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Engine won't hold rpm
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New User
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Engine won't hold rpm
Hope fully I'm posting this in the right section.
Truck specs
63 f100
4.9l 300 w/Aod transmission
Holley 4150 carb
Ford 9" rear end
OK so here's the problem the engine will not hold speed in OD gear. Truck will shift into OD at 60mph and from there it slowly loses rpms no matter how much gas I give it until it downshifts into 3rd then it gains rpms. I've already set the pressure on the TV cable to 35psi. Is this a timing issue or carb issue. I'm at loss as here. Any help is greatly appreciated
Truck specs
63 f100
4.9l 300 w/Aod transmission
Holley 4150 carb
Ford 9" rear end
OK so here's the problem the engine will not hold speed in OD gear. Truck will shift into OD at 60mph and from there it slowly loses rpms no matter how much gas I give it until it downshifts into 3rd then it gains rpms. I've already set the pressure on the TV cable to 35psi. Is this a timing issue or carb issue. I'm at loss as here. Any help is greatly appreciated
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
If you can operate the truck in 1 to 1 ok but it looses rpms in OD, makes me believe it's the rear ratio.
Sounds like you may have something like a 3.00 ratio when you need a 3.75. Sounds like it is dropping the rpms to a level that it can't maintain.
John
Sounds like you may have something like a 3.00 ratio when you need a 3.75. Sounds like it is dropping the rpms to a level that it can't maintain.
John
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Thanks for the help.
Senior User
Joined: Jun 2015
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Clogged air cleaner.
Vacuum problem.
Timing could need adjustment.
Or just a worn out engine.
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New User
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Engine is a fresh rebuild. I've ran thru the basics as you mentioned except for the fuel filter. I would think that if it was the fuel filter it would act like it does through out all the gears?
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Fleet Owner
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Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Then it's most likely the ratio. The engine is turning rpms below it's power band and can't keep up. Get it out on a big road and drive it 70 mph or more and try OD. Bet it will work there.
John
John
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If it makes a difference the problem is at highway speeds.
Logistics Pro
Joined: Feb 2011
Posts: 3,522
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From: Townsend, MT
Yup. Just that if it has something like 3.25's in the rear end, you will have to be going a little over 80 MPH to even be turning 2000 RPM. If it has 3.00's you will have to do 90 to get to 2000 rpm.
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