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1967 - 1972 F-100 & Larger F-Series Trucks Discuss the Bumpsides Ford Truck

I can't figure out why my truck is running rough

 
  #1  
Old 07-10-2019, 11:31 AM
71 F-100 Custom
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Unhappy I can't figure out why my truck is running rough

Any help will be greatly appreciated!

My truck is running rough. At first it was barely noticeable, but it has gotten worse and worse. It starts right up, but it backfires and misses. It idles OK, but runs worse at higher RPMs. It shudders when you try to accelerate. It runs worse after the engine warms up. It smells like unburned gasoline when it runs. It dies.

I canít figure out what the problem is, and I donít know what tests to do to identify the problem.

My truck is a 1971 F100 Custom with a 302, automatic transmission, non-power steering, and non- power brakes. It is stock except for an aftermarket air cleaner and an aftermarket HEI distributor. It has about 89,000 original miles. The in line fuel filter was replaced about 1 year and 5,000 miles ago. The battery was replaced 15 months ago.

These are the things I have done:
- I replaced the air filter.
- It wasnít driven much for about 6 months, so I gave it a fresh tank of name brand gasoline.
- I replaced the spark plugs. I used Autolite 124 plugs gapped to 0.040Ē.
- I replaced the distributor cap and rotor, but not the coil.
- I double checked that the spark plug wires were connected to the correct plugs.
- I hooked up my timing light to each one of the spark plug wires. All cylinders are getting spark.
- The timing marks on the old harmonic balancer were so faint I couldn't see them, so I replaced it with a brand new one.
- I set the timing to 6 degrees BTDC with the vacuum advance disconnected and the hose plugged.
- I removed the fuel filter in the fuel pump. There is not another fuel filter screwed into the carburetor inlet.
- I replaced the PCV valve.

Thanks in advance for any help you can give me!

Below is a photo of one of the old plugs.
 
  #2  
Old 07-10-2019, 12:22 PM
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kenny nunez
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Running rough

An HEI distributor requires a full 12 volts . You will have to wire up a 30 amp power relay to feed the distributor using the original resistor wire to energize the relay. If you are still using the original resistor wire that were used with points that could be your problem.
 
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Old 07-11-2019, 07:13 PM
71 F-100 Custom
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Thanks for your suggestion.

The HEI distributor was on the truck when I got it. The truck used to run great, so it seems to me that the voltage going to the HEI distributor probably isn't the source of my trouble.

I checked the voltage at the distributor and got about 13 volts DC at idle. It goes down to about 10 - 11 volts when the engine is revved. I've never checked this voltage before, so I don't know what the voltage was before the truck started running rough. It shows 14 volts when I put my meter across the posts of the battery while the truck is running.
 
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Old 07-11-2019, 07:26 PM
Jigs_n_Fixtures
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Check that the coil is good. There are trouble shooting guides all over the web on how to do it, and instructional videos on YouTube.
 
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Old 07-11-2019, 09:48 PM
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H E I

Just to be sure get a good jumper wire and go from the + on the battery to the spade connector on the H E I and see if there is any improvement.
 
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Old 07-12-2019, 01:34 AM
1TonBasecamp
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Next time you have it idling, check for vacuum leaks using one of the usual suspects. I prefer carburetor cleaner, but some use brake cleaner, while others use WD40 or even just water in a spray bottle.
The point is to spray it around the vacuum lines, gaskets, carburetor and intake manifold. If you spray it somewhere and the idle changes tone, whether going up, down, smoothing out or getting rougher, you've found a vacuum leak. And they're way too common an issue to ignore at this early stage.

Definitely check/test the coil if you find the proper procedure. Never done it myself, so don't know for sure to tell how it's done.
When you say HEI though, we're assuming for now you mean the big GM based large cap setup with the coil mounted to the top of the distributor? Not the later model Ford factory "hei" ignition called Dura Spark, or one of the other brands?
I'm guessing that's correct, but it's come up recently so it's good to be clear what you have.

A new damper does not always net you the correct timing marks for a 302 unfortunately. Sounds like yours is the correct one for your model based on what you're saying, but they are often misboxed with other 302 dampers and the timing marks are in different spots.
But at the very least you can play with your timing a bit. Rotate the distributor to advance the timing a few more degrees just to see what you get.

Was this engine rebuilt, or is it still original?

Thanks. Good luck.

Paul
 
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Old 07-12-2019, 10:13 AM
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Running rough

Have you ever rebuilt the carburetor? Your problem might just be a blown out power valve. The carburetor on your engine is the easiest to rebuild.
 
 


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