Bneezey

I have a 1979 Corvette that has a 383 Stroker with a 750 Holley double pumper, comp cam 274, scat crank, 195cc heads and a lot more. Just bought the car with all of these upgrades already installed.

The car runs and idles fine. Shifts through all the gears and has plenty of power. My issue has been that when I stomp on the throttle the engine wants to die. This happens in park or driving at any speed. I just changed the power valve which only helped slightly. I can't tell if I'm running into an over lean or too rich of a mixture. Any thoughts of what might be bogging down in the carbureter?

toobroketoretire

Its because the air velocity thru that huge mechanical secondary carburetor comes to a standstill. Had you put a 600 cfm vacuum secondary carburetor on it you wouldn't have that problem. Mechanical secondary carburetors aren't intended to be used on street engines.

SB64

A couple of other things. Does you car require the pre bent S hose for you fuel line? Put a psi gauge on to determine what the psi is. Should be 7-9.
R

F22

Run a length of fuel line temporarily to the base of the windshield and tape a small fuel pressure gauge to the base of the windshield. Drive it to around and watch what happens. I did this and what happened was that when I get stomped on it, it is would get up and go, then die, because the fuel pump pressure on went to zero. The fuel filter would let enough fuel in it to run normally, but on demand, it would drain the bowls dry.

Check your float level too. If the float bowl is set too low, there could be a defecit to begin with.

johnt365

So, if you are in the final transmission gear going about 50mph and you gradually go to wide open throttle, does it scream all the way up to 5.5-6k rpm?

My first thought was accelerator pump tuning on the Holley. This circuit on a Holley is very adjustable at two points.

1. Pump nozzles or Pump squirters

2. Pump cams.

You might check out what pump nozzle is in the primary side right now and report back the number. Also note what color pump cam is installed and report that back also.

John

F22

I disagree. I'm running a Holley 650. Mechanical Secondaries, double pumper and divorced choke. It's been in the car door three years. Runs like a scalded cat, idles smoothly at 900 rpm and it doesn't bog or die, when you floor it. Starts instantly, even after sitting for two weeks. Three pumps, hold it down, just a little and Vroom!

My only complaint was the stock jetting. They put the tiniest jets in the primaries and oversized the jets in mixture the secondaries. How did I know this? Because I run an AEM wideband Air/Fuel ratio gauge on the steering column. One of the best things I ever did to this car. It was super easy to install and even came with a bung, so you could weld the O2 sensor to the exhaust. You can tell when it's running too lean, too rich, or just right. Sure helps,setting the idle correctly. Along with a vacuum gauge, where the clock used to be, I know exactly what my engine is doing.

F22

That's a good thought too. If it has the squirt lever for the secondaries, it's not surprising that they come stock, with the nut barely showing any thread, which means any touch on the lever and it squirts a whole bunch of fuel into the secondaries. Ratcheting down the nut a good 1/4 inch or more helps. Heck, I've run with it disconnected, as the engine will pick up the fuel through the secondaries.

COOLTED

That was my first thought as well. Just needs a little tuning.

SH-60B

First of all, welcome to the CF. I'm with johnt365, check the accelerator pump as he described. What is your ignition timing set-up? Are you using a vacuum advance, how much is the initial, etc.? What transmission, converter?

And BTW, 750 CFM is not too much for your combination, quite to the contrary it's spot on.

toobroketoretire

Mechanical secondary carburetors are track racing carburetors designed for continuous 4000+ rpm engine speeds. As street engines typically operate in a wide 550 to 5500 rpm range only vacuum secondary carburetors are suitable for street engines. That's why the most powerful engines like the 426 HEMI's, 440 Magnums, 426 Ram Chargers, L-88's, LT-1's, and Z-28's came from the factories with vacuum secondary carburetors.

COOLTED

Blah, blah, blah...here we go again.

The double pumper will work just fine.

F22

https://www.holley.com/products/fuel...s/carburetors/

Double Pumper $404.96-$499.46

*Tire turning power for hot street or race vehicles.

*Mechanical secondaries means your foot controls rear throttle blade Opening

*For light vehicles with low gearing and manual trans or automatics with high stall converter

*600 to 850 CFM

diehrd

Check float level and fuel pressure , and a double pumper is fine dont let tobroke get in your head

bluedawg

I love my double pumper and it runs really well on the street....

COOLTED

You know...come to think of it...didn't the L-88, LT-1 and Z-28 have double pumpers?

Bneezey

Thanks for all the responses! Honestly I am very new to the carburetor world so I'm going to have to check everything that I'm running on it. First things first I'm going to try and tune it. Starting with the jet size (due to the fact that I have no idea what the previous owner may have put on) and the pump cam.

SH-60B

I know the L88 did have a double pumper: http://www.google.com/url?sa=t&rct=j...hOu0hAfJuPj_rw

Just another example of toodumbtoretire BS

toobroketoretire

When an engine is idling the air velocity thru the primary bores is "X" and when all 4 barrels are suddenly opened that X velocity drops to almost ZERO.

SH-60B

No. Vacuum drops to nearly ZERO, not "air velocity". Big difference.

74modified

First thing I would do is check all the other parts of the tune - timming, fuel pressure, float level.
If all of that is good, I would try tuning the Holley's accelerator pumps. The jets are part of tunning the carb, but are less likely to cause bog than the pumps. Don't start changing things that are not causing the problem, or you can get so messed up it will not be funny. And... some broke people only know how to swap parts, and not tune them, so their opinion is good for limited situations.