4160 Holley adjustments - Impala Tech
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post #1 of 3 (permalink) Old 03-28-2020, 02:31 PM Thread Starter
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4160 Holley adjustments

My 1966 Impala bogs down at wide open throttle. If I am cruising and step on it, it hesitates bad. When I let off and continue to cruise, no problem. The carb is a 4160 Holley 4 barrel. I tried to raise the fuel level in the secondary bowl as described by 'the book', but I cannot really see any gas. Of course, looking thru the small hole is really difficult. Any suggestions?
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post #2 of 3 (permalink) Old 03-28-2020, 07:34 PM
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but I cannot really see any gas. Of course, looking thru the small hole is really difficult. Any suggestions?
Hold a match up to the hole so you can see inside. Naw definitely kidding!

You know the fuel is at the top of the sight hole not by looking in the hole (so why do they call it a sight hole?), but by having a trickle of gas fall over the bottom lip as the engine idles. If you have plastic sight hole plugs, you can see the bottom edge turn dark as the gas rises to touch it.

Bogging can be caused by too much carburation (what is your cfm rating?). Or by too much carburation due to a manual secondary on a heavy car with an automatic and low numbered rear gears. A vacuum secondary is best for the street. You can tune the secondary by changing the springs inside the secondary vacuum diaphragm.

Further you are not complaining of back firing at the hit of the throttle so I know you are not going lean. Your solution is more air and less gas to get rid of the bog. So tinkering with accelerator pumps, cam position or squirter size may not be needed at all.

Accelerator pump shot is a measure of the volume of fuel added to the motor at the hit of the throttle. It is the average of ten strokes so a 30 cc pump actually only delivers 3 cc of gas a time. So a 50 cc Reo pump adds 5 cc of gas (66% increase). The accelerator pump cam location determines at which point when you jump on the gas the accelerator pump activates. The accelerator pump cam size (there are eight of them) determines the rate of discharge and the volume. Squirter size (the number stamped on the barrel of the squirter, determines the time it takes for the gas from the accelerator pump to enter the motor.

Finally your car might have been modified from stock. A clue to this is the fact that you have a Holley carburetor which wasn't used on an Impala. If in fact your cam isn't stone stock, then you might have a cam that changes the driving characteristics of your car in order to make it "sound" racey even though doing so allow a Smart car to beat you off the light.

Extending the the duration of a cam makes the idle rough. This is because your car engine is trying to run with the valves held open. It can not build compression if the intake valve never closes. Instead the rising piston on what should be the compression stroke is pushing the air and gas you sucked in on the intake stroke out of the engine. This pressurizes your intake that is supposed to have a vacuum in it. Carburetors operate on the differential between the manifold vacuum and the atmospheric pressure above it. No vacuum, no workee!

People with big cams turn the idle speed adjustment screws up higher to get their car to run at all (even though it still has a rough idle with a cam too large for the street). This uncovers the transition slot located in the carb base plate that allows the carburetor to transition off of the idle circuit over to the main jets. You have to either open the secondary blades a little more (requires taking the cab off the motor and adjusting the secondary blades from the bottom), or drilling holes in the primary blades to allow more air to enter with the throttle blades closed to where they need to be.

Big Dave
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post #3 of 3 (permalink) Old 04-05-2020, 10:37 AM Thread Starter
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Dave: thanks. Figured it out. Secondary bowl was practically empty. Adjusted too much and had a fuel fountain. Backed it down and now have trickle. Stomped it when cruising and wife spilled her beer. I chewed a small hole in my seat.
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