I'm sorry but the carb does care how much load the engine is under.

You can see this by the fact that cars with vacuum secondaries or weighted restrictor plates will only move a fraction when revved stationary compared to driving conditions. This shows the lack of vacuum compared to load conditions.

I'd agree with you that a carb will match the airflow required by an engine at any given time, but how much fuel the engine asks for is very different under load to when it's revved stationary.

 

I'll try to catch up on this later. I have a meeting I'm late for.

 

sorry been busy for a few days, the engine has a little bigger of a cam bored 35 over edlebrock performer intake dyno at 410hp, cant think of what timing is set at but I think around 12deg ptdc. When I say she falls on her face I mean when cruising at any speed and I stomp on the pedal The car will almost die unless i let up.

 

Check timing first if you're not sure. Do not guess. Even if it's not the problem, it's very important to know that it is correctly set.
12-18° initial should be OK, but what you want is about 36° total timing ( WITHOUT vacuum advance ) all in @ 2500 - 3000 RPM max.
The initial timing is secondary. Vacuum advance + total timing should be 52- 54°.
Most "carb issues" are timing related. See Lars papers.
Assuming timing is OK :
Do you have any backfire when you stomp on the pedal ?
Do you have a fuel pressure gauge ? Fuel pressure should always be between 4 and 7 PSI.
Did you check and adjust fuel level in the fuel bowls ?
Do you have a fuel filter ?
Do you have rust, sludge, dirt or debris in the fuel bowls ? Barb fittings can sometimes hurt hoses and tear small rubber pieces off...
New fuel lines / tanks are not always clean either.
If everything above is OK, check accelerator pump and discharge nozzles. You should have fuel coming from the nozzles at the slightest movement of the carb butterflies.
If your carburetor has vacuum secondaries and a noticeable bog on full-throttle acceleration, the secondary spring may be too weak.
Test and tune the performance with stiffer secondary springs that open up more slowly (as engine speed and vacuum warrants).
Engine size (displacement) also affects the opening point.
If power enrichment works, check main jets numbers and check spark plugs color. Try richer, according to the plugs. A wideband AFR Meter is a great help for fine-tuning, especially on a new rebuild...

Hope this helps.

 

Most likely causes: non-functional or marginal accelerator pump operation (in the carb); 'weak' timing set-up in distributor (not for performance use); too much fuel being dumped into the carb at low RPM WOT {yes, this is the opposite of the first item, but it can cause similar results for the 'seat of your pants' feel}; cam misindexed to the crank {just a few degrees can make a big difference}; weak spark condition (coil and/or plug gapping problem).

 

thanks for the tips ill see what i can come up with

 

with nitronick, carb knows pressure DP and flow through venturi to determine fuel needs. Load will change throtle position but fuel needs will still be determined by the above.

Holley tunning book says to do the free load test to adjust idel feed restriction on modded motors, it works.

 

I once had a bike that would suddenly stutter under heavy load. It would lose rpm and seem like it would die until I backed off the throttle. It would rev gently, but not under load. Mechanic friend said "change the plugs". I said they are low mileage and look good (proper type, gap, colour. Started fine. Ran fine cold). He insisted - he was right. I never understood why. Over the next 10 years years I had plugs last more than 3 times as long and I only ever changed them for the heck of it.

 

Too much fuel being dumped into the carb at low RPM WOT will cause a lazzy throttle response and a puff of black smoke.
This is why I wanted to know if you had any backfire ( which would indicate a temporary lean condition ) or not.

 

Vacuum advance is non-functional (fully retarded) at wide open throttle (WOT), so the problem is not your vacuum advance. Inadequate total timing can contribute to a stumble going into WOT - be sure you're running about 36 degrees total timing before you start playing with the carb.

But if your total timing is anywhere in the ballpark and the car is falling on its face going into the secondaries you might want to make sure you have enough rpm to get into the secondaries: A mechanical secondary carb cannot be slammed into the secondaries unless you have enough rpm to handle it - if you slam into the secondaries at anything under 3000 rpm you're likely to fall flat on your face until you let up on the gas.

If it's falling flat even at elevated rpm you need to take a look at the carb setup, fuel supply, and fuel pressure. You can e-mail me for my Holley and BG setup papers if you want to make sure you're set up right.

Lars
V8FastCars@msn.com

 

As Lars is da man, I will totally agree with him and also confirm his thought about the secondaries from personal experience on a friends car.

My friend had a rod fitted with a 69 SBC and correct qjet, it was auto, not sure what the OP has.
Anyways, he had it set so that a WOT the secondaries would open when parked, in N and stomping on the gas.
The result was exactly as the OP has stated. The car would drive nicely until he tried to get on it at which point it would quite literally just die.

A quick adjutment of the secondary valve spring and the problem went away. Nice and easy, cheap fix!