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MY1PATH

SU carbs will not sustain Heavy load

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I have run out of ideas on this, car idles and cruises good, single gear WOT to redline 6k+ rpm is fine. Try to do it again and it cuts out. I have installed a wideband O2 to see what's going on and it's running lean but not right away.
Here are some examples, all under WOT:
1st gear to 6k, AFR avg 12:1  -> 2nd gear starts cutting out at about 5k, AFR 18+:1
1st gear to 4.5k, AFR avg 12:1  -> 2st gear to 4.5k, AFR avg 12:1  -> 3rd gear cuts out almost instantly(3k?) 16~18+:1

Almost as if I'm sucking the carbs dry...

Under half throttle my only issue is that it's not as fast 😂. A balance between mid high throttle and earlier shifting can be found to get through the gears semi-quickly but it's not compromise want to keep making.
Setup: L28 9.75:1 (flat top pistons + shaved P90 head, mild port work), L26 "C" cam, 1972? SU carbs, 3-2-1 header, 2.25 exhaust, 280zx "match box" ignition, no smog equipment.
What else has been done (fuel system):
Tank removed and cleaned, inlet outlet fittings fished with wire brush on string.
Fuel hardlines blasted with compressed air from both ends (repeatedly, each time seemingly free of obstruction)
YES fuel filters have been changed regularly, if not unnecessarily.
Fuel hardlines around the valve cover were compressed air blasted before and after electrolysis cleaning and powdercoating.
There are no filter screens in the banjo fittings on these carbs to be cleaned and the banjos and banjo bolts are free of obstruction as well as the passage to the float needle, the needle valve, spigot to the  nozzle and nozzle itself. (checked AGAIN yesterday)
Floats were set using view through clear hose method under pump pressure (I can check it again, has been a long time)
Problem occurs with mech fuel pump, electric fuel pump and Mech+Electric combined. All 3 combos show 3.5-6 PSI going into the rail and pulling a hose shows plenty of flow.

That's pretty much the fuel system. Please tell me if I'm missing something. I don't recall these issues on the L26 but have been one of those things I've been putting up with for a while on my L28.
Any ideas? What's the limit on SU carbs? Have I gone past that? 
Contemplating higher lift cam, If I'm not over the limit will that put me over?

 

Edited by MY1PATH

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The SU's have a pretty high limit, so that's not my first thought. there are guys racing with them. My first thought is you have a restriction somewhere. If you're comfortable with rigging some kind of temporary fuel source and see if the problem goes away. I have rigged up systems with a 1 gallon fuel can. If it goes away, work your way towards the tank to id the problem. If it doesn't go away, then I would assume it's in the carbs, maybe the fuel level is set too low or the banjo filters are dirty. Some testing would really help to isolate it.

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3 hours ago, Patcon said:

The SU's have a pretty high limit, so that's not my first thought. there are guys racing with them. My first thought is you have a restriction somewhere. If you're comfortable with rigging some kind of temporary fuel source and see if the problem goes away. I have rigged up systems with a 1 gallon fuel can. If it goes away, work your way towards the tank to id the problem. If it doesn't go away, then I would assume it's in the carbs, maybe the fuel level is set too low or the banjo filters are dirty. Some testing would really help to isolate it.

No screens in my banjos. But a remote fuel source could help, thanks.

I did expand my thinking to not just liquid fuel... Reading on pistion springs 1/3 the way down... https://zparts.com/index.php/resources/su-carburetors-explained/
When the carbs were on the L26 there was a THICK neoprene washer stretched over the base of the dashpot tube in the piston. I do recall it kept the piston from raising all the way up into the bore cutting off the top 0.25 inch or so...
When I went to the L28 I replaced this with a thinner nylon washer that fit close but slid on freely. Two things are happening here, this piston is able to raise higher and the spring preload is a little bit lower. I imagine both could contribute to a drop in vacuum across the nozzle thus reducing fuel pulled into the engine. Or even not letting the piston drop fast enough to the lower engine speed after a shift. 
That's one more "new" thing I can look into when I get back to the car this week but I'm open to other ideas as well.

Edited by MY1PATH

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Sounds like an annoying issue, and I agree with your thoughts that it sure sounds like you're sucking the bowls dry. First thing I would do is try to confirm that's what is going on.

Assuming you don't have the early carbs with the bowl drain holes,... I would let the engine idle for a minute or so to make sure everything was stable. Then shut off the engine and drain each bowl into some sort of small graduated container, and measure how much fuel was in each one.

Then I would make your WOT run until your O2 sensor went lean and then quickly kill the motor (coast to a stop) and then do the same fuel volume measurement again. In theory, if your fuel supply system was able to keep up under high load conditions, you should have about the same amount of fuel as when you tested it at idle.

If you get very little out of the bowls, then you have at confirmed the root problem of fuel starvation.

You could do the same kind of test with the clear tube "Teed" into the bowl outlet, but I'm not sure I'd want to go driving around like that .

Other thoughts?

I once had similar problems, and traced it to debris partially clogging the needle valve. I was running from a small engine compartment fuel source and I was frequently wiping it clean and dry with paper towels. I was not running any banjo filters either, and the fibers from the paper towels were building up at the needle valve until they created restrictions. I know you said you checked your needle valves, but thought I would throw that out there anyway.

And lastly, I don't think it has anything to do with a washer under the suction piston spring.

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When I figured out my starvation problem it was while going up a mountain.  The car would barely run.  I could roll it backwards and cut the wheel, perpendicular with the road and let the bowls fill back up then it ran fine for a few more minutes.

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MY1PATH.....I assure you that your SU’s are adequate to supply your motor build. I run them on my Datsun Spirit Stroker Stage III motor with no problems throughout the power ban. I would not be so sure that carbs are your problem. I just fixed a high speed bog problem with new plugs (who would have thunk it?) I tried everything first.....points, condenser, tuned carbs etc. A new set of plugs and she kicks it again! Sometimes the little caps on your spark plugs can loosen causing a high rpm miss, or a cracked distributor cap or rotor. A vacuum leak can make it miss and run lean also. On a modified engine, you should probably be 3 1/2 turns down. It’s fun when you solve a problem......good luck.

Edited by Diseazd
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 After running it through the gears at WOT and the engine won't rev to high RPMs under a load, will it rev to 6 grand in neutral? 

 I'd let the engine idle, shut it off and pull the domes and pistons. Then pull the choke lever back and determine the fuel level in the nozzles. You should be able to see the fuel level with the nozzles pulled down. After determining the fuel levels, reassemble the carbs and run it hard until it cuts out. Shut it off immediately and pull the domes and pistons again. Pull the choke lever back to drop the nozzles and see if the fuel level is substantially lower than before. That should prove or disprove that fuel starvation is the problem.

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12 hours ago, siteunseen said:

When I figured out my starvation problem it was while going up a mountain.  The car would barely run.  I could roll it backwards and cut the wheel, perpendicular with the road and let the bowls fill back up then it ran fine for a few more minutes.

 

when It leans out letting off the throttle part way for a few seconds lets it recover. Uphill this means holding speed instead of accelerating at a lower rate. Hopefully I'll get to drive it this week and run a check after a hard pull.

 

Edited by MY1PATH

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Well to my untrained ear it continues to sound like high load fuel starvation.

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?

As an alternative way to address the problem... There is an adjustable stop on the gas pedal to control how far down the pedal goes. You could run that stop up some.     :ph34r:

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You were right Captain Obvious it was not the change of washer effecting the carb vacuum.

This is all I had time for today... Killed the engine at warm stable idol and again when it leaned out: found that both carbs were approximately half the normal level.

So now I know for certain that I'm draining the bowls and that it's not just one carb. Despite the appearance of free flow when the pump is running and with compressed air (probably just not free enough) I am going to fish a line through the fuel rail and through the Hardline going under the car.

If both those come out clean, this weekend I will pull the fuel tank and fish/inspect the pickup line.

KIMG0447.JPG
Edit: jar isn't level in picture so fuel looks lower than my line...

Edited by MY1PATH
grammar and detail

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2 minutes ago, siteunseen said:

Why not bend the float tabs?

I think running a higher than necessary float level (and re-tuning the engine to suit the higher fuel level) is more of a bandaid than a solution.

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

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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!  LOL

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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 by 240260280
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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. 

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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.

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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 by MY1PATH

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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...

KIMG0448.JPG
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 by MY1PATH
Pic via Mobile Txt via PC
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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?

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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)

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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).

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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.

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I checked, fuel can flow one way trough the pump when it is off.

I only drove the car once the electric pump in line but not powered (was chasing wires) and the was no noticeable decrease in performance. Before that it was exclusively mechanical and after that if it was inline it was powered. I also just remembered that when I added the pump I had to tighten my connection to the chassis hard line right after the pump...it was leaking pretty good with the pump powered.... Maybe another indicator...

 

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