Problem: car chokes when I smash the gas pedal

[Rck68]

hey All,

Got a 440 w a brand new 750 double pumper Holley carb in my 68 charger.
Recently got car up and running. Still messing w setting proper timing and rpm.

Problem:
When I mash the gas pedal as I’m going a comfy 30-40mph, engine feels like it chokes for half a second. By choke I mean almost stall but doesn’t.

I lay off the pedal and the engine comes back kickin fine.
What could be the reason?

Best!

 

[Kern Dog]

The accelerator pump is not adequate.
When you crack the throttle, the engine immediately sees a large increase in airflow. If that is not met with an adequate accelerator pump shot, the engine runs lean and bogs down. The accelerator pump temporarily fills the gap before the primary jets and power valve can react to increase the flow of fuel.

 

[turbine68rt]

This may help. What KD was describing.
https://www.holley.com/blog/post/how_to_adjust_the_accelerator_pump_arm_on_a_holley_carburetor/

 

[Rck68]

Thanks so much guys! That helps a lot!

This forum is the best

 

[Rck68]

The accelerator pump is not adequate.
When you crack the throttle, the engine immediately sees a large increase in airflow. If that is not met with an adequate accelerator pump shot, the engine runs lean and bogs down. The accelerator pump temporarily fills the gap before the primary jets and power valve can react to increase the flow of fuel.

So you think I should change my accelerator pump from my 30 cc to the 50 cc?
Much appreciate your reply!

 

[Kern Dog]

I'd need to know more about the combination but you can test my theory. With he engine idling and in Park or neutral, remove the air cleaner and look down the throat of the carburetor while opening the throttle. Start by opening it slowly, then try it a little faster, then faster. If it is functioning normally, you'll see fuel shoot from the nozzles as you open the throttle. In my car with a 440/493, as I crack the throttle open faster, the stream breaks up from the incoming airflow.
An air/fuel meter would be a great tuning aid for you too. It would give a real time readout of how rich or lean you are running at any given time. For instance, to prove my accelerator pump theory, in your case, when the throttle opens and the car bogs down, the reading would go from 13.8 to 14.5 up to the 17 to 18 range instead of dropping below 13.0, a number that many people feel is a good ratio for power.

 

[Rck68]

I'd need to know more about the combination but you can test my theory. With he engine idling and in Park or neutral, remove the air cleaner and look down the throat of the carburetor while opening the throttle. Start by opening it slowly, then try it a little faster, then faster. If it is functioning normally, you'll see fuel shoot from the nozzles as you open the throttle. In my car with a 440/493, as I crack the throttle open faster, the stream breaks up from the incoming airflow.
An air/fuel meter would be a great tuning aid for you too. It would give a real time readout of how rich or lean you are running at any given time. For instance, to prove my accelerator pump theory, in your case, when the throttle opens and the car bogs down, the reading would go from 13.8 to 14.5 up to the 17 to 18 range instead of dropping below 13.0, a number that many people feel is a good ratio for power.

So you had the same problem? And you solved it?
Thanks for the response I will try this tomorrow!
What other specs would you need to know?

 

[Kern Dog]

You might want to go back and check post # 3 here. It should guide you to see if your carburetor is functioning right.
Old cars are almost entirely tuned by a person. Modern fuel injected cars have computers and sensors that communicate to keep the engine running right, assuming all the parts are in good working order.
For instance, many older engines with stock cams can usually run fine with 5-8 degrees of distributor advance at idle. That same engine with a rowdy cam will want 15, 18 degrees or more at idle to feel right.
My car is a '70 Charger with a 440/493, 10 to 1 compression and a moderate solid camshaft and my distributor is set to 18-19 degrees at idle and a total advance of around 34 degrees. This works well for my engine but each combination is a little different and may require a different timing spec. I have found though that even fairly stock engines feel a bit more snappy with at least 10 degrees of advance at idle.
Bad gasoline will give you the bog that you describe as well. Old gasoline in our cars is often noticed immediately...the car runs a bit rough, power is down and even the exhaust can smell funny. New cars with the computers, they compensate for this and will often run okay, just a little less power.

 

[Rck68]

You might want to go back and check post # 3 here. It should guide you to see if your carburetor is functioning right.
Old cars are almost entirely tuned by a person. Modern fuel injected cars have computers and sensors that communicate to keep the engine running right, assuming all the parts are in good working order.
For instance, many older engines with stock cams can usually run fine with 5-8 degrees of distributor advance at idle. That same engine with a rowdy cam will want 15, 18 degrees or more at idle to feel right.
My car is a '70 Charger with a 440/493, 10 to 1 compression and a moderate solid camshaft and my distributor is set to 18-19 degrees at idle and a total advance of around 34 degrees. This works well for my engine but each combination is a little different and may require a different timing spec. I have found though that even fairly stock engines feel a bit more snappy with at least 10 degrees of advance at idle.
Bad gasoline will give you the bog that you describe as well. Old gasoline in our cars is often noticed immediately...the car runs a bit rough, power is down and even the exhaust can smell funny. New cars with the computers, they compensate for this and will often run okay, just a little less power.

Alright thank you, all fuel and ignition system is brand new including the carb. I will have to perform some tuning adjustments to begin then.

 

[Kern Dog]

I am by NO means a tuning expert. Forums like these have been a huge help. I am great at buying new stuff and still having trouble with it!

 

[beanhead]

What happens with double-pumpers when you slam the secondaries open, is the engine gets a big sudden gulp of air; without enough fuel at that moment to match, it bogs. You shouldn't need a 50cc pump. First make sure the pump arms are adjusted properly. If they're good, a more aggressive pump cam will likely cure your bog. Pump cam assortment kits are fairly cheap and you can try the different ones to get the response you want. I'd bet you need a bigger-number power valve as well, holley's generic method for sizing the PV is bullcrap...to really get it dialed you need a vacuum gauge and a long enough hose to be able to read it in the car while driving, and learn to read your plugs. It's a bit of time to get it tuned right but worth it!

 

[MoparLeo]

What is the history of what we have here ? Stock 440 ? Just got the car ? Did it ever run good before ? How many things did you change at the same time ? Do you have a service manual ? Have you done timing, carb/vacuum, adjustments before ? Why the Holley double pumper ? Somebody recommended it ? These cars ran fine when first manufactured without this stuff so why now ? Tuning a car is more about adjustments and understanding what makes certain things happen, than it is about putting new things on it. Most people will come to realize that the more you mess with something and change it from stock, the more time you will spend trying to make it work like it was designed to work before you messed with it. Diagnostics is all about step by step not doing more than one thing at a a time and then trying to figure out why something happened and which thing that you did caused it.
First things first. Baseline timing, idle speed and idle enrichment need to be set before you "mash the pedal" and wonder what happened. You need to know what is working properly before you can fix what is not. Getting recommendations on any forum, assumes that the people giving advice somehow know exactly what YOUR combination is and have seen/heard it run. Anything else is just guesses based on what they might have or have seen. Probably won't directly apply to you. Do your research. Read your service manual. Watch some videos if possible of the engine combination that YOU have. Take your time and take notes for the future. It is all about learning.

 

[myk r sanchez]

These cars ran fine when first manufactured without this stuff so why now ?

If man never tried to tinker with and modify their machines then they would never have made it to the moon. Just sayin...

 

[MoparLeo]

Nobody went to the moon in something they put together in their garage. Lots of science and research and testing. Lots of $$$ spent and a few lives lost. The average guy is generally not more informed than the engineers who develop and test the product package sold in the 100,000's . The after market sells Parts. It is up to the buyer to determine the fitness of the application and the result is on him. That is why Doctors have to go to school for so many years. You don't ask questions on internet forums or just watch a few videos in order to do surgeries. Just sayin..

 

[myk r sanchez]

These cars ran fine when first manufactured without this stuff so why now ?

Nobody went to the moon in something they put together in their garage. Lots of science and research and testing. Lots of $$$ spent and a few lives lost. The average guy is generally not more informed than the engineers who develop and test the product package sold in the 100,000's . The after market sells Parts. It is up to the buyer to determine the fitness of the application and the result is on him. That is why Doctors have to go to school for so many years. You don't ask questions on internet forums or just watch a few videos in order to do surgeries. Just sayin..

You're completely missing the point. It doesn't matter if we're discussing a big block mopar in someone's garage or a multimillion dollar rocket headed for the stars; the important thing is the IDEA of man wanting to take things like machines, and improving them, making them stronger, faster, more efficient, BETTER. If mankind stuck to the "if it ain't broke, dont fix it" mentality then progress would never be made and we'd still have horse drawn carriages for transportation and muskets for the 2nd amendment, and mankind most CERTAINLY would never have developed the SR71 in your avatar...

 

[Rck68]

This is all great advice guys, thanks for taking the time for the input!

I will begin with checking timing and vacuum, making sure those are correct then moving on to the carb.

It didn’t run well at all when I first got the car. I’ve made this 440 run so much better now! This is the last little hiccup I still have.

Lots to learn for sure! I’m glad you old school guys are still around to help and teach. Thank you!

 

[HEMI-ITIS]