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1975 Ford F-100 Running Rough
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Grif
User
Feb 16, 2009, 7:05 PM
Post #1 of 5
(3136 views)
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1975 Ford F-100 Running Rough
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Hey folks,
You all helped me solve a problem with this truck before, so I'm coming back to ask for more advice.
Here's the info:
1975 Ford F-100
390 engine (miles unknown)
Auto transmission
The truck starts and idles fine, but when I press the gas pedal, the truck accelerates for a second or two then hesitates for a half second, then resumes acceleration for a second or two, then hesitates, and so on. This happens whether I'm taking off from a stand still or heading down the highway at 70 mph. I am aware that the trouble spot for the vintage was carb and choke issues. I'm suspicious that this is a carb issue as the engine feels like the fuel delivery is being repeatedly interrupted for a brief moment then resuming, then being cut off again. Maybe the carb needs tuning, but I've never done this before and am not sure what all is involved with this. Maybe the fuel pump is malfunctioning?
Any ideas would be appreciated.
Thanks in advance for helping me out, everybody!
Nick in Kalamazoo
(This post was edited by Grif on Feb 16, 2009, 7:06 PM)
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Loren Champlain Sr
Veteran
/ Moderator
Feb 16, 2009, 7:17 PM
Post #2 of 5
(3134 views)
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Re: 1975 Ford F-100 Running Rough
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Nick; You are right, the carbs were the 'trouble spot'. However, not so sure this is a carb issue. Try disconnecting the vacuum advance and drive it again. The pickup coil in the distributor was another trouble spot.
If the problem goes away, that'd be it. The accelerator pump diaghragm is a weak point and will cause a hesitation on tip in, but shouldn't be an issue at speed. The diaghragm is easily replaced and readily available through most parts stores (definitely NAPA). The power valve (or economizer valve) is also a weak point, but generally will cause a rich condition if it leaks. It is mounted on the underside of the carb under a similar four bolt cover like the accelerator pump. Also, easily replaced and readily available.
Loren
SW Washington
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Grif
User
Feb 17, 2009, 5:19 AM
Post #3 of 5
(3131 views)
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Re: 1975 Ford F-100 Running Rough
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Loren,
Thanks for your advice. I'll try disconnecting the vacuum advance as you recommended.
I should add that the engine backfires occasionally. Maybe this is a timing issue. Would improper timing cause the hesitation as I described?
Thanks for helping me out, everybody. I'm a young guy and this is my first truck, so I'm learning as I go.
Nick
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 17, 2009, 6:51 AM
Post #4 of 5
(3126 views)
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Re: 1975 Ford F-100 Running Rough
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Nick - re: timing possible issue:
If timing is changing all the time while driving it could do this. With unknown miles now and unknown on some history no doubt it could be a jumping timing chain.
Find the marks on damper/lower pulley and line them up where there are #s just as a reference. Find the socket size that fits the lower crank/damper bolt and turn it one way to a #ed index spot. Then take dist cap off and watch rotor while slowly turning the crank the other way and see how many degrees it turns the crank before it turns the dist rotor. You may be surprised it's over 10 degrees now! 5 would be plenty. You really can't see the chain and how loose it really is or the condition till you are right there looking at it. Some can be "felt" thru where fuel pumps go in but not sure on this.
I have no idea how these can jump for a moment or a while and manage to jump back to a spot that will run fairly well for a while and not just quit the first time but they can??!!
They are one of those things that with higher miles AND age cause hair loss to figure out sometimes. Check first and see what you find. If you want to do that job it's fairly cheap as things go but lots of stuff to do.
Just a maybe. The carb issues with choke and so forth should be done with once warmed up and driving along except the accellerator pump as Loren suggested. It's a quick job for a new one for those - they just add a squirt of fuel for speed changes or "pumping" gas pedal and fail quite readily and even when failed would run steady ok - just changing speeds gets messed up,
T
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Loren Champlain Sr
Veteran
/ Moderator
Feb 17, 2009, 5:02 PM
Post #5 of 5
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Re: 1975 Ford F-100 Running Rough
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Nick; Check timing chain slop as Tom suggested. If it backfires and hesitates at the same time, that can be caused by a lean condition, such as the accelerator pump diaghragm. Also, if the timing is retarded too far, can cause similar symptoms, but I'd think you notice it running rather sluggish all of the time. You could also have a plug or a wire or even a carbon track in the distributor cap that can cause intermittent misfires. With the engine off, look into the carb while moving the throttle. You should be able to see a 'squirt' of fuel each time you move the throttle. If you don't, replace the accel. pump diaghragm. Let us know what you find out.
Loren
SW Washington
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