WoT engine sputtering

[Dirka]

1977 K10 newly rebuilt 400 (by new it's been running for about 6months, less than 1500 miles on it) It's fairly heavily modified, aluminum heads, aluminum intake, bigger cam, 850cfm double pumper holley carb, the specifics I can't get into, I didn't do the build and don't have the sheet with all that info on it.  If it matters I'm anywhere from 485hp to 530hp depending on which version of the conversion reference you use online based from the dyno results of 316whp.
Anywho that's a rough idea of what I'm working with.  Now my WoT issues only started a little while ago, and first started in the winter, which I thought was just side effect of the below freezing temps.  I live in Nebraska.  But this week it did it again, and I was driving in 60*+ temps So the cold weather guess is out the window. 
I'm Cruising along about 20mph and give it WoT about 3000-3300rpm it sputters and on occasion has actually killed the engine completely(has happened twice) I'm assuming it's got to be a fuel issue, but if I'm at a higher rpm then go WoT this issue doesn't happen. And it doesn't happen everytime either.  I'm having some other issues to, but that's for another thread.
Thanks!

[Irish_Alley]

what fuel you using?

[Dirka]

91 octane

[FlatBlack77]

how new is the carburetor?

[Dirka]

everything motor wise is 6 months old. It's been tuned and rejetted and all those things i don't understand by a local shop here in Lincoln, NE.

[Irish_Alley]

something to check maybe the cold cracked a vacuum line?

[FlatBlack77]

something to check maybe the cold cracked a vacuum line?

that gives me an idea.

maybe a vacuum line such as the vacuum advance line isnt the right kind of line. like its the thin stuff that can clamp shut when it gets hot. maybe it took awhile for it to happen because of the cold

[Captkaos]

Out of curiosity have you asked the people that built the motor?  Did they tune it on the dyno?  Did they build it for high rpm power or low rpm torque...

I would suspect if you are running a 850cfm DOUBLE PUMPER carb, that is the issue.   It is way over carb'd unless it redlines at 8000+rpms. (unless you are trying to get max power out of it)  I would imagine it was jetted WAY down.  Realistically you shouldn't be hammering it at low rpm with a double pumper as it is opening it up for max air with less than optimum RPM which is going to make it hesitate and over carb the motor.  If you plan to drive this way you need to switch to a vacuum secondary setup, or stop driving like that.

In general Double pumpers are not suited well with Heavy vehicles and automatic transmissions or motor built for low end power.

[Dirka]

All my vacuum lines are rubber hoses, which doesn't mean those can't happen. will look into that.
Cap't - I haven't talked to him about it, but it was tuned once in truck on dyno from a local shop that the builder recommended. And it did run good for awhile. Ran it out of gas once, and it really hasn't been the same since.
I may also to need to reroute my fuel line from the fuel pump to carb, it hugs a couple of water hoses fairly tight, and maybe just getting too hot?

[FlatBlack77]

checked/replaced the fuel filter since it ran out?

makes me think it could have sucked up some dirt or gunk from the bottom of thr tank and partially clogged up the filter

[rich weyand]

Something to check is whether the accelerator pump is working properly.  If you stomp on it and the air rushes in without a sufficient shot of gas from the accelerator pump, it will go dead lean and the fire will go out.

Especially with the big double doors on that carb.

BTW, 485hp to 530hp on a naturally aspirated 400 means 730 to 790 cfm through the carb at 7000 to 7600 rpm.  Are you really winding it that tight, or is somebody blowing smoke up your skirt?

[Dirka]

Flat- I have not checked that, but I did take it to the shop who tuned it after that happened.
Rich- I have noticed some issues with my accelerator in general, I will look into that as well.  As far as the "winding it that tight" I'm personally not doing that, but the dyno sheet said 316hp to the wheel, so I wanted to see if I could find out approximately what the engine was putting out.  and that's where the 485-530hp comes from. I don't have those true numbers from and engine only dyno.

[rich weyand]

316 rwhp ought to be more like 393 at the crank.  Nothing wrong with that from a naturally aspirated 400.
http://www.mk5cortinaestate.co.uk/calculator4.php

393 at the crank should be 589 cfm at 5650 rpm, which sounds much more reasonable.
http://www.mk5cortinaestate.co.uk/calculator3.php

And yeah, Chris, the 850 double pumper is way over-carbed for that rpm.

I'm still betting on the accelerator pump.  Not enough squirt.

[Captkaos]

Depending on the transmission (auto or manual) you can assume you are loosing 25% though an auto, 20% though a manual.  These are general answers...

[roger97338]

If you download the factory service manual for your pickup, it'll have step-by-step troubleshooting for any problem that General Motors foresaw you having.

If women came with troubleshooting guides like that, I wouldn't have been called so many horrible things throughout my life.