Throttle return spring question
Postmaster
Joined: Oct 2008
Posts: 3,026
Likes: 0
From: Renton, WA
It has to do with the load placed on the throttle shaft.
The way the factory has it set up the throttle linkage pulls back to open the throttle. This will place X load on the backside of the throttle shaft bore in the carb. This load will cause the bore to wear.
To close the throttle there is a spring to pull the throttle shut. This spring hooks to the bottom part of the throttle arm, below the throttle shaft centerline. This means that to close the throttle the spring tension also pulls back towards the firewall, effectively doubling the load on the backside of the throttle shaft bore. All the load is placed on the backside of the throttle shaft bore. This is diecast aluminum or zinc and is relatively soft compared to the steel throttle shaft. This will make the bore wear egg shaped, causing air leakage, idle mixture problems, etc.
However, if you mount the return spring in front of the carb and attach it to the top of the throttle arm so it pulls forward to close the throttle, this effectively splits the load between the pull back of the gas pedal and the pulling forward of the return spring. This kinda evens things out reducing the wear on the throttle bore.
The way the factory has it set up the throttle linkage pulls back to open the throttle. This will place X load on the backside of the throttle shaft bore in the carb. This load will cause the bore to wear.
To close the throttle there is a spring to pull the throttle shut. This spring hooks to the bottom part of the throttle arm, below the throttle shaft centerline. This means that to close the throttle the spring tension also pulls back towards the firewall, effectively doubling the load on the backside of the throttle shaft bore. All the load is placed on the backside of the throttle shaft bore. This is diecast aluminum or zinc and is relatively soft compared to the steel throttle shaft. This will make the bore wear egg shaped, causing air leakage, idle mixture problems, etc.
However, if you mount the return spring in front of the carb and attach it to the top of the throttle arm so it pulls forward to close the throttle, this effectively splits the load between the pull back of the gas pedal and the pulling forward of the return spring. This kinda evens things out reducing the wear on the throttle bore.
Yup, looks right, except hooking the springS to the throttle linkage pin/ball in the narrow area in between the linkage and the arm would probably be a little better.
New User
Joined: Mar 2020
Posts: 22
Likes: 1
Were can one get the same type of throttle return spring you have on your FE 390 mikeo0o0o0. Is that the a period correct throttle return spring? I have a 390 on a 1973 ford f250 and its got the same bracket- I just need the same spring you have. Ford calls for part number D3TZ 9737-D.
Mountain Pass
Joined: Mar 2022
Posts: 114
Likes: 11
From: Mendocino county Califor
bolt hole goes toward the front of the engine and blue is on top. Bracket bend faces the throttle
Mountain Pass
Joined: Mar 2022
Posts: 114
Likes: 11
From: Mendocino county Califor
do you know if both brackets are factory and if so was two springs the stock set up? Just trying to understand what this is about. Looks like a aftermarket or dealer installed for something really looks like a spring bracket.
Hotshot
Joined: Feb 2004
Posts: 10,837
Likes: 2,612
From: The Shenandoah Valley
I added cruise control and a extension for kick down when putting the intake and a Holley on long ago.
Mountain Pass
Joined: Mar 2022
Posts: 114
Likes: 11
From: Mendocino county Califor
Do you recall if the bottom of the forward bracket was orange or blue underneath? It's just odd to have something not blue when everything else is. Not a big deal but definitely gets me thinking it was added. For someone needing a second spring do you have a aftermarket suggestion something you found similar to its strength?
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Hotshot
Joined: Feb 2004
Posts: 10,837
Likes: 2,612
From: The Shenandoah Valley
Do you recall if the bottom of the forward bracket was orange or blue underneath? It's just odd to have something not blue when everything else is. Not a big deal but definitely gets me thinking it was added. For someone needing a second spring do you have a aftermarket suggestion something you found similar to its strength?
Mountain Pass
Joined: Mar 2022
Posts: 114
Likes: 11
From: Mendocino county Califor
Don't recall color underneath. In looking closer at those springs on mine, I almost see red on the blue and blue on the red ... never noticed that before. Just get a pack of springs at Advance Auto off the "Help Line" aisle, experiment. You want something that pulls it shut but doesn't cause fatigue on a drive.
Hotshot
Joined: Feb 2004
Posts: 10,837
Likes: 2,612
From: The Shenandoah Valley
I like at least one spring in front to better balance the load on the throttle shaft (steel) going through the carburetor base (aluminum or zinc) ... partly why I keep the red one. Another reason for two, I've had one break while driving a school bus, so I've known for 50+ years that it happens.
Mountain Pass
Joined: Mar 2022
Posts: 114
Likes: 11
From: Mendocino county Califor
I like at least one spring in front to better balance the load on the throttle shaft (steel) going through the carburetor base (aluminum or zinc) ... partly why I keep the red one. Another reason for two, I've had one break while driving a school bus, so I've known for 50+ years that it happens.
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