SU carbs will not sustain Heavy load

[MY1PATH]

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 May 27, 2018 by MY1PATH

[Patcon]

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.

[MY1PATH]

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 May 28, 2018 by MY1PATH

[Captain Obvious]

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.

[siteunseen]

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.

[Diseazd]

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 May 28, 2018 by Diseazd

[Mark Maras]

 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.

[MY1PATH]

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 May 29, 2018 by MY1PATH

[Captain Obvious]

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.    

[MY1PATH]

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.

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

Edited May 30, 2018 by MY1PATH
grammar and detail

[siteunseen]

Why not bend the float tabs?

Edited May 30, 2018 by siteunseen

[MY1PATH]

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.