1978 Ford F-150

Tiny
BLIZZARDELF
  • MEMBER
  • 1978 FORD F-150
  • V8
  • 2WD
  • AUTOMATIC
  • 197,000 MILES
When accelerating from a dead stop, (like pulling out into traffic) the throttle stays open and and won't return when I let off the pedal so I have to either throw it into neutral or turn the key off. It only does this in drive. I can make this happen while sitting in drive with the brakes applied and then slowly giving it gas and then the throttle "catches". There are no throttle sticking problems with this holley 2 barrel carb. The return spring is tight. While in park there are no problems with it ever catching or stayng open. Also when cruising down the road I can accelerate suddenly with no problems and shifting is fine. The kickdown rod seems to be working properly (WOT). I have made adjustments to the vacuum modulator and the transmission responds like it should to these adjustments. Obviously this is a dangerous situation that I would like to get repaired, just not sure what. Thank You.
Friday, April 3rd, 2009 AT 9:39 AM

14 Replies

Tiny
RACEFAN966
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OK if you could include a pic of the carb without the air cleaner on it so I can see what all is there so I can help determine what the problem is that would be great. The throttle return spring should only be tight enough to close the throttle no more. If the return spring is to tight it will wear the shaft out and make it stick when it hits the worn spot and only do this when the return spring is hook up. Get back to me with the pic and we will go from there.
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Friday, April 3rd, 2009 AT 10:15 AM
Tiny
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http://www.2carpros.com/forum/automotive_pictures/339553_IMG_3626_1.jpg



http://www.2carpros.com/forum/automotive_pictures/339553_IMG_3627_1.jpg


Does not seem to be a carb issue because it only happens when the transmission is in drive.
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Friday, April 3rd, 2009 AT 11:04 AM
Tiny
RACEFAN966
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Ok if I see right there looks to be a vacuum hose under the throttle linkage is it toughing anything? Does there look to be any rub on it form the linkage? Next thing is are the engine mounts in good shape and not broken? How about the trans mount? Now this is an auto trans does any of the linkage going down to the pedal binding with any of the shifter linkage around or near the steering column? How about the bell crank at the end of the manifold is it in good shape and not groved? Get back to me with what you find and we will go from there.
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Saturday, April 4th, 2009 AT 8:54 AM
Tiny
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There is nothing binding any of the throttle linkage or kickdown rod. The only time this happens is in drive, on sudden accelleration. If I just steadily start accellerating I can continue all the way to wide open throttle with no issue. Motor mounts and transmission mounts are intact. Don't know about the bell crank and not sure how it would cause this. There are no other driveability issues.
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Saturday, April 4th, 2009 AT 11:04 AM
Tiny
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Is it possible that this is vacuum related? Also, could the vacuum modulator ever cause something like this to happen?

Thank You for the help.
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Sunday, April 5th, 2009 AT 7:29 PM
Tiny
RACEFAN966
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OK no it is not vacuum related. When under load the engine is either moving up on the left (drivers) side and binding the linkage. I really need you to look really good at the drivers motor mount even if it means jacking up the engine with a floor jack and piece of 2x6 between the jack and the oil pan and see if that drivers side picks up at all. I really think there is something allowing to much movement thats why its binding only under hard acceloration.
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Monday, April 6th, 2009 AT 9:24 PM
Tiny
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I'm sorry, I haven't done the best job describing this. It's hard to describe. This only happens with the transmission in drive. And it's not hard acceleration, just steady pressure on the pedal (with the brake firmly applied) then it "catches". But I am only moving the pedal about an inch. Not even enough to make the motor tilt. There's lots of clearance all around the throttle linkage down to the firewall. When it's in park, I can try all kinds of ways to duplicate this, but nothing related to the throttle ever sticks or catches. I've tried sudden moves on the pedal and just even presuure to almost wide open throttle with no problems. I would love to find something binding or a broken motor mount. But it's just not there.
Thanks again for your time.
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Monday, April 6th, 2009 AT 11:37 PM
Tiny
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Have you jacked up the engine to make sure it isn't seperated? Have you checked the trany mount for the same? If it only sticks under pressure that means that the engine is moving enough to cause something to bind up. So if you would jack the engine up just a little and then move the lingage by hand and see what if anything is changing. It could be that there is a bad bushing in the bellcrank between the pedal lingage and the carb linkage and it pulls under engine lift.
Also if you can block the wheels and then with in not running put it in gear and see if anything from the shifter is moving to a point of getting in the way. The linkage at the end of the column may be getting in to just the right position that it is causing this. See the cam at the bottom of the column that moves as you probably know so check to see that is clearing everything and not binding with the lingage any where.


http://www.2carpros.com/forum/automotive_pictures/249564_Graphic_64.jpg

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Tuesday, April 7th, 2009 AT 10:25 AM
Tiny
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Thank you. I will try again to see if the engine moving is causing the throttle to be in a bind. It's just that the engine moves a little while in park also and this throttle issue never happens. I have moved the transmission selector (to all positions) but do not see any clearance problems. Is it ok for this 400 motor and automatic c6 transmission to run without the kickdown rod connected? I have not tried this yet but I am curious if this could be the problem. Thanks again.
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Tuesday, April 7th, 2009 AT 8:30 PM
Tiny
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Yea it will be ok you just won't have passing gear thats all. Now the engine won't move much in park as the engine has no load on it. The only way to creat a load would be to put the trans in drive and then hold the brake and slowly step on the gas but I don't want you to that because if the engine lifts and the throttle sticks then it could be a bad thing. Thats why I am asking you to jack up the engine as discribed in out previous conversation. Let me know what you find.
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Wednesday, April 8th, 2009 AT 10:15 AM
Tiny
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Had to use a 4x4 block to get the floor jack in the right position under the oil pan. The motor mounts held and can also put some pressure at the bell crank without movement. The throttle shaft on the carb does not appear to be worn. Did replace the accelerator pump cam. I'm trying to avoid taking this in to a shop because I have to listen to them about everything else that is wrong. Some of it I've been hearing for 25 years (when I first started driving this truck). Thank you again for your patience and I will appreciate any additional advice.
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Saturday, April 11th, 2009 AT 4:24 PM
Tiny
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Ok then I need you to check the pedal and the linkage from the gas pedal to the bell crank and then the bell crank to the carb. Something here must be hanging up. Have you pulled the kickdown lever to see if that changes it any yet? Let me know what you find ok. This is really frustrating isn't it? It is for me we should have found something by now weather is be bent or worn part or something.
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Monday, April 13th, 2009 AT 6:26 PM
Tiny
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Thank you for staying with me on this. You have been right on target. The cruise control unit mounting bar had gotten bent letting the unit fall in the way after the hood was closed. I feel like an idiot now. Again, thank you for being patient with this even though you knew I just wasn't looking close enough.
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Thursday, April 16th, 2009 AT 9:23 AM
Tiny
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Wow I am not sure I would have guessed that one. Anyway glad to hear it is fixed and doing well again. Thanks for staying with it and keeping me informed. It is a little more difficult when you do this site unseen then when the car is infront you. Again thanks and have a great day.
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Thursday, April 16th, 2009 AT 10:11 AM

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