I had a similar issue due to poor breathing of the fuel tank, believe it or not, a simple change to a vented fuel cap and the car idled and stopped flooding out, but it drove me nuts for weeks !

 

Another problem may be fuel boiling in overheated carb.s... if the intake manifold is heated by exhaust crossover, but you're never going to drive it in winter weather, block the crossover ports off via the readily available little port blockers... shown in the middle of these two gaskets:

 

Could be simply a fuel pressure problem.
Eddy carbs don’t like more than 5 psi.

 

I had a similar issue on my 67 fairlanes 390 and it had an aftermarket intake with no exhaust crossover. The high heat under the hood would cause the gas to flash boil as the pressure dropped from 5 psi to 0 when it got to the fuel bowl. When i figured it out gas was bubbling out everywhere from the top of the carb, and dripping from the aircleaner.

Winter gas was worse than summer blends too, so if you're still running gas bought a month or more ago that could be some of it.

I eventually had to put a return line on my Fairlane to stop it completely. May not be your problem but its something to check.

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I had a similar issue on my 67 fairlanes 390 and it had an aftermarket intake with no exhaust crossover. The high heat under the hood would cause the gas to flash boil as the pressure dropped from 5 psi to 0 when it got to the fuel bowl.

Jay Leno mentions the same problem with his Ford and his Dad having the same problem with the same car back in the day... at 4:45 in this video:

 

Might want to change the fuel filter just as a precaution as well, seen partially obstructed filters cause similar issues.

 

Might want to get bigger carbs designed for bigger engines?
Bad luck with the cam, I'm guessing hydraulic flat tappet?

couple 850 or better Holleys for serious power. Couple 750s for a cruiser

 

Fresh build, or did this just develop?

Vacuum leak? They are tough to check though with a cam that doesn't make any vacuum at idle with anything other than a can of ether or carb cleaner spraying all over your motor .


Where do you have the timing at? Initial, all in degree and at what rpm?

 

trying to respond to all the great comments:
I do not have an exhaust cross-over
If I had a fuel pressure or filter issue it should not accelerate to 2000 rpms fine. The stall out comes when it is below 2000 while in park. I would think that the problem is in the idle circuitry.
The initial timing is set at approx. 15 at idle when I can get the car to idle before the test run. I haven't checked the all-in timing.
I will verify the fuel pressure again, but it runs around 6.
Fuel cell is vented but I will check it again.
Is there something on the vacuum side that will cause the stall out idle when the engine heats up?
thanks everyone and keep the suggestions coming.

 

If I had a fuel pressure or filter issue it should not accelerate to 2000 rpms fine. The stall out comes when it is below 2000 while in park. I would think that the problem is in the idle circuitry.

I haven't checked the all-in timing.

The first item above is incorrect. With a pump making 2 psi and 5 gallons per hour will allow the engine to run well past 2000 rpm's because the bowls hold a lot of fuel. Especially on a dual carb set up. Basically, there is no good reason not to confirm that the fuel pressure is in spec - 4-6psi and enough volume to make through 5-10 second WOT blast.

15 degrees initial with a Thumper is probably not enough initial (guess only), but you NEED to confirm/set your all in timing before too much driving over 2500 rpms is done IMO.

You didn't answer - fresh build or did this just develop?

 

the 2000 rpm is sustainable for several minutes without stall.
This just happened and the engine has approx. 2000 miles on it since rebuild.
What initial timing do you suggest?
How should the idle screws on both carbs be set?

 

After you get your engine warmed up try and set your timing to at least 20*. It may take more just experiment. Once you have the timing set adjust your idle to 900-1000 rpm. If it were my engine I would get rid of that Thumper cam. It is a known fact that the big block Chevrolet engine will wipe the lobes off a hydraulic camshaft. Google it to find out all the information. I would get a Camshaft like this Howards Cams. https://www.summitracing.com/parts/hrs-cl120245-12 They are really good camshafts made here in the USA. One thing not to forget is to change valve springs to match your camshaft. You will also need retainers and locks. If you don't work on engines just take your heads to your machinist to install the valve springs. I always use roller rocker arms on my big blocks like these. https://www.summitracing.com/parts/scc-scp1014/overview/

 

I did not build this engine. It was put together by a guy in Fla and I bought it as is. Ran good with a holley 650 with vacuum secondaries, but have problems with the dual quad 500 Elde's. This also makes me think that the situation can be resolved fairly easy as it did fine with the holley.
I will try setting the timing to 20 and then the idle rpms. I am very much aware of the cam lobe and lifter disaster. I had this to happen to me year before last in a 350. I did order the Howard's cam kit for the rebuild using hyd. roller cam and lifters in addition to roller rocker arms. This is a great product..

 

FYI we just ran into a similar issue with my sons V8 Ranger, pressure was good at idle and revving in park. Driving the vehicle hard it would flatten out and have trouble making it past 5000 rpm and when you dropped the throttle it would want to stall.

Pulled the fuel filter and changed it, pressure went up 1PSI...we cut the filter apart and it was so plugged you couldn't see light through it. Also now it doesn't hold pressure at the regulator for hours until it bleeds off...now its instant.

Just saying, don't discount the simple and obvious until you know for sure.

 

thanks I will replace the filter and try again