emptying into a second jerry can.  

Both of you are correct I am missing the restrictor on the return line.  As it turns out the stock restrictor is quite small which would only allow for a very small trickle being returned to the tank.  I have a choice I can either replace the fuel rail, or build a restrictor for the end of the rail that I have.  I think I have an old jet that might work.  This will certainly cut down on the volume of fuel being returned to the tank.  If it is still too much then I can always replace the fuel rail with a used one.

Zed Head - thanks for taking the time to post the test procedure.  Like you I thought that the fuel pump has some relief mechanism that would be triggered by sufficient back pressure and cause the pump to stop sucking gas from the tank.  I have been told that there is no distinct pressure relief. As I understand it when back pressure is higher than the 3-4 lbs the mechanical pump simply can't overcome the back pressure, but the pump continues to try to suck fuel into the rail; it just will not succeed until the back pressure is lower than the output of the pump.  The end of the return line continuously weeps fuel and if the pressure in the fuel rail decreases below 3-4 lbs then the mechanical pump can push more into the rail.

 

Once again thanks for the help and the education.

 

Regards,

ron

[Captain Obvious]

Yeah, with no restriction in the line, I can easily see the pump being able to move fuel from one jerry can to another pretty quickly. That's good news.... Your fuel pump works!    The carbs were consuming a tiny bit of that fuel along the way, but most of it was just being pumped from one can to the other.

I also do not believe there is an "on purpose" fuel pressure relief built into the mechanical fuel pumps.  They have a pair of check valves that just won't work without enough pressure differential across them and the pump will loose efficiency. I haven't been inside one of the original pumps, but that's my read.

The "real" solution to the problem would be to install a fuel pressure regulator. That can be in the form of a restriction in the return line, or can be something more glamorous like a real regulator that works at carb pressures. In any event, it sounds like you're well on your way to a solution. Good luck!