choke removal effects and operation question
Thread Starter
Racer
Joined: Jul 2004
Posts: 444
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From: Dallas TX
choke removal effects and operation question
I had a 800 CFM Holley 4150 with no choke and I'm replacing it with a Holley 600 CFM 80457 which comes with an electric choke. I'd like to understand what the choke is actually doing before considering removing it just to clean up the look.
All it does is close that flapper plate above the primaries and push on the vacuum actuator to increase the idle speed, right? I just want to make sure it doesn't have any fuel circuit such that the mixture will go out of whack if it is taken off. Also, what determines how much the plate is closed and the idle is bumped? Is it engine temperature?
All it does is close that flapper plate above the primaries and push on the vacuum actuator to increase the idle speed, right? I just want to make sure it doesn't have any fuel circuit such that the mixture will go out of whack if it is taken off. Also, what determines how much the plate is closed and the idle is bumped? Is it engine temperature?
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
The choke, when closed, richens the mixture by restricting airflow. Most engines need a richer mixture when cold in order to operate smoothly and reliably. Yes, the choke affects mixture.
The choke pulloff determines how far the choke is cracked open upon engine start. This is adjustable.
Most Holley chokes are electric. The choke opens after a certain duration regardless of engine temperature. Closure of the choke sets the fast idle cam, and fast idle speed is determined by the relationship between the fast idle cam and the fast idle screw.
Lars
The choke pulloff determines how far the choke is cracked open upon engine start. This is adjustable.
Most Holley chokes are electric. The choke opens after a certain duration regardless of engine temperature. Closure of the choke sets the fast idle cam, and fast idle speed is determined by the relationship between the fast idle cam and the fast idle screw.
Lars
Thread Starter
Racer
Joined: Jul 2004
Posts: 444
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From: Dallas TX
Thanks for the quick reply Lars.
Let me make sure I've got this right. When the choke is closed it creates a pressure differential between the throat and the bowl which pushes extra fuel into the carb through the main metering circuit. So the choke has no direct access to a fuel passage like the idle screws do, just indirect access to the main circuit through varying throat pressure.
When the car was cold, I would push the gas once or twice before starting to let the squirters add some fuel. That shouldn't be necessary with the new carb since that extra enrichment is taken care of with the choke.
The idle speed is then upped to help the car warm up faster and the extra airflow helps with fuel atomization, right?
Let me make sure I've got this right. When the choke is closed it creates a pressure differential between the throat and the bowl which pushes extra fuel into the carb through the main metering circuit. So the choke has no direct access to a fuel passage like the idle screws do, just indirect access to the main circuit through varying throat pressure.
When the car was cold, I would push the gas once or twice before starting to let the squirters add some fuel. That shouldn't be necessary with the new carb since that extra enrichment is taken care of with the choke.
The idle speed is then upped to help the car warm up faster and the extra airflow helps with fuel atomization, right?
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Close, but not exact...
At idle, there is no fuel discharged from the main discharge circuit. Fuel is only discharged from the idle and transition circuits. The discharge points for these circuits is below the throttle blades, so the amount of fuel pulled from the circuit is based on pressure differential and not on the bernoulli effect through the venturi. So by closing the choke plate, more fuel is drawn out these curcuits with less air allowed through the carb.
You need to pump the gas pedal prior to cold start whether or not you have a choke.
At idle, there is no fuel discharged from the main discharge circuit. Fuel is only discharged from the idle and transition circuits. The discharge points for these circuits is below the throttle blades, so the amount of fuel pulled from the circuit is based on pressure differential and not on the bernoulli effect through the venturi. So by closing the choke plate, more fuel is drawn out these curcuits with less air allowed through the carb.
You need to pump the gas pedal prior to cold start whether or not you have a choke.
Thread Starter
Racer
Joined: Jul 2004
Posts: 444
Likes: 0
From: Dallas TX
Ok, so the pressure difference caused by the choke is not enough to push fuel up the main circuit, but is enough to allow more fuel through the idle and idle transition.
I think I got it now. Thanks Lars!
I think I got it now. Thanks Lars!
