Engine stalls when floored
Thread Starter
Intermediate
Joined: Nov 2015
Posts: 30
Likes: 4
From: Atlanta
Engine stalls when floored
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?
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?
Last edited by Bneezey; Nov 28, 2015 at 01:58 AM.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
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.
Le Mans Master
Joined: Oct 2006
Posts: 6,842
Likes: 285
From: Palmdale CA
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.
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
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
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
Le Mans Master
Joined: Oct 2006
Posts: 6,842
Likes: 285
From: Palmdale CA
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.
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.
Le Mans Master
Joined: Oct 2006
Posts: 6,842
Likes: 285
From: Palmdale CA
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
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
Pro
Joined: Mar 2015
Posts: 681
Likes: 31
From: Nebraska
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
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
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Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
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?
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?
And BTW, 750 CFM is not too much for your combination, quite to the contrary it's spot on.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
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.
Pro
Joined: Mar 2015
Posts: 681
Likes: 31
From: Nebraska
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.
The double pumper will work just fine.
Last edited by COOLTED; Nov 28, 2015 at 11:10 AM.
Le Mans Master
Joined: Oct 2006
Posts: 6,842
Likes: 285
From: Palmdale CA
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.
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
Last edited by F22; Nov 28, 2015 at 10:59 AM.
Safety Car
Joined: Jan 2009
Posts: 4,000
Likes: 299
From: New York
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?
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?
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
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.
Thread Starter
Intermediate
Joined: Nov 2015
Posts: 30
Likes: 4
From: Atlanta
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.
Last edited by Bneezey; Nov 28, 2015 at 01:42 PM.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
Just another example of toodumbtoretire BS
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Why Mechanical Secondary Carburetors are NOT INTENDED FOR STREET USE
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.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
Burning Brakes
Joined: Oct 2008
Posts: 1,037
Likes: 14
From: Mobile Alabama
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.
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.