SU carbs will not sustain Heavy load

[Captain Obvious]

I agree. If your floats are correct at idle, then setting them extra-high to account for the fuel usage at high load is just a Band-Aid and isn't addressing the root issue. And besides... All that will do is move the "time x load product" (a technical term I just made up) up a little bit. By that, I mean you will still be pulling fuel out of the bowls faster than you are putting it in, and as long as that's the case, you can eventually run them dry.

I know you know this already, but what you really, really need to do is make sure your fuel supply system has enough capacity to be able to put the fuel in faster than you could possibly take it out.

So let me make sure I understand your gas jar picture... You dumped the rear carb into the jar first and marked a line. Then you dumped the front carb on top of the rear and marked a higher line, right? If that's the case, then the important things to note are:

1) Good to see that the total seems to be about twice as what you got from just the rear. In theory, that means the two are about the same (as they should be). And...

2) The volumes under load are much lower than when at idle. You have sucked the bowl level down below where the venturi can pull fuel and you go way lean.

So tell us some more about the fuel delivery system? What type(s) of pumps? Where are they located? And you didn't answer anything about a pressure regulator. You had previously mentioned that all three of your fuel pump combos show 3.5-6 PSI going into the rail. Do you have a fuel pressure regulator installed somewhere? If so, how about a couple pics?

Edited May 30, 2018 by Captain Obvious
Over-agressive spell checker - I typed venturi, and I really meant "venturi" and not "venture"

[Captain Obvious]

Oh, and great testing BTW. I love the jar!  Somewhere around here I've got pics that look just like that from a number of years ago! 

[240260280]

If the fuel rail is a standard 240z one: Does the fuel rail still have the small orifice on its exit so that most fuel goes to the bowls rather than return to the tank?

Try running with just the mechanical pump and the return line dead-headed.

Edited May 30, 2018 by 240260280

[MY1PATH]

No regulator, just the original small orifice restrictor on the rail return i believe it has a spring ball in it to prevent drain back when psi=0.

More detail on pumps...

Mech: std location (with STD insulator thickness) max 6psi but pulses a lot below 1500rpm (to be expected)

Elec: Airtex E8251 solid state diaphragm, rated 30gph for 2.5 to 4.5 psi applications. Located where factory elec pump bracket used to be. Shows about 3.5psi at idle and neutral rev too 6k.

Noted driving differences:

Mech only: slow starts after sitting for a few days.

Mech and elec: faster starts after sitting, low rpm psi pulse doesn't dip as low.

Elec only: faster starts after sitting, seems to be enough pump despite the lower peak pressure.

Using the electric pump both combined and by itself where my most recent attempts before this thread to combat the leaning issue. 

[240260280]

That is all I could think of.

 

The mechanical pump has a high pressure limit so it should work on its own with the return line dead headed.  Not sure how high the pressure will go as I never tested this but it may be a way to rule out short cycle of fuel bypassing the bowls.

[siteunseen]

Could he clamp off the return hose and see what happens? Simple way to see if they're getting bypassed, the float bowls I mean.

[MY1PATH]

8 hours ago, 240260280 said:

Try running with just the mechanical pump and the return line dead-headed.

I don't want to deadhead the rail (permanently) because that increases the fuel temperature.  I'm sure my elec pump would peak at 4.5psi if I did but we get 115F days out here in the summer.

 

Edited May 30, 2018 by MY1PATH

[MY1PATH]

Today I pulled all the hoses off the rail, I noticed a tiny crumbly piece what I think was black rubber sticking out of the return orifice. I fished around the orifice area with some wire (turns out there is no check valve) and blew compressed air through all the fittings. Then I drug a cloth mop through the rail from the rear fitting to the inlet, from the front fitting to the inlet and vise versa. I found a few more tiny crumbly bits (not pictured) and some normal white/tan fuel system residue. The mop drug smooth the whole way. 

Because of the black crumbly bits I imagine that there is some remnants of old rotted hose in the chasis hardline that may be causing a restriction. Maybe Friday I'll run the mop through the 3 chassis hardlines(feed, return & vent) for good measure. If that's not it, I guess the tank is coming out again...


I used compressed air to launch my string through the rail to drag my mop I use this trick to clean out aircraft tubing after production.

Edited May 31, 2018 by MY1PATH
Pic via Mobile Txt via PC

[Captain Obvious]

Like the string and the angioplasty treatment.      And yeah, I was going to tell you there wasn't any spring or ball on the stock orifice. Just an orifice. I'm no fuel pump expert, but i's clear there is a capacity issue somewhere. I hope it isn't a problem with the tank. 

I did a little looking into the Airtex E8251, and there are plenty of people out there who seem to think it's fine to run in series with the stock mechanical pump. 30 gph is one-half gallon per minute. How much fuel do you think you're putting through the rail in a thirty second WOT foray? And how much of that goes through the orifice and back to the tank?

You could clamp off the return line and run dead headed just to see if the WOT problem goes away. Although, if you had a goober partially blocking off the stock orifice, you may have been running close to dead headed already. But the goober does bring up a question... What is the fuel pressure in the rail with the goober removed? Do you have a permanently mounted pressure gauge?

PS - Out of curiosity... Anyone know what does it mean when they say that pump is "Solid State"?  I'm guessing they mean the driver circuit for the solenoid pump does not contain any vacuum tubes?

[MY1PATH]

1 hour ago, Captain Obvious said:

Like the string and the angioplasty treatment.      And yeah, I was going to tell you there wasn't any spring or ball on the stock orifice. Just an orifice. I'm no fuel pump expert, but i's clear there is a capacity issue somewhere. I hope it isn't a problem with the tank. 

I did a little looking into the Airtex E8251, and there are plenty of people out there who seem to think it's fine to run in series with the stock mechanical pump. 30 gph is one-half gallon per minute. How much fuel do you think you're putting through the rail in a thirty second WOT foray? And how much of that goes through the orifice and back to the tank?

You could clamp off the return line and run dead headed just to see if the WOT problem goes away. Although, if you had a goober partially blocking off the stock orifice, you may have been running close to dead headed already. But the goober does bring up a question... What is the fuel pressure in the rail with the goober removed? Do you have a permanently mounted pressure gauge?

PS - Out of curiosity... Anyone know what does it mean when they say that pump is "Solid State"?  I'm guessing they mean the driver circuit for the solenoid pump does not contain any vacuum tubes?

I think the little bits I found were too small and crumbly to block the orifice unless there was a large number of them are a larger more solid piece somewhere else. The wire I ran through the orifice was corkscrew twisted so I could feel if there were any larger obstructions. It's just a soldered on cap for the orifice so if need be it could have been removed and reinstalled for more extensive cleaning.

Yes I said it looks like it flows "plenty"  but I did not take any specific measurement. I doubt any  of L6 could drink 1/2 gallon in a minute (refill every 20 minutes at the track?) so even if 30 GPH is probably its freeflow rate its still plenty if its even half that @ 3.5-4.5 psi.

PS- this is my understanding...
Solid state is commonly misunderstood as "no moving parts" which in many classes is close. What it really means is "no moving electrical parts"...
In terms of a diaphragm pump it means that the magnetic coil that moves the diaphragm board controlled electronically pulsed via solid state controller instead of using an analog relay-like mechanism that physically moves electrical contacts to make and break the circuit for the magnetic pulse.
In rotary pumps you may instead see the term "brushless motor". But again (more or less) no moving electrical parts, a circuit board dictates which set of coils around the housing are powered to  advance the permanent magnets on the shaft and at what speed. (although some still use an analog hall effect sensor to relay the motors position)

[Captain Obvious]

Couple observations... First off, Airtex's website sucks. Bad.

Other thoughts?

You have suggested a couple times that the pump is a diaphragm type. I haven't seen any pics of the insides of that Airtex pump, but Airtex says it is a "Type: Solenoid" (not diaphragm). From that suggestion, I suspect it is a reciprocating piston where the piston is magnetically driven (solenoid). That is how the original 240/260 pump worked, and (based on the description and mechanical shape) I suspect the Airtex is the same. I do not believe there are any diaphragms inside that assembly.

The reference to "solid state" exists in a number of places on-line, but curiously enough, not as part of Airtex's own description of the pump. So it seems that part of the description was either made up by someone other than Airtex, or it used to be called that by Airtex some time in the past, but is not anymore.

Just so I don't have to keep hunting for them on Airtex's crappy website, here are a few (mostly useless) documents from Airtex:
http://test.showmetheparts.us/BIN/documents/Airtex/Universal_Fuel_Pump_TSB.pdf
http://test.showmetheparts.us/BIN/documents/Airtex/X93273.pdf

(Note that if you take a look at the "Make the final check" section of this second document, they talk about potential noise caused by a reciprocating piston).

And a couple parting thoughts about the pumping capacity of that pump... They say it will do 30 gph, but they say that is the "Freel flow" (GPH (Free Flow): 30 ). I'm not sure what they mean by "free flow". I'm guessing that it either means  1) what the pump will provide if it is pumping directly into an open container like a bucket with zero head pressure to fight against. Or 2) what the pump will allow to be pulled through it if the pump is off.

In any event, I agree that it is difficult to believe that an L6 could drink 1/2 gallon in a minute, but don't forget that a lot of that flow is simply going through the restriction orifice and back to the tank. The question is when you shunt that flow back to the tank, is it conceivable to believe that the L6 can drink the remainder (and want more).

[Captain Obvious]

Forgot... You mentioned in your original post that you tried mechanical pump only, electric only, and Mech+Electric combined.

When you were trying the mechanical pump only, was the electric pump in the circuit at all, or was it bypassed? In other words, were you pulling through it even though it was not running, or was it completely non-existent?

I'm guessing it was non-existent, and you installed it in an effort to alleviate the problem we are discussing. But I figured I would check just to be sure.