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1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks Discuss the Early Eighties Bullnose Ford Truck

1981 F100 Misfire & Run-On Problem

 
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Old 02-09-2019, 02:17 PM
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1981 F100 Misfire & Run-On Problem







I recently purchased this F100 w/300 Straight 6 running and driving back in January as a project. No power brake, no power steering, no A/C. Manual transmission 3 on the tree. A friend and I went through and changed the oil, the radiator fluid, the fuel filter, the air filter, replaced the fuel tank and fuel pump, bought a new battery and replaced the spark plugs with new ones (yes we gapped them). I put sea foam in the gas tank, as well as ran a little through the carb. We didn't do this all at once but over a couple weeks.

The first thing we noticed was the run on when shutting it off. It would rattle and catch and act like it didn't want to shut off. We thought maybe it was a vacuum leak, so we took some carb cleaner and sprayed around the vacuum lines, replaced every rubber cap we could find and it stopped for a while. Then we noticed the missfire. It seemed like overnight the truck had jumped out of time. Also, now when the truck got warm the run on was worse, it rattles for a good 20 seconds before spitting white smoke out of the carb and finally shutting down.

SO we pulled the vacuum line on the distributor and we used a timing gun to time the truck... And other than idling better and sounding better nothing changed. There is still a missfire and it still spits and chugs on shut off.
Now I have replaced the distributor cap, as well as the spark plug wires, rotor, and the ignition coil. and still I can't figure it out. I doubt this is anything but I'll mention that the heater core is bypassed. I don't see a gage to check the RPM's at idle or I would include that bit of information as well. Any ideas?
I know this is long but here is a complete list of things I've replaced.

Fuel Tank, Fuel Pump, Fuel Filter, Radiator Fluid, Oil, Oil Filter, Spark Plugs, Plug Wires, Ignition Coil, distrubutor cap, and the rotor, Battery, Air Filter, and Rubber Caps on the carb.
 
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Old 02-09-2019, 03:42 PM
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Not to be obtuse or rude or unfriendly here and this is going to sound bad, but... if that yellow line is correctly following that rubber hose, somebody clearly has no idea what they were doing there.


 
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Old 02-09-2019, 03:45 PM
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First welcome to FTE.
I thought my 81 F100 flare side was a strip model with only power steering. I do have the flare side bed so think that was extra and a T18 4 speed with the granny low.
I have done some upgrading like added AC (swap fire walls), and power brakes (from parts truck and added a duel rear tank (from parts truck) never offered on a flare side.

Now your problem(s)
I wonder why people replace the coil when something does not run right? I have yet to see one that stopped working.
I would also look for the right size air filter housing. That little 3" thing is just about better than nothing. BTW my truck HAD the same one, was able to get one from a member on here.

So timing is set but you have run on. this can be caused by different things.
Most of the time the idle speed is to high. I know you said no tach but you need to find a hand held one as it can be used for other things also. My old school meter is a tach / dwell / volts / amp set up.
Try a paw shop to see what they may have for cheap or Amazon / Ebay.
For now just try turning the idle speed down till it just about stalls then back up just a bit to see what that does for run on.
Has this truck been sitting for a long time? Could the gas be bad? Bad gas will make it run like crap (miss) but not really cause run on.

Have you taken it out for a good blast down the high way to "blow it out"?
Hot carbon that the Sea Foam is knocking loose can cause run on.
Also the wrong heat range spark plugs staying hot, glow plugs, can also be a cause.

When you changed the plugs did you do a compression test on the motor? If you have a dead hole that can cause a miss.
It is also nice to know what shape the motor is in with the compression test.
Did this miss start after you replaced some of the vacuum hoses? I say this because that EGR hose looks new and don't think, almost 100% sure, it does not goes to the carb.
If me and did not need it for smog check, I don't thing you do either or you would not have the little air filter and some of the caps on vacuum lines, I would remove that hose from the EGR and cap the hose.
So no vacuum to the EGR and you may want to pull it off and make a blank off plate from a pop can and put the EGR back on so you know it cant cause any problems.
Do you have a vacuum line to the dist for advance? I want to say that is where that hose should go, need to check mine.

I see a large cap on the front of the carb that is the fuel bowl vent. Some do cap this and have had issues with hard hot starts. If left open had issues with gas fumes.
There should be a hose on it to the charcoal canister(s) that should be under the battery tray. There is a Tee in that line and the open side of the tee goes to a vacuum source, the top of the PCV.
From the factory it had a solenoid then the vacuum source but mine was long gone so I have it like above.

The other small cap should go to a metal line that goes to the exh. manifold. They like to rust off at the manifold but Dorman makes a repair kit you can get at parts stores to fix this.
That line is the fresh clean air supply from the carb thru the manifold to heat the air then up to the choke.
I saw the line at the choke does it go to the exh manifold? This is the hot air side and with out this set up the choke will not open all the way.
There should also be a wire from the ALT, S lug to help open the choke.

On the back lower side of the intake is a vacuum tree. I see a long hose off of it but don't see where it goes? Is it plugged?
Also check that tree really good as there is a port that is really hard to see pointing to the motor that the cap likes to dry, crack and fall off causing a vacuum leak and could cause a miss.

I think that is enough for you to check for a start.
Good luck and keep us posted on what you find.
Dave ----
 
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Old 02-09-2019, 03:51 PM
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Originally Posted by ctubutis View Post
Not to be obtuse or rude or unfriendly here and this is going to sound bad, but... if that yellow line is correctly following that rubber hose, somebody clearly has no idea what they were doing there.


See what happens when you write a book
That was the EGR hose I was talking about.
Also I see the dist. has a metal line up over the valve cover but I cant tell where it goes on the manifold side of the motor?
Is it connected to anything? It may go to that EGR port on the carb if it gets ported vacuum when above idle speed.
Dave ----

edit: I see you have a sticker on the radiator support, that should have the vacuum hose routing on it.
I would only worry about the dist. hose and the carb bowl vent hose and don't worry about anything between them that the sticker shows as long as they get a vacuum supply and end at the device.
 
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Old 02-09-2019, 04:27 PM
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Thank you both for your response.
Here are some of the things I can answer then I'll go and take a look at some of those other sugestions and get back to you.
YES! That air filter is ridiculous, I've been shopping for one online but stopped at auto zone for one just so I had a clean one. The last one was trashed.
I'll look for a tach or call around and see if someone I know has one I could borrow.
It sat for about 3 years, as far as I know according to the last owner.
It's got good gas, and a new fuel tank, the last one was almost rusted through.
I did not compression test it when we did the spark plugs, nor do I own a comp tester. So I'll look for one of those as well.
That EGR hose was stumping the hell out of me as well but I didn't know enough to take it off.
I'll be right back with more information.
** I'll add this disclaimer ** I got this truck as a project to learn and teach myself this skill set. I've always been interested in motors and trucks but never had anyone to show me. So I had a son and decided that I needed to learn so that one day I can teach him.
 
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Old 02-09-2019, 04:32 PM
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Old 02-09-2019, 04:46 PM
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My dads dad worked on big rigs and construction equipment, my dad did not like to get greasy so he leaned to do body work, dad taught me body work but hate it and I went to school (tech high school) to learn the mechanical side but only did it for a living for a few years.
I tried to teach my son, he was 5 when I could have him with me to help, what I know but was hard because he did not live with me he is 28 now.
Now with computers he can look up any thing he wants and asks me just how I would do it.
But to tell you he has done a lot on his YJ Jeep by him self and done it well.

It is good you want to learn and then teach your son it is great to bond that way. If you can learn how to fix the truck with out a computer it will make you and son a better person if / when you take a computer car / truck in for repairs. I don't fix my 02 Durango but I have told the dealer every time what was wrong before they put the computer to it to tell them what was wrong.
It is the skills to trouble shoot the older stuff that I use on the new stuff and it all runs the same way - gas / air / spark = run.
You just need to know what the new parts do, EFI over carb, and what the computer need to make the EFI work.
Dave ----
 
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Old 02-09-2019, 04:47 PM
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There is a metal line that runs to the intake manifold and a smaller line that runs to the exhaust manifold.
A fuel line.
Distributor line and my throttle cord running over the valve cover.
And a wire from the back of my carb that joins a harness to the solenoid and the alternator.
There are two other lines that are bypassed that I'm assuming used to run to the heater core.
Also a manual choke to the top of the valve cover? I think?
 
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Old 02-09-2019, 04:56 PM
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Ok I see how the dist line is run I just don't know if it is hooked to ported supply or if where the EGR was hooked to is ported.
Ported has no supply at idle but if you give it throttle the port will have vacuum supply, dist. gets hooked to that one and the other port caped.
DO NOT forget to cap the port the EGR hose came off from.

I also see a cap on a white thing under the carb bowl vent (2nd picture) that is on a bracket. Is there a nipple on the other side and if so you can remove that cap as there should not be any vacuum to it.
You may need that cap for that vacuum tree on the rear under side of the intake manifold.
Better yet see that long hose that goes down to the screw on the hose clamp holding the plug in the air injection hose use that cap on the tree and remove that long hose.
Dave ----
 
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Old 02-09-2019, 05:03 PM
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Originally Posted by Jakehx F100 View Post
There is a metal line that runs to the intake manifold and a smaller line that runs to the exhaust manifold. Can you point out the metal lines?
A fuel line. I see that
Distributor line and my throttle cord running over the valve cover. I now see that also
And a wire from the back of my carb that joins a harness to the solenoid and the alternator. Yes that is right
There are two other lines that are bypassed that I'm assuming used to run to the heater core. Can you point them out also
Originally Posted by FuzzFace2 View Post
Ok I see how the dist line is run I just don't know if it is hooked to ported supply or if where the EGR was hooked to is ported.
Ported has no supply at idle but if you give it throttle the port will have vacuum supply, dist. gets hooked to that one and the other port caped.
DO NOT forget to cap the port the EGR hose came off from.

I also see a cap on a white thing under the carb bowl vent (2nd picture) that is on a bracket. Is there a nipple on the other side and if so you can remove that cap as there should not be any vacuum to it.
edit I think this white thing is a throttle vent. When the throttle is at WOT the flap hits a stem on this valve and think would dump vacuum. So yes that cap can be removed.
You may need that cap for that vacuum tree on the rear under side of the intake manifold.
Better yet see that long hose that goes down to the screw on the hose clamp holding the plug in the air injection hose use that cap on the tree and remove that long hose.
Dave ----
I should wait for you to post the notes LOL
 
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Old 02-09-2019, 05:22 PM
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This is one of the metal lines to the manifolds.

This is the other metal hose, i know it looks like rubber lol

These are the two hoses that are bypassed, the big one has a bolt in it and the small long one is on the screw of the hose clamp.
 
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Old 02-09-2019, 05:25 PM
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This is what the distributor hose is hooked to
 
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Old 02-09-2019, 05:25 PM
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Now I'll re cap the tree.

Question if the manual choke was malfunctioning, would that have any effect?
 
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Old 02-09-2019, 07:59 PM
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Originally Posted by Jakehx F100 View Post
Now I'll re cap the tree.

Question if the manual choke was malfunctioning, would that have any effect?
I don't think you have a manual choke. Do you have a cable going to the choke housing? If not then it is an auto choke.
You also posted you have a wire from a harness that runs from the ALT. so it would be an auto choke.
Below are pictures of the exh manifold with a coat hanger going in the top & bottom where the metal line should be.


The metal line to the carb with the fitting is the top line, the hot line.
Now I cant make out your bottom line but it should go into the bottom of the exh manifold and gets hooked to the carb here, you have a rubber plug on it now.

Dave ----
 
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Old 02-09-2019, 08:20 PM
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Yep. I'm missing that one.
 

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