hello! This is my first post on here but idk where else to turn. I have owned this 1986 F150 for almost 3 years now and never had an issue and it always ran fine. But last August the truck died at a stop light while I was idling. It will run only when the accelerator is pressed down but as soon as RPM drops it dies.

the truck does not have the original feedback carb but it always ran fine with the replacement. I did buy a new carb just to see if that was the problem/ maybe a vacuum leak around carb but still nothing.

i also replaced the fuel pump and brass in line fuel filter. Attaching a video for reference.

 

Welcome to FTE

It sounds like it has a miss or a vary large vacuum leak
When was the last tune up done? Plugs cap / rotor and air filter not that will fix it but cant hurt if it has been a bit.
Time can go by fast as I had a miss under load and found it was a bad spark plug with 28K on the tune up.

What I would do is air filter off grab the top of the carb and try and move it.
The straight six carbs like to come loose and cause issues. Still dont think that is it ....yet.

Pull the EGR valve off the base and make a plate from a tin can to block off the port and bolt the valve back in place.
See if this fixes it as I think this is the cause. Valve stuck open causing a lean issue and why it will not idle.
With the block off plate the valve will no longer work.

If the valve & carbon is not too bad you might be able to tap the valve with a hammer to get it to close but the next time it opens it may do the same thing?
Could pull and plug the vacuum line to the valve it the hammer trick works to keep the valve from opening.

Let us know what happens from there.
Dave ----

 

Good call on the EGR valve.

Another possibility is the PCV valve. It’s supposed to close at idle. If not, that’s a major vacuum leak. There are methods to test the PCV valve, but the easiest is to gamble on a quality replacement and see what happens.

 

I will check plugs today. I’m not sure when they were changed last honestly. I too thought it sounded like a bad vacuum leak. Will check EGR and carb. May upload some pics.

 

I dont think I have ever seen a PCV stick open to cause a vacuum leak? But guess it could happen.
On the 300 six motor the valve had a small nipple at top and on some years it is used for the carb bowl vent evac.
I had mine hooked to this small nipple with out the factory switching valves and that caused a vacuum leak.
I installed the smallest carb jet I had to lessen the leak

The way yours started it am sure this next one is not what you have going on.
If you have a vacuum leak but cant find it at the carb, intake gaskets or vacuum hoses check the "metal juice can" on the right inner fender.
Mine had pin holes all across the bottom of the can.
I pulled it off, removed the bracket and used fiberglass rosin to coat the can sealing it.
Painted it black and put the bracket back on and good as new.

When you got the plugs out it would be a good time to run a compression check.
With my miss and plugs were out I ran one just so I would know if that 1 bad plug hole had a issue.
My tester was the push in type with a rubber end. The rubber was a little hard, had it since the early 80's, to get a good seal but that hole was no worst than the others.
Yes I have since bought a screw in type, I could not find the adapter for my old screw in type I also had since the early 80's.
Dave ----

 

When it's fully warmed up and the choke is straight up, are you setting the curb idle on the new carbs? The original carb had a little dc motor that was controlled by the computer to set the idle. Of course none that works now. I do not like this mix/match setup you have there, I think you should go ahead and put another distributor in it also. Your fuel mileage will go up and it will pay for itself in a few months as high as fuel is.

 

Dave the way I read it he had a non-feed back carb on the motor that was running fine up to last Aug.
He also bought a 2nd non-feed back to try and has the same issue.

It could be a spark plug or 2 are fouled?
Had that happen on a chevy 350. Was running fine on the high way got off to get gas and it started missing running poorly.
My 300 only started missing when I hooked up to a dual axle enclosed trailer! First when under load pulling it to missing at idle on the way back.
Dave ----

 

my plan is to switch to HEI distributor but as mentioned, it was running fine without the feedback carb and then suddenly stopped. If I can get her to at least idle, I’ll start on the HEI. Pulling plugs this evening. Also checking for any vacuum leaks.

 

Here are some pics of the carb and around EGR. Do any of these need to be plugged/



corrected for vacuum?

 

All need to be plugged. C is a huge vacuum leak. If you can get the engine running stick your finger on it, it should suck your finger right to the nipple. That is a very big leak.

B needs to be plugged just to keep dirty unfiltered air from entering the engine. Otherwise it's not a serious offender.

A. Do you get exhaust out of that or suction on it? If it's doing nothing, it probably will someday. Best thing is to unscrew that out of the bottom of the carb, but that is going to be a nasty terrible job I am afraid.

 

thank you so much for your quick responses. Should I just plug A with a large bolt or something after it’s removed?

 

Depends on what thread it is. I am not familiar with that particular setup, it may be pressed in place. You may be better off to take a sawzall and cut it off behind the rusted piece leaving as much of the pipe as possible, and then taking vise grips and pinching the end of the pipe shut, and then try to fold it over. Then take a hammer and a metal block and smash it shut. If all that is possible. Just to seal it off. If you had a welder you could smash the end and then weld it.

 

so I plugged those vacuum areas except for the area beside EGR. And it still dies out. I was able to get jump the starter so that I could try and hear/see what was happening and yes there is air coming out of the big rusted area.

 

Sounds like Dave is all over this one. When that happens, it’s best just to let him finish:



One YUGE suggestion, though, while you work through his suggestions . Put away your jumper cables and charge that poor battery!

The only time you should need jumper cables is for emergency use away from home. Even then, they are only suitable for temporary situations with a healthy battery that ran down just a little below the minimum needed to crank the starter. For example, leaving your lights on accidentally while parked for several hours. If the battery is fully dead, jumper cables won’t help much either.

If you’re at home and the starter can’t get the crankshaft spinning at a good clip (regardless of whether the engine actually runs), the first step should be to charge the battery.

For all we know, the charging system could be subpar and not keeping up at low RPM. This could lead to a weak spark at idle, causing the engine to die. In addition, the battery would not be getting a full charge and causing slow starter cranking speed.

I’m not suggesting getting too sidetracked with such a possibility right now. But do charge that poor battery and see if the situation improves.

Edit: Forgot to mention another important detail. If repeatedly trying the starter and it just won't get the crankshaft spinning properly (with or without jumper cables), don't keep trying and trying. You'd run the very real risk of burning out a perfectly good starter. That's all you need to pile onto whatever fault is causing the engine to quit at idle.

Now back to Dave.

 

For "C" get a short hose that fits that nipple and then find a bolt to fit the other end of the hose as a plug and stick it on the nipple.
It should fit the nipple snug so no need for a clamp.

Use the same for "B" to keep dirty air from being pulled into the carb.

Heck do the same for "A" after you cut that big rusted thing off the end and just have a straight pipe.

I am 99% sure after "C" is plugged off the motor will run and idle again as that is a huge vacuum leak.
BTW see that rubber cap at C? I bet there was one on that open nipple and it cracked and fell off so you may want to do the hose & bolt / screw on that rubber cap one before it falls off too.
Dave ----