Holly 4bbl problem
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Tuned
Joined: Dec 2001
Posts: 418
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From: Greeneville TN USA
Holly 4bbl problem
My 1968 N750 with a 361 and Holly 4bbl (not originally equipped with that carb, as the 4bbl was only on the 391). Anyway, I did rebuild the carb at one point, and it seemed to be in good shape but with a bit more clearance throttle shaft-wise than I would have liked, and maybe that is my problem. The throttle does not return well to idle and wants to drop the idle speed to 1000 rpm rather than 550 or so. As soon as the engine is shut off the throttle comes back against the stop. I have a stiff return spring on it which helps, but not enough. If I remember correctly, I did install an O ring on the shaft placed against the base to limit air admittance at that point, but that did not seem to help much if any.
All suggestions appreciated!
Tom
All suggestions appreciated!
Tom
Last edited by Tomc750; Dec 22, 2021 at 09:04 PM. Reason: spelling
Super Moderator
Joined: Apr 2004
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So when the engine is running, you can't make the throttle go back against the stop?
Is there a governor under that carb? What kind of vacuum hoses are running in/out?
Have you gone through the linkage from the pad on the pedal to the carb? Lots of places for things to bind up.
Is the shaft binding in the carb body due to the vacuum against it at idle?
Is there a governor under that carb? What kind of vacuum hoses are running in/out?
Have you gone through the linkage from the pad on the pedal to the carb? Lots of places for things to bind up.
Is the shaft binding in the carb body due to the vacuum against it at idle?
Cargo Master
Joined: Oct 2020
Posts: 2,685
Likes: 789
My 1968 N750 with a 361 and Holly 4bbl (not originally equipped with that carb, as the 4bbl was only on the 391). Anyway, I did rebuild the carb at one point, and it seemed to be in good shape but with a bit more clearance throttle shaft-wise than I would have liked, and maybe that is my problem. The throttle does not return well to idle and wants to drop the idle speed to 1000 rpm rather than 550 or so. As soon as the engine is shut off the throttle comes back against the stop. I have a stiff return spring on it which helps, but not enough. If I remember correctly, I did install an O ring on the shaft placed against the base to limit air admittance at that point, but that did not seem to help much if any.
All suggestions appreciated!
Tom
All suggestions appreciated!
Tom
Thread Starter
|
Tuned
Joined: Dec 2001
Posts: 418
Likes: 24
From: Greeneville TN USA
So when the engine is running, you can't make the throttle go back against the stop?
Is there a governor under that carb? What kind of vacuum hoses are running in/out?
Have you gone through the linkage from the pad on the pedal to the carb? Lots of places for things to bind up.
Is the shaft binding in the carb body due to the vacuum against it at idle?
Is there a governor under that carb? What kind of vacuum hoses are running in/out?
Have you gone through the linkage from the pad on the pedal to the carb? Lots of places for things to bind up.
Is the shaft binding in the carb body due to the vacuum against it at idle?
Thread Starter
|
Tuned
Joined: Dec 2001
Posts: 418
Likes: 24
From: Greeneville TN USA
Your throttle plates might need to be realigned. Take the throttle plate off of the carburetor, back off the curb idle screw and then loosen the screws that hold the throttle plates, just crack them loose. Then gently tap on them while you hold the throttle plate up to the light, you'll be able to see them get centered in the bores, when they're good tighten up the screws. Now give it a try and see if it's better. If not the throttle shaft might have to be rebushed which isn't a big deal for most carburetor shops.
Cargo Master
Joined: Oct 2020
Posts: 2,685
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Fleet Mechanic
Joined: Feb 2007
Posts: 1,652
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From: Mooresville, NC
High idle is typically a vacuum leak --- I'd run a comprehensive vacuum inspection / diagnostic (since it's pulling back on shut down), then go thru the idle circuits one more time, recheck primary and secondary fuel bowls / levels (via the sight plugs), plus I'd also equip it with a Air/Fuel Ratio Gauge w/O2 sensor.
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Cargo Master
Joined: Dec 2015
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From: Harned, Ky
Although the 361FT normally used a 2 bbl Holley the later L series came both ways
I have a cousin with a 72 LN700 that was 361 with 4 bbl Holley
If you can pull up on the pedal and slow the engine down you have a linkage issue
Will it idle down with the linkage disconnected
I have a cousin with a 72 LN700 that was 361 with 4 bbl Holley
If you can pull up on the pedal and slow the engine down you have a linkage issue
Will it idle down with the linkage disconnected
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Thread Starter
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Tuned
Joined: Dec 2001
Posts: 418
Likes: 24
From: Greeneville TN USA
Although the 361FT normally used a 2 bbl Holley the later L series came both ways
I have a cousin with a 72 LN700 that was 361 with 4 bbl Holley
If you can pull up on the pedal and slow the engine down you have a linkage issue
Will it idle down with the linkage disconnected
I have a cousin with a 72 LN700 that was 361 with 4 bbl Holley
If you can pull up on the pedal and slow the engine down you have a linkage issue
Will it idle down with the linkage disconnected
Thread Starter
|
Tuned
Joined: Dec 2001
Posts: 418
Likes: 24
From: Greeneville TN USA
High idle is typically a vacuum leak --- I'd run a comprehensive vacuum inspection / diagnostic (since it's pulling back on shut down), then go thru the idle circuits one more time, recheck primary and secondary fuel bowls / levels (via the sight plugs), plus I'd also equip it with a Air/Fuel Ratio Gauge w/O2 sensor.
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